A BOTTOM GRAVITY SURVEY OF THE CONTINENTAL
SHELF BETWEEN POINT LOBOS AND POINT SUR,
CALIFORNIA
Walter Browne Woodson
Na^Pos^raauate Scnoo^
Monterey, California 93940
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A BOTTOM GRAVITY SURVEY OF THE
CONTINENTAL SHELF BETWEEN POINT LOBOS
AND POINT SUR, CALIFORNIA
by
Walter Browne Woodson, III
Thesis Advisor
R. S. Andrews
J. J. von Schwind
September 1973
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A Bottom Gravity Survey of the
Continental Shelf Between Point Lobos
and Point Sur, California
by
Walter Browne Woodson, III
Lieutenant, United States Navy
B.A., University of Mississippi, 1966
Submitted in partial fulfillment of the
requirements for the degree of
MASTER OF SCIENCE IN OCEANOGRAPHY
from the
NAVAL POSTGRADUATE SCHOOL
September 1973
Libra
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ABSTRACT
From an occupation of 68 ocean bottom and 38 land gravity stations
between Pt. Lobos and Pt. Sur, California, a complete Bouguer anomaly
map was produced and analyzed. The steps in data reduction leading
to the complete Bouguer anomaly field is presented, unique features
of which are associated with bottom gravimetry.
The geological interpretation of the gravity data shows excellent
correlation with earlier seismic records of the proposed offshore
extension of the Serra Hill fault, a structure long associated with the
Sur-Nacimiento fault zone. Two dimensional models of gravity
anomaly profiles were constructed across this fault and another fault
located several kilometers to the northwest and extending into the
western tributary of the Carmel Canyon. The results indicate a
minimum vertical displacement of the basement of approximately
2 km on the southwest sides, It was concluded that these two faults
are one in the same. Evidence is presented which indicates that the
Palo Colorado fault zone, located approximately 2 km to the east,
parallels the Serra Hill fault and subsequently leads into the eastern
tributary of the Carmel Canyon.
TABLE OF CONTENTS
I. INTRODUCTION 10
A. OBJECTIVES 10
B. LOCATION AND TOPOGRAPHY ■ 10
C. PREVIOUS WORK 14
II. SURVEY PROCEDURES 25
A. UNDERWATER GRAVIMETRY 25
1. Equipment 25
2. Calibration 29
B. STATION SELECTION AND LOCATION 33
C. SURVEY OPERATIONS 34
1. Measurement Procedures 34
2. Navigation 37
3. Equipment Reliability 3 8
D. COASTAL SURVEY 38
III. DATA REDUCTION 42
A. OBSERVED GRAVITY 42
1. Instrument Drift Correction 44
2. Earth Tide Correction 45
3. Curvature Correction 45
B. THEORETICAL GRAVITY 46
C. TOTAL UNDERWATER REDUCTION 46
1. Corrections 47
a. Initial Bouguer Correction 47
b. Free-Air Correction 48
c. Secondary Bouguer Correction 49
d. Terrain Correction 50
2. Gravity Anomalies 56
a. Free-Air Anomaly 56
b. Simple Bouguer Anomaly 57
c. Complete Bouguer Anomaly 57
D. PARTIAL UNDERWATER REDUCTION TO COMPARE
WITH SEA SURFACE GRAVIMETRY 58
1. Elevation Correction 58
2. Mass Adjusted Free-Air Anomaly 59
E. LAND REDUCTION 59
1. Corrections 59
a. Bouguer Correction 59
b. Free-Air Correction 60
c. Terrain Correction 60
2. Gravity Anomalies 62
IV. DATA PRESENTATION AND ANALYSIS 63
A. GENERAL DISCUSSION 63
B. CBA ANALYSIS 65
C. TWO-DIMENSIONAL PROFILES 71
D. CONCLUSIONS _- 89
4
V. FUTURE WORK 92
APPENDIX A: DATA REDUCTION CORRECTION VALUES
FOR INDIVIDUAL STATIONS 93
APPENDIX B: VARIOUS GRAVITY ANOMALIES AND
LOCATIONS FOR INDIVIDUAL STATIONS 98
APPENDIX C: COMPUTER PROGRAM 103
REFERENCES CITED 106
INITIAL DISTRIBUTION LIST 109
FORM DD 1473 ppp
LIST OF TABLES
Table Page
I Explanation of Geologic Abbreviations 21
II Possible Errors in Complete Bouguer
Anomaly Calculation (values in milligals) 66
LIST OF FIGURES
Figure
1 Bathymetry of the Survey Area 11
2 Survey Area Limits and Location 13
3 Geological Boundaries of the Salinian Block
(after Greene et al. , 1973) 16
4 Regional Fault Map (after Greene et al. , 1973) 19
5 Regional Geology and Fault Location
(after Dohrewend, 1971) (geologic abbreviations
listed in Table I) 20
6 Simplified Diagram of the LaCoste and Romberg
Gravimeter (after LaCoste, 1967) 26
7 Model HG6 Gravimeter Ready for Use 27
8 Internal View of the Model HG6 Gravimeter 28
9 Schematic Diagram of the Auxiliary Equipment 30
10 Auxiliary Equipment Installed Aboard the
R/V ACANIA 31
11 Naval Postgraduate School's Oceanographic
Research Vessel R/V ACANIA 32
12 Ocean Bottom and Land Station Density and
Location 35
13 LaCoste and Romberg Model G-08 Geodetic
Land Gravimeter 40
14 Schematic Representation of the Steps
Necessary to Compute the Elevation Correction
( O = density in grams per cubic centimeters) 51
15 Schematic Diagram Showing Areas Involved in
Terrain Corrections for Ocean Bottom Stations 54
16 CBA Distribution for the Continental Shelf
and Adjacent Coastline Between Pt. Lobos
and Pt. Sur (values in milligals, contour
interval 2 mgal) 64
17 PDR Profile Locations and Fault Scarp
Position from a Bathymetric Study 72
18 PDR Profile A_A' 73
19 .PDR Profile B-B» 74
20 PDR Profile D-D' 75
21 PDR Profile E-E' 76
22 PDR Profile F_F' 77
23 PDR Profile H_H' 78
24 Location of Two-Dimensional Modeling Profiles 79
25 Depth to Basement of Profile Model A_A* 82
26 Calculated and Observed Gravity for Profile A-A1 83
27 Depth to Basement of Profile Model B-B1 84
28 Calculated and Observed Gravity for Profile B-B1 85
29 Depth to Basement of Profile Model C-C 86
30 Calculated and Observed Gravity for Profile C-C 87
31 Summary Fault Map Indicating Proposed Locations 90
ACKNOWLEDGEMENTS
The author wishes to express his appreciation for the professional
guidance and cheerful support provided by his thesis advisors, Dr.
Robert S. Andrews and Dr. J. J. von Schwind of the Naval Postgraduate
School (NPS) Department of Oceanography. To Capt. "Woody" Reynolds
and the crew of the R/V ACANIA go a heartfelt thanks for their timely
assistance in all aspects of the gravity survey operations and for their
never-ending good sense of humor. Dr. Howard Oliver, Dr. S. L.
Robbins, and Mr. Richard Farwell of the U. S. Geological Survey
(USGS) provided the land gravimeter, computer programs, and much
helpful information needed to conduct the survey. Mr. H. Gary Greene,
USGS marine geologist, made available much unpublished material
pertaining to the local geology of the survey area. Acknowledgements
are also made to Mr. 'Yogi' Parks and Mr. Joe Bighorse of LaCoste
and Romberg for their patience and guidance in the maintenance of
the underwater gravimeter. ST1 Rick Desgrange and Mr. Dana May-
berry of the NPS Oceanography Department were essential in maintain-
ing the underwater gravimeter in working order. The NPS Public Works
Department and Mr. Pete Wisler provided necessary assistance on a
number of occasions. Finally, this work would not have been possible
without the assistance of Lt. Henry Spikes, co-worker and good friend.
Partial funding for this project was provided by NPS Research
Foundation from Office of Naval Research Resources.
I. INTRODUCTION
A. OBJECTIVES
The continental shelf between Point Lobos and Point Sur, Cali-
fornia (Fig. 1), is an area in whicb the geology has only been super-
ficially examined. In contrast, the coastal region of this part of the
California coast has been studied extensively in the past, and continues
to be in the present, due to increased public awareness of probable
future earthquakes and proposed residential and commercial construction.
Seismically active fault zones exist along the continental shelf of
Central California. The area between Pt. Lobos and Pt. Sur must be
included as one of these zones. Although no major earthquakes have
occurred in this region throughout the period of record (since 1926),
there is a good probability for future seismicity based on the recent
mapping of offshore structures by Greene et al. [1973].
The present gravity survey was undertaken to add to the sparse
geological data that presently exists and in hopes that the composite
will, in the future, produce a detailed and complete understanding of
the structure of the continental shelf and surrounding area.
B. LOCATION AND TOPOGRAPHY
The area of study encompassed in this research is that of the
continental shelf, from the coastline west to the 100 fathom (183 m)
contour and from Pt. Sur north to Pt. Lobos, California. This area
10
Figure 1. Bathymetry of the Survey Area
(depth contours in fathoms)
11
is bounded by latitude 36°18.5' N and 36°31.5'N and by longitude
121°54' W and 122°58. 8" W (Fig. 2). In this region the shelf break
occurs at approximately the 70 fathom (128 m) contour at an approxi-
mate distance of 2 km from the shoreline in the north to over 11 km
to the south off of the Pt. Sur tombolo.
The shelf is interrupted in the north by two tributaries of the
Carmel Canyon located approximately 3.7 km southwest of Pt. Lobos,
by an extension of the Monterey Canyon to the west of the central
portion of the area, and by the Sur Canyon almost due south of Pt. Sur.
The area generally exhibits flat terrain over the continental shelf with
the exception of one or two northwest trending fault scarps of approxi-
mately 20 m relief and numerous rock outcroppings marking the sea-
ward extensions of the many rocky headlands. Dohrewend [1971], from
bathymetric and seismic profiling, determined the slope of the shelf
o
to be approximately 1.5 over 90% of the area.
Immediately to the east of the area, the Santa Lucia Range
rises abruptly attaining frontal heights of over 800 m. This rugged
range extends the entire length of the area attaining a maximum
separation from the coastline of 16 km at Pt. Sur. Two major streams,
the Big Sur and Little Sur Rivers, are found on the western half of the
range. Both have only seasonal flow, and like all streams in the area,
are separated from the eastern side of the mountains.
Two well defined marine terraces are evident along the coast-
line, one averaging 25-30 m above present sea level, and the other,
12
122 u00
•30
f36°24'
OB OS
? km f
nmi
OBERANES PT
ASLAR PT
OCKY PT
IXBY LDG
RRICANE PT
Little Sur
River
Figure 2. Survey Area Limits and Location
13
approximately 65 m [Trask, 1926; Phifer, 1972], These well defined
marine terraces are probable evidence of the recent emergence of the
northwest portion of the Santa Lucia Range. This uplift is most likely-
continuing at the present time [Phifer, 1972].
C. PREVIOUS WORK
It has only been within the past 3 or 4 years that any geological
work has been carried out within the study area. Dohrewend [1971]
and Ellsworth [1971] utilized seismic reflection profile records,
precision depth recording (PDR) traces, and core and grab samples
to describe the geology of the continental shelf between Pt. Lobos and
Pt. Sur. Greene et al. [1973] conducted a study of faults and earth-
quakes in the Monterey Bay region. Most of their work within the
region appears to be a compilation of previous onshore studies, coupled
with some offshore seismic reflection profiles and dredge hauls used
to approximate offshore extensions of onshore faults. Colomb [1973]
made a study of recent sediments on the shelf between Pt. Lobos and
Pt. Sur, and in conjunction with this, conducted two bathymetric and
seismic profiling cruises through the area. These studies represent,
as far as can be ascertained, the extent of the scientific investigation
of the continental shelf between Pt. Lobos and Pt. Sur.
Before attempting to understand any of the offshore geology it is
first necessary to have a thorough appreciation of the onshore geology
of the surrounding area. There exist several geological investigations
14
of the adjacent coastal regions as well as some offshore studies to
the north.
Trask [1926] mapped and studied the geology of the Pt. Sur
Quadrangle. Shepard [1948], Martin [1964], Martin and Emery [ 1967],
and Greene [1970] investigated the geology of Monterey Bay and the
Monterey Bay Submarine Canyon. Martin and Emery's work contains
a brief description of the continental shelf just to the south of the
Carmel Canyon. Page [1970] describes the geology of the area sur-
rounding the Sur-Nacimiento fault zone at the southern edge of the study
area including a probable time sequence of events in the formation of
the present day geology. The following geological summary of the area
is based on the above works.
Three major fault zones that exist in the vicinity of the area of
study influence the local geology. These include the San Andreas, the
Palo Colorado-San Gregario, and the Sur-Nacimiento fault zones. It
is generally believed that the boundary of the Salinian Block is the San
Andreas fault to the northeast and the Sur-Nacimiento fault to the
southwest. The Salinian Block is comprised chiefly of Cretaceous
granitic- metamorphic rocks with oceanic crust of Franciscan assem-
blage to either side (Fig. 3). Overlying the granitic basement rocks
of the Salinian Block is a layer of Tertiary strata, primarily sedimen-
tary rocks.
The Sur-Nacimiento fault zone extends northwest through the
southern and central Coast Ranges of California and presumably extends
15
TT
122*00'
~W
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TNueyo.
-V
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iz,
1*
L37°00
'SALI I H A N
IV
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Santa
Cruz
36°45
.30' 9.
-!* nmi
^
£
.0,
Moss
Landing.
Figure 3. Geological Boundaries of the Salinian Block
(after Greene et al. , 1973)
16
offshore on the continental shelf just north of Pt. Sur [Page, 1970].
The Sur-Nacimiento fault zone consists of the Sur fault zone, which
can be traced for 67 km southeast from Pt. Sur, and the Nacimiento
fault, the southern extension of the Sur fault zone which continues down
to the southern portion of the Coast Range of California. Page pos-
tulates that the Sur fault zone was the result of a tectonic collision
between Pacific oceanic crust and the Salinian Block granite along the
continental coast. He stated that at least part of the Sur fault zone
marks the former margin of the continent; that it is probable that oceanic
trench deposits which accumulated out in the Pacific moved into contact
with the continent. The oceanic portion moved either northeasterly or
easterly relative to the North American continent as the result of sea
floor spreading along the Pacific Rise while the continental portion moved
as the result of sea floor spreading from the Atlantic Ridge. Page also
wrote that the oceanic portion of the Franciscan assemblage was down-
thrust under the edge of the continental plate thus creating the northeast
dipping Sur fault zone.
Trask [1926] named the metamorphic rocks of the Salinian Block
to the northeast of the Sur-Nacimiento fault zone the Sur Series.
This series consists of quartzites, schists, gneisses, marbles, and
granulites. The Sur Series throughout the Salinian Block is intruded
by granitic rocks composed of quartz diorite, granodiorite, and
admellite. The granitic rocks are believed to be generally younger
17
than the Franciscan rocks which they now border. Page asserts that
fault displacements existed to explain the fact that the Franciscan
assemblage was generally unaffected by the intrusion of the granites.
The Franciscan assemblage, found only on the southwest side of the
Sur-Nacimiento fault, consists primarily of graywake, shale, volcanic
greenstone, and some interspersed serpentine.
The Palo Colorado fault (Fig. 4) occupies a narrow (approxi-
mately 3 km wide) fault zone connecting in the south with the onland
Serra Hill [Sierra Hill of Trask, 1926]-Palo Colorado fault complex
near Kaslar and Hurricane Points and in the north with the San
Gregario fault and a thrust fault on Ano Nuevo Point [Greene et al. ,
1973]. Trask [1926], in defining the geological setting of the Pt. Sur
Quadrangle, described the macrostructure of the area with a series
of northwest-southeast trending fault-bound blocks. Two of these
boundaries are the San Andreas fault zone to the northeast and the
Sur-Nacimiento fault zone to the southwest.- The third boundary is
the Palo Colorado fault zone which subdivides the Salinian Block
/between the San Andreas and Sur-Nacimiento zones. To the north-
west of the Palo Colorado zone, Santa Lucia quartz diorite dominates
entirely, and to the southwest, the Sur Series with some quartz
diorite and some Cretaceous sedimentary rocks can be found (Fig. 5
and Table I).
18
Figure 4. Regional Fault Map
(after Greene et al. , 1973)
19
122°00 W
PT LOBOS
, %
i % 1 TOu L
1 ' i i '
\vv> — - --' r~'oo — <x
7 r^
#.
.+
50
Nautical Miles
1
Yards
O 1000 2000 3000
PT SUR
AFTER DOHRENWEND (1971)
Figure 5. Regional Geology and Fault Location
(after Dohrewend, 1971) (geologic abbreviations listed in Table I)
20
TABLE I
EXPLANATION OF GEOLOGIC ABBREVIATIONS
TQu Tertiary - Quaternary undifferentiated
Mm Miocene marine (Monterey Formation)
Qm Pleistocene marine
Qs Quaternary dune sand
Ep Paleocene marine (Carmelo Formation)
Ku Upper Cretaceous marine
KJf Franciscan Assemblage
gr Cretaceous granitic rocks (Santa Lucia granodiorite
and quartz diorite)
m Pre-Cretaceous metamorphic rocks (Sur Series)
Sedimentary rock isopach contour line (meters) 400
21
Both Trask [1926] and Page [1970] consider the Serra Hill fault
part of the Sur fault zone, while Greene et al. [1973] believe it may
be a southern extension of the Palo Colorado-San Gregario fault zone.
This fault zone leaves the coast near Hurricane Point and is lost
in a zone of seismic incoherency, but becomes well defined in the
central and northern regions. The fault dips 50-60 NE near Hurricane
Pt. with granodiorite northeast of the fault thrust over upper Miocene
sandstone to the southwest and with an estimated vertical separation
of 300 m [Gilbert, 1971]. The probable offshore extension exhibits
the same characteristics and in the north can be traced to the western
tributary of the Carmel Canyon and may have controlled the location
of this submarine canyon [Greene et al. , 1973],
Northeast of this fault another fault leaves the coast in the
vicinity of Kaslar Pt. Greene et al. [1973] believe that this fault may
bend eastward and connect with the Palo Colorado fault on land. This
idea is supported by Trask's description of the location of the Palo
Colorado fault. Dohrewend [1971] and Ellsworth [1971] obtained seis-
mic profiles (7. 5 kHZ) of a well-formed west facing scarp in this area
giving further support of a seaward extension of the Palo Colorado
fault. Dohrewend calculated that Pliocene and Pleistocene sedimen-
tary rocks approximately 200 m thick on the southwestern side are in
fault contact with the quartz diorite on the northeastern side. Greene
et al. [1973] state that the Palo Colorado fault zone is well defined
22
offshore of Pt. Lobos across the eastern tributary of the Carmel Can-
yon. This conclusion was based on seismic reflection profiles and
dredge hauls across the canyon which indicated at least 120 m sep-
aration with relative upper movement of the east wall. Just 1.5 km
to the southwest, the probable offshore extension of the Serra Hill
fault can be traced into the western tributary of the Carmel Canyon.
The authors included this fault as part of the Palo Colorado-San
Gregorio fault zone. Trask, in describing the onshore Palo Colorado
fault zone, states that the fault is a high angle thrust which has been
traced 25 km to the southeast and crosses the coast about 200 m
north of Garrapatas Creek (located just north of Hurricane Pt. ).
Phifer [1972] states that the Palo Colorado fault crosses 150 m north
of Doud Creek since outcrops of the Santa Lucia quartz diorite on
either side of the gap has been severely sheared. He places the
crossing of the Palo Colorado fault 600 m to the north of where Trask
originally mapped it. Near the cove just north of Kaslar Pt. there is
further evidence of more faulting with a strike N40W in which quartz
diorite is thrust over sandstone and conglomerates. Near Rocky
Creek there is evidence of a series of thrust faults covering a zone
800 m wide bordering both sides of the creek mouth. The southern
blocks are overthrusted. A fracture on the north side of the zone
appears to extend northwest and run into a fault zone crossing Rocky
Pt. This latter zone appears to be associated with some part of the
Serra Hill fault.
23
From the above description one can see that the geology of the
region is more complex than the idealized three block macrostructure.
Within the area there appear to be seven or eight major faults and
numerous minor ones. The interrelationships and the exact seaward
extensions of these faults and the types of structure which they border,
is in most cases little more than conjecture. The question still arises
as to the seaward location of the Sur Fault. Page [1970] believes that
it branches out to the west across the continental shelf in order to
pass to the west of the Farrallon Islands, which are made up of
granites. Both Page and Greene et al. contend that the Sur Fault
zone lies to the west of the Palo Colorado- San Gregorio fault zone.
But if, as stated before, the seaward extension of the Sur fault zone
begins in the vicinity of Hurricane Pt. , there is no evidence of it
proceeding out to the west in order to parallel the Palo Colorado-
San Gregorio fault zone, although there is evidence of numerous
smaller fault zones in the deeper waters of the continental shelf.
24
II. SURVEY PROCEDURES
A. UNDERWATER GRAVIMETRY
The object of underwater gravimetry is no different from the
object of sea surface or land gravimetry; i. e. , to measure the spatial
variation of the earth's gravitational field. The modern day precision
instrumentation used for relative gravimetry measurements, whether
on the sea floor, on the sea surface, or on land, all involve the same
basic principle of measuring the elongation of a sensitive spring with
a known mass or beam attached at one end (Fig. 6). Underwater
gravimetry does, however, involve some unique equipment modifica-
tions, procedural techniques, and data reduction methods. These will
be discussed in the sections which follow.
1 . Equipment
A LaCoste and Romberg Model HG6 underwater gravimeter
on loan from the Naval Oceanographic Office was utilized for this
ocean bottom survey. Figure 7 shows the gravimeter ready for use,
while Fig. 8 is an internal view of the meter with the top hemisphere
removed. Under laboratory conditions the manufacturer specifies a
reading precision of + 0. 02 mgal. Experience from past surveys
[Brooks, 1973; Cronyn, 1973], as well as the present one, indicates
an operational accuracy of + 0. 10 mgal. The meter is similar in
design to the LaCoste and Romberg land gravimeter and has a
25
Measuring
Screw
Figure 6 . Simplified Diagram of the LaCoste
and Romberg Gravimeter (after LaCoste,
1967) .
26
Figure 7. Model HG6 Gravimeter Ready for Use
27
u
V
6
>
o
0
K
i— i
O
V
^
O
• r-l
1—1
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0)
pi
28
comparable 7000 mgal range. The modification of a land meter to
permit underwater measurements includes the mounting of the meter
within two thick aluminum hemispheres, the inclusion of an automatic
leveling system and depth sensing unit, and an insulated multi-conductor
shielded by an armored cable. The cable enables measurements to be
taken remotely at a control box. Remote operation through the control
box consists of various step functions which include nulling of the
pressure sensor depth unit, high and low speed meter leveling, flood
and tilt indication, clamping/unclamping of the gravimeter mass,
nulling of the mass position, and the gravity counter display giving
the measuring screw adjustment to null the mass.
Auxiliary equipment includes a specialized winch with a
secondary termination at the bitter end of the armored cable through
a set of slip rings. A marine gasoline engine is coupled to a hydraulic
pump which is used to position an A-frame and to run the hydraulic
winch. Power to the meter itself is supplied by the ship's 115 vac
system through a rectifier and an isolation transformer to the control
box and the meter. Figure 9 is a schematic diagram of the auxiliary
equipment while Fig. 10 illustrates the equipment as installed on
NPS R/V ACANIA (Fig. 11). The engine, winch, and A-frame assembly
were mounted to the after upper deck of the ACANIA.
2. Calibration
The meter itself was calibrated by utilizing two standard-
ization bases located in the immediate area. The first base, WA-84,
29
Directional
TO
GRAVIMETER
Isolation
Transformer
Ship's
115vac
Power
Figure 9 . Schematic Diagram of the Auxiliary Equipment
30
ro
O
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>
CD
x:
O
CD
Q
Oh
O
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CD
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o
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60
31
32
is located at the Monterey County Airport, and the second, WH-29,
at the base of the steel tower at the end of the Monterey Coast Guard
pier [Wollard and Rose, 1963]. The published gravity difference
between the two stations is 22.5 mgal. The readings obtained by the
author showed a 22. 1 mgal difference. The discrepancy of 0. 4 mgal
is probably due to the recent construction of a new terminal at the
airport and in the inability to locate the exact position of the bench-
mark. Since the absolute gravity reading at the airport is less than
that at the pier, and since the addition of a new concrete foundation
on top of the airport benchmark would introduce a greater gravity
reading and therefore a decreased difference between the two stations,
it is felt that the readings were within the tolerance of 0. 1 mgal.
Upon completion of the calibration check, the gravity meter
was connected to the armored cable onboard the ACANIA. A trial
run was conducted and successfully completed in the shallow waters
adjacent to the Coast Guard pier.
B. STATION SELECTION AND LOCATION
Station selection within the area between Pt. Lobos and Pt. Sur
was originally based on a grid of 0. 5 nmi (0. 9 km) interval between
stations with the constraints of the 100 fm (183 m) contour to the west
and the practical depth limitation of the research vessel of not less
than 5 fm (9 m) to the east. It was intended to bracket the fault scarp
33
identified by Dohrewend [1971] and by the author from shipboard PDR
traces. It was felt that this particular scarp runs into the western
tributary of the Carmel Canyon as previously mentioned.
It later became necessary to modify the 0. 5 nmi grid due to
equipment malfunctions and decreased ship availability. A final grid
of approximately 0. 5 nmi spacing in the northern portion of the area,
and approximately 1. 0 nmi (1. 9 km) spacing in the southern portion
resulted in a total of 68 ocean bottom stations. It is believed that the
fault scarp was bracketed a total of seven times (i. e. , seven stations
on each side for a total of 14 stations). Figure 12 illustrates the ocean
bottom and land station densities; the coordinates of each station are
given in Appendix B.
C. SURVEY OPERATIONS
1. Measurement Procedures
The survey included a total of four 12_hour ship-days over
a 2 month period from 29 May 1973 to 27 July 1973.
Prior to getting underway each day, a base station reading
was taken at the ship's mooring location approximately 25 m south of
the Coast Guard pier in Monterey Harbor, Monterey, California.
Conditions permitting, this check was repeated at the conclusion of
the day's operation in order to determine meter drift.
Upon arrival at a particular station the meter was lowered
directly to the bottom at an approximate rate of 30 m/min. Bottom
34
•30'
.36°25'
-20'
122°00'
f nmt
f km
Figure 12. Ocean Bottom and Land Station Density and Location
35
arrival was monitored through the use of the depth nulling galvanometer
on the control box. Originally the operating procedure called for obtain-
ing a pressure sensor depth reading while the meter was suspended at
the sea surface. This was found to be impractical due to motion in-
duced by sea and swell. Subsequently, a check of the depth reading
was taken when the meter was in the two-block position at the A-frame.
The depth reading here was essentially the same as that obtained just
prior to the meter entering the water. This value was used as the
surface depth counter reading.
Once the meter reached the bottom it was necessary for the
winch operator to continually pay out cable to allow for ship drift and
preclude dragging the meter on the bottom. The control box operator
would obtain an ocean bottom pressure depth sensor reading and, con-
currently, a reading of the ship's fathometer was recorded. Next, a
check of the flood and tilt indicators was required. If the meter was
o
tilted more than 15 from the horizontal it was necessary to reposition
it at a different location. This occurred a number of times throughout
the survey, particularly in shallow rocky areas. No flood indication
ever occurred. After the meter was leveled and the mass undamped,
a gravity counter reading was taken. It is worth noting here that the
time involved in taking a reading as well as the accuracy of the reading
was a function of meter depth and sea state. Even at depths as great
as 90 m, the influence of the water motion above the meter could be
36
discerned. This effect varied directly with the sea state and inversely
with depth and required, in some cases, an averaging procedure to
obtain a reading.
Once the gravity reading was recorded and the mass clamped,
the meter was raised to the surface. During periods of strong winds it
became apparent that the ship had drifted a noticeable distance from
the original lowering position and it was necessary to maneuver the
ship back as near as possible to this position in order to attempt to
lift the meter off the bottom vertically to prevent dragging it. Because
of various difficulties, this procedure was not always successful.
It is felt that the meter was dragged over the bottom for short
distance on a few occasions, apparently with no damage.
Under ideal conditions of calm, windless seas, the entire
operation of obtaining a reading took approximately 15 min. ; under
adverse conditions the time increased to 30 min.
2. Navigation
All navigation was conducted by the crew of the research
vessel. This was necessary since it required both the author and a
co-worker to operate the gravity equipment.
Visual navigation was utilized throughout the entire survey
for bearing information, while radar provided a check of range on all
fixes. Normally, a three-point fix was obtained at each station from
three lines of bearing. In areas where this was not possible, radar
37
ranges were used to supplement bearing information. Examination
of the station charts indicated that three-line fix triangles were
accurate to within a 0. 1 nmi (0. 19 km) or less on a side. In the
north-south direction this would introduce a maximum error of 0. 14
mgal to the final complete Bouquer anomaly (CBA) values.
3. Equipment Reliability
On seven separate occasions it became necessary to re-
terminate the electrical connections located at the bitter end of the
armored cable. A rubber boot with two watertight clamps at either
was designed to prevent salt-water leakage to the electrical terminals.
It is believed that at operational depths increased pressures caused a
leakage through the clamps at the ends of the boot. Various salt-water
seals were attempted but none proved entirely satisfactory. After
each retermination a re_occupation of either base station WH 29 or
the ship's mooring location was accomplished to determine if the
absolute base counter reading had changed. No such change was ever
observed.
D. COASTAL SURVEY
A coastal gravimetric survey was conducted concurrently with
the ocean bottom survey for the purpose of tieing-in the complete
Bouguer anomaly (CBA) values obtained on the continental shelf with
those obtained by the U. S. Geological Survey (USGS) along the Cali-
fornia coast. It was also felt that the gravity readings along the coast
38
would provide a verification of the ocean bottom readings and facilitate
in the identification of any trends.
A total of 38 stations were occupied using a LaCoste and Romberg
Model G-08 gravimeter on loan from USGS at Menlo Park, California.
The meter has a 7000 mgal range and an optimum accuracy of + 0. 01
mgal [LaCoste and Romberg, 1970] (Fig. 13). Station selection was
based on the ability to accurately define the elevation of any given
location and on the accessibility of that location. For this reason the
majority of the stations selected were located at USGS monumented
benchmarks as noted on USGS topographical charts. The remainder
of the stations were located at road intersections where elevation was
noted on the chart or along the beach as close as possible to the water
level. The beach locations were selected so as to bracket the location
of the Palo Colorado fault. Station elevation relative to mean sea
level at these locations was later calculated from tidal information.
Prior to use within the study area, a calibration run was made
from USGS headquarters in Menlo Park (USGS 1 JD) to Skeggs Point
(USGS B-388), spanning a range of 137.2 mgal [Chapman, 1966a],
Reduction of the data from the calibration run yielded a 137. 13 mgal
range for a difference of -0.07 mgal. A subsequent tie-in was made
at base station WH-29 at the Coast Guard pier to determine a counter
reading on the meter corresponding to the known absolute gravity value
at WH-29. This value was determined to be 3405. 63 counter units.
39
Figure 13. LaCoste and Romberg Model G-08 Geodetic Land Gravimeter
40
This process was necessary since readings prior to and subsequent to
each day's survey were difficult to accurately obtain. Water motion
around the Coast Guard pier coupled with the sensitivity of the instru-
ment caused considerable oscillation of the spirit level of the meter
and only average values could be obtained. This factor made the
determination of any meter drift impossible. Counter readings
varied randomly about 3405. 63 with a maximum deviation of 0. 11
counter units. Therefore, meter drift was not considered measurable
and 3405.63 was used as a base reading for all land gravimetry.
41
III. DATA REDUCTION
The data obtained from both the underwater survey and the coastal
survey had to be converted from counter units to milligals. Corrections
were then made for elevation, topography, earth curvature, earth tides,
and latitude. This process is necessary in order to obtain gravity
values which can be compared with measurements obtained at any
location on the surface of the earth. These values are used to calculate
the CBA. There are intermediate anomalies which may be useful within
a particular region but the CBA is the ultimate goal for most surveys,
this one included. The following section is devoted to the methods used
to obtain it.
A. OBSERVED GRAVITY
Observed gravity (OG), for the purposes of this paper, is defined
as the value of gravity at a given location corrected for earth tide,
meter drift, and curvature of the earth.
In order to convert counter readings from the meter to observed
gravity, it is first necessary to convert counter readings to equivalent
milligal readings. This is done through the use of a conversion table
for each individual meter which gives calibration factors for each
100-increment counter reading [LaCoste and Romberg, Inc. , 1970].
Since all of the author's counter values fell within the range of 3300-
42
3400, only the one calibration factor of 1.03985 was required. The
next step involves transforming the equivalent milligal reading to an
uncorrected observed gravity (OGfi). In order to do this a base value
of absolute gravity and the corresponding counter reading must be
known. For the entire survey the absolute gravity at base station
WH-29 of 979891. 7 mgal [Wollard and Rose, 1963] was used as a
reference. The counter reading corresponding to this value was
3323.66 as measured prior to the start of the survey. The above
steps can be combined to produce the formula for obtaining uncorrected
observed gravity:
OG - 979891.7 + (CV - 3323. 66) (1. 03985) mgal f (1)
0 0
where CV is the control box counter reading recorded at each station.
It became apparent at the conclusion of one day's work that a tare
had occurred at some point during the day. A tare is defined as a
sudden jump in the readings between observations. Scrutiny of the
data worksheets lead to the conclusion that the tare occurred when
the gravity meter struck the A-frame during a heavy roll of the ship.
This conclusion was verified when the original data for that day was
reduced to CBA values and a sudden jump in the values was noted.
Subsequently, the counter reading corresponding to the absolute gravity
of WH-29 had to be modified. Measurement at WH-29 resulted in a
new value of 3327. 67, a difference of 4. 01 units from the original
3323.66. The value of 3327.67 was utilized as the base reading from
43
the time when the tare occurred until the conclusion of the survey.
Equation (1) was modified accordingly.
1. Instrument Drift Correction
On only one of the 4 days was it possible to obtain meter drift
readings. Instrument malfunction on two of the days precluded obtain-
ing a final reading at the ship's mooring location, and the tare, dis-
cussed previously, occurred on one of the other days.
Calculation of meter drift (D) is possible when two readings
taken at the same location (the ship's mooring location in this case)
over a time span are corrected for the variables of earth and ocean
tides. On the one day, meter drift was calculated to be 0. 07 mgal
over a 12 hour period; this value was assumed to be negligible.
To determine the drift over the entire 4 days of the survey,
all readings taken at the mooring location were corrected for earth
and ocean tides. Between the first and third days, a drift of +0. 18
mgal was calculated. However for the interval from the beginning of
the third day to the end of the fourth day, the drift was determined to
be -0. 19 mgal. This resulted in a net drift of -0. 01 mgal for the 4 day
interval. Since the reading precision of the gravity counter on the
control box is 0. 10 units, it was felt that the non-linear jumps which
occurred within the 4 day period were probably a result of reading
error. For this reason, coupled with the fact that the 4 day variation
was only -0.01 mgal, meter drift corrections were neglected.
44
2. Earth Tide Correction
Since the earth is a non-rigid body, gravitational forces,
primarily as a result of attractional forces from the moon and the
sun, act to deform its shape. The adjustment made for this deform-
ation is known as the earth tide correction (ET). The variation is
cyclic with a range of 0. 3 mgal encompassing the tidal period. All
calculation of earth tides were computed using a USGS computer
program modified to be compatible with the NPS IBM 360 computer
system.
3. Curvature Correction
A discussion of the Bouguer correction is found in a later
section of this paper. This correction assumes that gravity measure-
ments were taken over a flat surface. This is valid only in those cases
where terrain effects on the gravity are computed out to short dis-
tances. However, for this survey, the large variations in topography
necessitated considering the terrain effects as far distant as 167 km
from the station. At these distances it becomes necessary to com-
pensate for the curvature of the earth. The following USGS equation
for curvature correction (CC) was utilized:
CC=-1. 376X10"4(Z_Zt) +3. 049X10'9(Z_Zt) -1. 110X10"17
(Z_Zt)mgal, (2)
where Z is gravimeter depth in meters (measured positively downward),
and Z is the height of the tide (measured positively upward) relative
45
to mean sea level in meters. For ocean bottom stations (Z > 0) this
correction is negative; for land stations (Z-Z < 0) the correction is
positive.
Observed gravity (OG) then is given by:
OG = OG +D+ET+CC . (3)
0
B. THEORETICAL GRAVITY
The reference spheroid utilized for this survey was chosen so as
to be compatible with the work of the USGS and the California State
Division of Mines and Geology. The constants used for the equation
of the ellipsoid were those of the 1930 International Spheroid [Dobrin,
I960]. The formula for the theoretical gravity (GTH) with the constants
incorporated is:
2 2
GTH = 978049. 0(1+0. 0052884 sin L - 0. 0000059 sin 2L)
mgal, (4)
where L is the latitude. This formula points out the necessity of
accurate navigation since a north-south variation of 1 km results in a
difference in GTH of 0. 81 mgal.
C. TOTAL UNDERWATER REDUCTION
Up to this point the reduction of data for either a land station or
an ocean bottom station is similar with the one exception of the sign
of the curvature correction (plus for land, minus for underwater).
The remainder of the data reduction exhibits some unique differences
46
depending upon whether the measurement was made on land or on the
ocean bottom. This is particularly true for the terrain correction
which not only is the most time consuming, but also the most difficult
to fully comprehend.
The majority of the literature dealing with the reduction of gravity
data is written from the standpoint of a land reduction. Although this
survey involved both land and ocean bottom environments, it was
primarily an ocean bottom survey, and the following sections are
written from that viewpoint. Land reduction methods are included in
a separate section. Many of the methods and concepts described were
derived from a paper by Andrews [1973].
1. Corrections
a. Initial Bouguer Correction
The Bouguer correction assumes that measurements were
made on an infinitely flat surface with no regional terrain irregularities
The initial Bouguer correction (BC^) has the effect of removing the
gravitational attraction of the water above the meter and replacing it
with air. The 'Bouguer plate' of water above the meter is assumed
to be of uniform composition and thickness with infinite length. Its
gravitational effect is given by:
BC1 - 27TG OwZ , (5)
- 8 3
where G is the universal gravitational constant (6.67 x 10 cm /
2
g-sec ), a is the density of the water, and Z is the meter depth.
w
47
3
For water of density 1. 027 gm/cm and Z in meters, equation (5)
reduces to:
BCj = (0.0430) Z mgal. (6)
This correction is positive since the water is attracting the gravimeter
mass upward.
b. Free-Air Correction
The free-air correction (FAC) is the vertical gradient of
gravity at MSL as determined in free space multiplied by some change
in elevation. The general formula for this correction is:
FAC = 2 GM (Z _ Z ) / R3 , (7)
where M is the mass of the Earth, R is the radius of the Earth, and
27
Z is the tidal height. For an Earth of mass of 5. 976 x 10 gm and
radius 6371 km [MacDonald, 1966] equation (7) reduces to:
FAC = -0.3083 (Z - Z ) mgal, (8)
where both Z and Z are in meters. The correction is negative (Z > 0
for bottom stations) since, in essence, the meter is being positioned
further from the center of the Earth. The FAC is the largest single
correction to be applied, and from equation (8) it can be seen that
accurate measurements of station depth and ocean tides are of prime
importance.
A pressure transducer, mounted on the inside of the bottom
hemisphere of the gravimeter and with an external opening to the out-
side, provides an indication of depth in the form of counter units on the
48
control box. The counter reading at the sea surface is subtracted
from the reading at the bottom, the difference being directly propor-
tional to depth. The proportionality constant is determined at the time
of calibration of the pressure sensor unit and was provided by the
manufacturer.
Tidal information was based on tidal heights at Carmel
Bay with Los Angeles as the reference station [U. S. Department of
Commerce, 1973]. The tide tables are based on a reference datum of
mean lower low water (MLLW), and in order to relate tidal heights to
MSL, it was necessary to determine the difference in height between
the two. This value was found to be 0. 884 m (Coast and Geodetic Sur-
vey Nautical Chart 5476).
c. Secondary Bouguer Correction
The gravimeter at this point may be envisioned as being
positioned at MSL directly above its original position. The Bouguer
plate directly below, originally filled with water, now consists of air.
In order to be compatible with land measurements, the Bouguer plate
must be filled with rock. The general formula for this secondary
Bouguer correction (BC?) is:
BC = 2 7TGO (Z - Z ) , (9)
2 r t
where O is the density of the rock. Using a common value of 2. 67
3
gm/cm [Dobrin, I960] for the density of the crustal rock, equation (9)
reduces to:
49
BC2 = 0. 119 (Z - Zt) mgal, (10)
where Z and Z are again in meters. This correction is positive since
mass is being added beneath the reference ellipsoid.
The initial Bouguer correction (BCJ, the free-air correc
tion (FAC), and the secondary Bouguer correction (BC?) are often
combined to produce the elevation correction (EC) [Nettleton, 1971]:
EC = BC + FAC + BC2 . (11)
3 3
Using densities of O = 1.027 gm/cm and o =2.67 gm/cm ,
equations (6), (8), and (10) combine and reduce to:
EC = (0. 1964 Z- 0. 1534 Z) mgal, (12)
where Z and Z are in meters. Figure 14illustrates the corrections
t &
necessary to determine EC.
d. Terrain Correction
In applying the preceding corrections it was assumed
that gravity measurements were made on an infinitely flat bottom with
an overlying Bouguer plate of uniform composition and thickness and
infinite length. Regional terrain irregularities were neglected. This
assumption may be valid at some location such as certain areas of the
continental shelf in the northern Gulf of Mexico. However, it does not
hold true in the area between Pt. Lobos and Pt. Sur where the con-
tinental shelf is transected by deep submarine canyons to the north,
south, and west, and where, to the east, the Santa Lucia Range rises
abruptly. Therefore, a topographic or terrain correction (TC) is
necessary.
50
LAND STATIONS
UNDERWATER STATIONS
air (<r=0.0)
SITUATION
AT
STATION
LOCATION
station
air {<r =0.0)
crust ^
MS
L___ J__?
crust (<T=2.67)
seawater
(<f=1.027)
/ / /
crust
(<r=2.
V station
6/)
<r=o.O
INITIAL
BOUGUER
CORREC-
TION
NOT APPLICABLE
_MSL_ _ f/t-°
<T=0.0
/^station
cr-2.67
<r-0.0
<**=o.o
FREE-
AIR
CORREC-
TION
//////
<T=2.67
iMSL ost-f— '
MS\
<T=2.67
<r--0.0
_ -^ilatipa-.
cr=o.o
//////////A
<T=2.67
<r=o.o
c=o.o
SECOND-
ARY
BOUGUER
CORREC-
TION
<T=0.0 N
_MSL_ _ _ ^stat]on_
\
(T=2.67
<r=o.o
(T=2.67
/ /// ////// A
(T=2.67
Figure 14. Schematic Representation of the Steps Necessary
to Compute the Elevation Correction ( O = density in grams
per cubic centimeters)
51
The effects of the terrain correction can be visualized in
either of two ways. The first stems from the fact that the assumption
of a flat, infinite Bouguer plate was made requiring that the Bouguer
correction be modified to fit the regional topography in the form of the
terrain correction. The second way is to reduce the topography to a
flat bottom prior to any other correction. This second method seems
to be the more logical and was the one followed by the author.
All mass above the horizontal plane of the meter due to
topographic or bathymetric relief introduces, at the meter, a vertical
component of gravitational attraction in the upward direction. Like-
wise, an absence of mass below the meter has the same effect. Thus,
removing the mass above the meter elevation and filling in the voids
below the meter elevation will have an additive effect on the observed
gravity. Since some of the mass surrounding the station level is im-
mersed in water (or both water and air when in proximity to the shore-
line), and since the voids below the station level are filled with water,
modifications of the gravitational attraction, as found in tables,
becomes necessary.
The terrain correction is normally calculated by employ-
ing a circular graticule or template divided into zones with each zone
further divided into a number of compartments- of varying dimensions.
The center of the graticule is placed over the station on a chart display,
ing topographic or bathymetric relief, and the average elevation or
52
depth relative to station level is visually estimated for each compart-
ment. It is then possible through use of tables of weighting factors for
each compartment to determine the vertical component of the gravita-
tional attraction of each compartment at the station by summing the
values in all of the compartments. The result is the terrain correction
for the individual station.
For the survey, a total of 15 zones (A through O) out to a
radian distance of 166. 7 km were utilized. This distance necessitated
the use of four separate graticules to fit four different scale charts
(1 : 24, 000; 1 : 40, 000; 1 : 210, 663; 1 : 820, 000). The tables used to
determine the attraction of each compartment were obtained from
USGS and are based on the original Hayford and Bowie [1912] tables
with the modifications of Bullard [1936] and Swick [1942],
Bottom gravity stations require, once the average ele-
vation or depth for each compartment has been determined, that the
tables be modified. The first step requires filling in the voids below
the station with rock (Fig. 15, AREA A). However, the rock would
be displacing the water that already exists there, so an adjustment to
3
the tables (which assume a 2. 67 gm/cm density for the compartments)
3
is necessary. Taking the density of the water as 1. 027 gm/cm this
modification is in the form of a multiplication factor as given by:
Or - Ow = 2. 67 - 1. 027 =0.615 . (13)
Ot 2.67
53
MSL _ ft
t
station depth £
AREA A
Figure 15. Schematic Diagram Showing Areas Involved in
Terrain Corrections for Ocean Bottom Stations
54
The second step requires removing the material above station elevation
(AREA B and C). The density of the rock is again assumed to be
3
2.67 gm/cm , and as this is the density upon which the tables are
based, no correction is required. However, AREA B is now filled with
air whereas the infinite Bouguer plate calls for water. It is therefore
necessary to refill this area with water. Assuming a density for water
3
of 1.027 gm/cm , the modification to the tables is the irmltiplication
factor:
°w
1'027 = 0.385 . (14)
ax 2.67
This adjustment must be subtracted as mass is being added to an area
above the meter.
If a compartment located within AREA B is underwater
(i. e. , on the continental shelf), it has been determined that the terrain
o
correction is probably unnecessary for a bottom slope of 3 or less
[Grant and West, 1965]. This proved to be the case in the author's
area where the shelf slope is 1. 5 or less over 90% of the area
[Dohrewend, 1971]. For example, for a station at a depth of 6 1 m
below MSL and a compartment in zone H (outer radius 5.2 km, inner
radius 3. 5 km) with an average elevation of 300 m above MSL, an
error of 0. 0019 mgal for that particular compartment would be
introduced. Although this portrays an average situation, it is the
author's opinion that the determination of average compartment
55
elevation/depth introduces a far larger error. This belief is substan-
tiated by Brooks [1973] who assumed an error of + 0. 02 mgal per zone,
with a total terrain correction error of _+ 0. 30 mgal for a survey area
having relatively flat topography. Between Pt. Lobos and Pt. Sur, an
area where relief is substantially greater, a more realistic value of
+ 0.5 mgal per terrain correction should be utilized. For this survey,
application of equation (14) was neglected. The corrections as calculated
for each station are listed in Appendix A.
2. Gravity Anomalies
The departure of a corrected gravity value from the theoret-
ical value of gravity at a given location is defined as a gravity anomaly.
The type of the anomaly depends on the corrections that have been
applied to the observed gravity. Using the corrections obtained thus
far, it is now possible to calculate the various intermediate gravity
anomalies, and finally, the complete Bouguer anomaly.
a. Free-Air Anomaly
Application of the free-air correction (FAC) to observed
gravity results in the free-air anomaly (FAA). In essence, this
anomaly brings all observed gravity readings to the level of the
reference ellipsoid (MSL), at the same time neglecting the effects of
both the surrounding topography and the material within the Bouguer
plate. The free-air anomaly is defined as:
FAA = OG+FAC - GTH . (15)
56
b. Simple Bouguer Anomaly-
Application of the elevation correction from equation (11)
to observed gravity results in the simple Bouguer anomaly (SBA):
SBA = OG+EC - GTH . (16)
The SBA can be viewed as a modification to the FAA with the inclusion
of the effects of the Bouguer plate on observed gravity:
SBA = FAA+BC +BC2 . (17)
In areas where the topography is relatively flat and uniform, the SBA
is adequate for gravity survey correlations.
c. Complete Bouguer Anomaly
Inclusion of the terrain correction (TC) to the SBA yields
the complete Bouguer anomaly (CBA). This anomaly considers all
corrections to observed gravity and may be used to correlate separate
gravity surveys from other locations. The plotted isolines of CBA,
used in conjunction with magnetic data, seismic data, and any other
available geological data, are a tool whereby density variations and
non- conformities in the near surface structure can be inferred. The
complete Bouguer anomaly is given by:
CBA = OG+EC+TC - GTH , (18)
or from the simple Bouguer anomaly:
CBA = SBA + TC . (19)
A listing of the various gravity anomalies for each station can be
found in Appendix B.
57
D. PARTIAL UNDERWATER REDUCTION TO COMPARE WITH
SEA-SURFACE GRAVIMETRY
At times it is convenient to compare ocean bottom gravity measure,
ments with those taken on the surface usinga sea surface gravimeter.
This can be done in conjunction with a sea-surface survey to check the
accuracy of the surface values since they are subject to errors result-
ing from the accelerations of the surface vessel caused by sea and
swell.
1. Elevation Correction
The concepts involved in the computation of this elevation
correction (EC ) are similar to the ones used in obtaining the elevation
correction in equation (11) for underwater reduction, except that the
last step involves replacing the air contained in the Botiguer plate with
water rather than rock, The elevation correction for an ocean bottom
station when it is to be compared with a sea surface station directly
above at MSL is:
EC = BC,+FAC+BC , (20]
I 3
wh
ere
BC, = 2 7T G <7 (Z-Z) . (21)
3 w x t v '
3
Using 1.027 gm/cm for the density of water, equation (21) reduces
to:
BC3 = 0.0430 (Z _ Z ) mgal , (22)
where Z and Z are in meters. Substitution of equations (6), (8), and
(22) yield:
58
EC =(0.2653Z .0.2223Z) mgal , (23)
where Z and Z are in meters.
The above effectively places the meter at MSL.
2. Mass-Adjusted Free-Air Anomaly
The mass -adjusted free-air anomaly (MFAA) is analogous to
the simple Bouguer anomaly of equation (16), and is perhaps a mis-
nomer since a more appropriate term would seem to be 'mass-adjusted
simple Bouguer anomaly'. Contours of the MFAA again allow correla-
tion of an ocean bottom survey with a sea surface survey. The simple
formula for the MFAA is:
MFAA - OG+EC ' _ GTH , (24)
or as a modification to the FAA:
MFAA = FAA+BC1+BC . (25)
Values of MFAA for each station are listed in Appendix B.
E. LAND REDUCTION
The reduction of land stations is, for the most part, a simplifica-
tion of that for underwater stations. Observed gravity is obtained in a
similar manner but with a different base station counter reading and
different calibration factors. Theoretical gravity is identical as it is
a function of latitude alone.
1. Corrections
a. Bouguer Correction
The Bouguer correction (BC) for land stations effectively
removes the mass between station elevation and the surface of the
59
reference ellipsoid (MSL) (i. e. , the mass contained in the Bouguer
plate). The general formula for this correction is:
BC - -2 7T G Ox H , (26)
where is the density of the rock being removed and h is the
3
station elevation, (h > 0) Using a density of 2.67 gm/cm , equation
(26) reduces to:
BC = -0. 1119h mgal , (27)
where h is in meters. This correction is now negative since it removes
mass below the meter.
b. Free-Air Correction
The free-air correction repositions the gravimeter from
station elevation to MSL. Modification of equation (8), the free-air
correction for underwater stations, gives the correction:
FAC = 0.3083h mgal , (28)
where h is in meters. This correction is now positive since the meter
is being repositioned closer to the center of the earth.
Combining equations (27) and (28) produces the elevation
correction for land stations:
EC = 0. 1964h mgal , (29)
where h is in meters.
c. Terrain Correction
As in the terrain correction (TC) for underwater stations,
for land stations the concept is to reduce the surrounding topography
60
to produce a flat, infinite surface on the same horizontal plane as
the station.
For compartments which overlie land alone, the modified
Hayford-Bowie tables can be used intact. This correction is positive.
For compartments which overlie water, it is necessary to modify the
correction due to existing density differentials. Calculation of the
attraction of each compartment involves two steps. First, it is nec-
essary to fill in the depression between station level and the ocean
bottom with rock. This correction is positive. However, after this
is carried out, a portion of the rock will have displaced a wedge of
water from MSL to the bottom; the previous correction will have been
too large. The next step requires removing the effect of this displaced
3
water. Assuming a density for water of 1.027 gm/cm , the modifica-
tion to the tables is again in the form of a constant multiplication
factor:
°™ = i-027 = 0.385 . (30)
a 2. 67
r
This correction must be subtracted as the attraction of the water below
station elevation is being removed.
In actual practice, since some of the land stations were
relatively close to MSL, and the depth of the ocean water within the
compartment was far greater (especially in the outer zones) than the
station elevation, the following multiplication factor was applied to
the values read from the tables, assuming standard densities:
61
vr - w
2.67 - 1.027 =o.615 . (31)
0W 2.67
The error introduced using this simplication results from the assump-
tion that a small wedge of water extends from MSL to station elevation
when, in fact, it is actually air. This error was determined to be
negligible. For stations of high elevation and for compartments in
shallow waters, equation (30) was utilized.
2. Gravity Anomalies
Employing the correct signs land station gravity anomalies
are calculated from equations (15), (16), and (18), using equations
(28) and (29) to determine FAC and EC.
62
IV. DATA PRESENTATION AND ANALYSIS
The procedures and discussion thus far have dealt primarily with
obtaining and reducing of gravity values. The interpretation of results
in terms of the geologic structure below the surface, a more qualitative
analysis, is discussed in this section.
A. GENERAL DISCUSSION
The CBA gravity values were plotted on a USGS topographic map
of the area (scale 1 : 24, 000) and CBA isolines were drawn by hand at a
2 mgal interval. The decision to adopt a 2 mgal interval was based
upon a desire to depict the greatest detail while still maintaining the
general trend. Transference and reduction of the gravity map on the
topographic chart to a larger scale map was accomplished through the
use of a pantograph on a 3/16-scale reduction. This proved to be
satisfactory in maintaining the detail and the general trends of the
gravity map (Fig. 16).
The accuracy of this map is dependent upon two factors: (1) the
accuracy of the CBA values themselves, and (2) the contouring of the
gravity field in the construction of the isoline pattern. From former
surveys utilizing the HG6 Model gravimeter [Brooks, 1973; Cronyn,
1973; Souto, 1973] and from the author's own analysis, the accuracy
in determination of CBA values is + 1. 04 mgal for sea stations and
63
Figure 16. CBA Distribution for the Continental Shelf and
Adjacent Coastline Between Pt. Lobos and Pt. Sur
(values in milligals, contour interval 2 mgal)
64
+0.72 mgal for land stations. Table II delineates possible error sources
and the reduction step to which they relate. The contouring of the iso-
lines is the author's interpretation of the 'best fit' of the gravity values
and is therefore subject to error since the process is partly subjective.
B. CBA ANALYSIS
Assuming that the effects of elevation, topography, and latitude
have been correctly removed, the anomalies in the map of the gravity
field are caused by horizontal variations of the density within the crust
and upper mantle of the earth. The magnitude of the anomaly is depend-
ent upon the density contrasts, the location of the contrast relative to
the gravity station, and the form or sharpness of the geologic discon-
tinuity. If there is no density contrast, no anomaly will exist. The
interpretation of a CBA map in terms of subsurface geologic structure
is not unique from utilization of gravity data alone. Other sources of
information such as seismic data, drill core data, or specific geologic
data from outcrops must be available. From this information density
values can be inferred and actual depths to a density contrast can be
obtained and subsequently related to the subsurface structure of the
surrounding area. The degree of uniqueness is dependent upon the
validity of the accepted geological data. In the case of this survey
specific geological data was not available; therefore, it was neces-
sary, when making a final analysis, to generalize.
65
INITIAL
ERROR
SOURCE
DATA
REDUCTION
STEP
ERROR IN
COASTAL
SURVEY
(Land Gravimetry
ERROR IN
CONTINENTAL
SHELF SURVEY
) (Bottom
Gravimetry)
GRAVIMETER
ACCURACY
OBSERVED
GRAVITY
+0.04
+ 0. 10
OPERATOR
READING
ACCURACY
OBSERVED
GRAVITY
+0.01
+0. 10
NAVIGATION
THEORETICAL
GRAVITY
+ 0. 05
+0. 14
ELEVATION/
DEPTH
CALCULATIONS
FREE-AIR
AND
BOUGUER
CORRECTIONS
+0. 12
+0. 20
ELEVATION/
DEPTH
CALCULATIONS
TERRAIN
CORRECTION
+ 0.50
+0. 50
TOTAL
COMPLETE
BOUGUER
ANOMALY
+0. 72
+ 1. 04
TABLE II.
POSSIBLE ERRORS IN COMPLETE BOUGUER ANOMALY CALCULATION
(Values in Milligals)
66
The CBA values along the California coast usually do not range
far from a value of about 0 mgal. They decrease eastward to high
negative values of -200 mgal or less in the vicinity of the Sierra
Nevada and Great Basin provinces [Chapman, 1 966]. From a gravity
profile analysis conducted by Thompson and Talwani [1964] for a
cross-section approximately 160 km to the northwest of the survey
area, the regional trend of the gravity anomaly along the profile from
the Pacific Basin to the continental shelf is highly negative, going from
approximately 250 mgal to less than 50 mgal. Thompson and Talwani
relate this strong negative gradient to an increase in thickness of the
crust as the continental margin is approached. Although this particular
profile was to the north of the author's area of interest, the regional
trend which it exhibits could be expected to be similar to the one
within the study area. However, from the author's CBA map, the trend
of the gravity anomaly is opposite, showing a strong positive west to
east gradient. This apparent contradiction can be related to local
anomalies and is believed to be caused, at least in part, by a rapid
rise of the granitic basement just offshore from Pt. Lobos to Pt. Sur.
From the CBA map partially closed gravity highs are evident in
the vicinity of Soberanes Pt. , Kaslar Pt. , and Pt. Sur (location of
these points is shown in Fig. 2). These highs are generally oriented
northwest-southeast and are congruent with the trend of the local
geology. They may represent granitic outcroppings, particularly the
67
most northern high which parallels the geographic trend of Pt. Lobos.
An alternative explanation is that these highs may be associated with
the fault zones which transect the area. This is especially true for the
34 mgal high at Kaslar Pt. , the axis of which is nearly parallel to the
Palo Colorado fault. The position of this high coincides with the 30
mgal high plotted on the Santa Cruz Sheet of Bishop and Chapman [1967],
The 30 mgal high at Pt. Sur exhibits generally lower gravity values
within the semi-closed isoline system in comparison to the other highs
and is possibly an indication of the lower density Franciscan assem-
blage which is reported to exist in this area on the southwest side of
the Sur-Nacimiento fault zone.
Offshore of Hurricane Pt. a definite ridging of the isolines is
evident. The orientation of the isolines to the north of this ridge is
northwest-southeast while to the south they exhibit a northeast- south-
west trend. The location of this ridge coincides with the offshore
extension of the Serra Hill fault as reported by Greene et al. [1973]
(Fig. 4). Judging from the gravity data alone, no further faulting
occurs between this ridge south to Pt. Sur. Since it is generally
accepted that the Sur-Nacimiento fault zone leaves the coast north of
Pt. Sur, the structure associated with this gravity ridge may be
related to its offshore extension. The conclusion that this isoline
configuration is the reflection of a fault is supported by both the
gradient and the orientation of the isolines which appear to indicate
68
either strike-slip or dip-slip motion where the southwestern side has
descended relative to the northeastern side. Gilbert [1971] states
that the fault which leaves the coast near Hurricane Pt. has a dip
o
50-60 NE with an estimated 300 m vertical displacement. This would
call for a normal fault where the Sur-Nacimiento fault zone is usually
associated with thrusting. Page [1970] contends that the formation and
movements of the Sur fault are more complex and that it cannot be
described as a simple thrust fault. Other mechanisms must be
introduced to understand its present state. He contends that thrust
faulting was only the initial stage in its development. Subsequent
normal faulting is believed to have occurred as a result of the collision
of the East Pacific Rise crest and the westward moving North American
Continent. Possible strike-slip motion followed when Pacific spreading
became oriented to a northwest-southeast direction. This occurred at
approximately the same time that the San Andreas fault became active.
Further evidence for strike- slip motion can be obtained from an exam-
ination of the bathymetry in the Pt. Sur area (Fig. 1). The continental
shelf is at its widest at a point almost due west of Pt. Sur. The ridg-
ing of the depth contours could have been influenced by compressional
forces that existed when the oceanic crust came into contact with the
Salinian Block, or it could be viewed as an elongation associated with
right-lateral strike- slip motion where the southwestern block has
moved northwesterly in relation to the northeastern block.
69
From the CBA map it is seen that the Hurricane Pt. ridging
abruptly terminates as the coastline is reached. No explanation can
be put forth as to why this is so. Serra Hill rises sharply at this point
and gravity measurements were taken only along its western edge.
More data are needed in this area, particularly along the eastern side
and top of Serra Hill.
Southwest of Pt. Lobos there is strong evidence from the gravity
field of a partially closed 10 mgal low. The gradient to the east of
this low is very steep, attaining maximum values of 12 _ 1 3 mgal/km.
This is a strong indication of a rapid rise in the basement, most
probably associated with faulting. The direction of ridging of the iso-
lines to the north and orientation of the partial low can be projected
into the western tributary of the Carmel Canyon. The form of the 10
through 20 mgal isolines to the west of this low is unknown beyond the
limits shown on the map. It is possible that these isolines continue
south and connect with the open isolines west of Pt. Sur. If this is the
case, a large low of 8 mgal or less would result. Insufficient data
precluded projection of the open isolines.
Just to the east of the 10 mgal low and coincident with its orienta-
tion, the extension of either the Palo Colorado fault zone [Dohrewend,
1971] or the Sur fault zone [Greene et al. , 1973] leads into the west-
ern tributary of the Carmel Canyon. Dohrewend obtained PDR traces
of a fault scarp extending for 6 km of approximate 20 m relief at a
70
depth of 82-101 m. He correlates this scarp with the offshore extension
of the Palo Colorado fault and projects it onshore just north of Kaslar
Pt. (Fig. 5). A bathymetric profile survey was conducted from the
ACANIA in an attempt to substantiate the location of this fault scarp.
Profile locations and the plotted position of the fault scarp are shown
in Fig. 17. The profiles are reproduced in Fig. 18-23. An attempt
to locate the position of the scarp south of profile A_A' was unsuccessful
and lack of time prevented a more thorough search. The fault scarp is
easily recognizable in profiles A-A', B-B', and D_D'. In profiles
F-F' and H-H1 the original scarp has trended into the western tributary
of the Carmel Canyon while a new scarp emerges on the shoreward
side of profile F-F'. This scarp is barely recognizable in profile
H-H' as it leads into the eastern tributary of the Carmel Canyon. In
both profiles granitic outcroppings are evident between the two canyons.
C. TWO DIMENSIONAL PROFILES
The evidence of faults leading into the tributaries of the Carmel
Canyon lead to the construction of three separate two-dimensional
modeling profiles, the locations of which are shown in Fig. 24. It
was hoped that the depth to the basement could be determined. The
computer program used in this effort, based on an earlier model by
Talwani, was developed by Cady [1972] of the USGS. The model
requires that the regional trend be filtered out of the total gravity
71
-30'
N
36°25'
0
i
122°00'
Sobercsnes Pt
Kaslar Pt
Figure 17. PDR Profile Locations and Fault Scarp Position
from a Bathymetric Study
72
40
.-.,
Depth, m
80 ^ 120
57/h€T i~M£ A JL.
{-oSa's. &,ck:S
lo/o
r I w
1. .-.—
\ "1
4
160
T
200
to
<
K,
-7 m
a»er-
Jd<g
'S-— * —
<£■
L.4
j
£+J£> Uu€ Pi'(\
1024
Zo^n
50
i'p
100 150
One-woy travel time, msec
200
*)
• UJ
250
Figure 18. PDR Profile A-A'
73
40
Depth, m
80 120
St-*^- irxie 6 '3
14XSL
- 6-
... »■*£
■U
C«f
160
C*3 t^^s £ Z.< *
Arf
200
**£B?'
; ' > :
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'•-I
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2Ch"
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;• . ,.; ■ ■".'.. ill f . ■ ■: ;; l
100 150~ ' 200 '
One-way travel timey msec
Figure 19. PDR Profile B-B'
■
250
74
Depth, m
80 120
160 200
V i i '■'-'• m-
i__$! JU& — ; .
100 150
One-way travel time, msec
Figure 20. PDR Profile D-D"
75
Depth, m
LU*^"5
100 150
One-way travel time, msec
Figure 21. PDR Profile E-E'
76
L °
I
.i*?-
40
i
\
Depth, m
80 120
160
9;q
200
- CO
<
LU
.
-i
i^C-
S
1
-
s?-;-
S>e
50
Eg
■w.
100 150
One- wry travel time,, msec
Figure 22. PDR Profile F-F'
200
oo
LU
250
77
Depth,
i -
3Ei
i&;
.- »
« ■*■■
..-»■ - ■---
^avi
0
SB*?
£«;■> t.«/«; /v//'
-m
<
IXI
50
o 10° * I ,• 150
One-way travel time, msec
Figure 23. PDR Profile H-H'
78
200
250
Figure 24. Location of Two-Dimensional Modeling Profiles
79
anomaly. In this case, however, the length of the profiles ranged
from 3. 48 to 3. 90 km and for such short distances it is impossible to
determine a regional trend. If a regional trend, such as the one
determined by Thompson and Talwani [1964] is assumed, the gradient
of the gravity anomaly input to the model would be increased slightly
but would not appreciably alter the end result. In order to successfully
run the computer program it is necessary to input the depth to the
basement at one or more points along the profile. In all three cases
the basement, assumed to be quartz diorite with a mean density of
3
2. 806 gm/cm [Daly, Manger, and Clark, 1966], was located at sur-
face level at the eastern boundary of each profile. This is in agreement
with actual coastal conditions found between Pt. Lobos and Pt. Sur
[Trask, 1926]. Dohrewend [1971] reports that the fault brings at least
200 m of Plio-Pleistocene sedimentary rocks into high angle contact
with the Cretaceous Santa Lucia quartz diorite. He also states that
Miocene marine rocks outcrop in the southern portion of the study
area and consist of claystone and shale. This unit is characteristically
overlain unconformably by marine mud and siltstones. Because Mio-
cene marine sedimentary rock was the deepest unit that Dohrewend
could distinguish seismically, he was unable to measure its thickness.
The Plio-Pleistocene marine rocks which are found on the south-
western side of the fault probably have a composition similar to that
of the Miocene marine unit found to the south (i. e. , claystone and
80
shale). An approximation of the density of nearly horizontal and un-
3
disturbed Miocene shale is 2.06 gm/cm [Daly et al. , 1 966] - This
was the second density value used in the model. The difference
between the two assumed density values, 0.746 gm/cm , constitutes
the density contrast parameter input. The results of the computer-
run models are illustrated in Fig. 25, 27, and 29, while Fig. 26, 28,
and 30 give the respective model-calculated and the observed gravity
profiles of each cross section.
Although by no means the only solution, these models represent
the best fit utilizing the above assumptions and existing data. Various
3
density contrasts ranging from 0. 0 to 0. 840 gm/cm were tried but
all attempts introduced errors greater than that which resulted from
3
the 0. 746 gm/cm contrast. The RMS errors for each model are
given in Fig. 26, 28, and 30. It is apparent that the greatest errors
for each profile occurred at the boundaries. The program was
designed for much longer profiles where the errors occurring at the
boundaries would be averaged out over the length of the profile. For
short distances these errors become readily apparent.
The position of the fault break for each section is shown on Fig.
24. The correlation between these points and the location of the off-
shore fault scarp as mapped by the author from the PDR profiles is
quite good. This is also true for the proposed offshore extension of
the Serra Hill fault across model profile C-C The computed minimum
Figure 25. Depth to Basement of Profile Model A-A1
82
1
>
i
1 --
•
= 0.2649 mgal
= 1.0953 mgal
*>
o
6
T
Mean Error
RMS Error :
-*-
_o
o
• o
Gravity V\.
CO.
-
t3
I
0)
o
E
cs-
"(
\
"5
O)
E
-.
\$\
CN
o
CN
»
•
o
Figure 26. Calculated and Observed Gravity for Profile A-A1
83
t r
T ~T
co
E
<
E
O
O
oo
CM
E
CM
CO
E
CN4
Figure 27. Depth to Basement of Profile Model B-B'
84
Figure 28. Calculated and Observed Gravity for Profile B-B
85
Figure 29. Depth to Basement of Profile Model C-C
86
Figure 30. Calculated and Observed Gravity for Profile C-Cl
87
displacement of the basement ranges from 1. 5 km for model profile
C-C to 2. 1 km for model profile A_A'. These values are much
greater than the ones proposed by Dohrewend [1971] and Greene et al.
[1973] whose conclusions were based on seismic data alone. However,
Thompson and Talwani [1964] for a 40 mgal negative residual anomaly
in the vicinity of the continental slope suggest that this relative low is
caused by a thickness of about 3 km of sedimentary rocks. The west-
ern most stations of model profile A_A' were located on the continental
slope and the deep depth to basement could at least be partially related
to the similar relative low southwest of Pt. Lobos. The fault which
separates this sedimentary boundary is believed to be the primary
cause of the deep basement. The model itself gives the appearance
of dip- slip motion where the southwestern block has dropped relative
to the northeastern block. Strike-slip motion could also be a cause
of the vertical separation; most likely, as with the Sur fault, it is a
combination of both dip-slip and strike-slip motion.
The most eastern scarp shown on PDR profiles F-F' and H-H'
(Fig. 17) is probably the northern extension of the fault running into
the eastern tributary of the Carmel Canyon in the north which Greene
et al. , [1973] believe to connect with the onland Palo Colorado fault.
It was hoped that some indication of this structure would be found in
the profile models; however, no such structure was evident. One
explanation is that a fault may indeed exist in this location but lacks
the density contrast necessary to bring it out in the model.
88
D. CONCLUSIONS
From interpretation of the two-dimensional profiles and the gravity
anomaly pattern, the author concludes that the offshore extension of the
Serra Hill fault as mapped by Greene et al. [1973] (Fig. 4) and the fault
trending into the western tributary of the Carmel Canyon are one in the
same (Fig. 31). This conclusion is based on similar minimum displace,
ments of the basement on the southwest side of each fault. Dohrewend
[1971] projects the offshore Palo Colorado fault into the western trib-
utary of the Carmel Canyon (Fig. 5). If this were true it could be
expected that the isoline pattern in the vicinity of Kaslar Pt. , the
location of the onland Palo Colorado fault, would indicate a basement
displacement closely approximating the one found just south of the
western tributary. No such correlation was found. Instead, the iso-
lines indicate a continuation further south to the vicinity of Hurricane
Pt. and the Serra Hill fault.
The fault scarp leading into the eastern tributary of the Carmel
Canyon is probably associated with the offshore extension of the Palo
Colorado fault. The close proximity of this scarp to the shoreline
precluded locating its position south of PDR profile F- F' and the
gravity data offered no additional information. The decreased CBA
gradient in the vicinity of the onland Palo Colorado fault is also evident
near the eastern tributary of the Carmel Canyon and may be indicative
of a small density contrast across this fault.
89
Position located from v
i._.
nmi
km
Position located from CBA
isoline ridging and model profile
Hurricane Pt
SERRA HILL
FAULT
Figure 31. Summary Fault Map Indicating Proposed Locations
90
If the Sur-Nacimiento fault zone leaves the California coast north
of Pt. Sur, there is no indication of this trend from an analysis of the
gravity data. However, onshore on the southern side of Serra Hill
there is an outcrop of Franciscan assemblage reported by Page [1970]
to be found only to the southwest of the Sur-Nacimiento fault zone. It
is concluded that this fault zone leaves the coast in the vicinity of this
outcrop and may in part be correlated with the gravity isoline ridging
just to the north. What becomes of the Sur-Nacimiento fault zone after
it leaves the coast is still unknown. In the author's viewpoint there
are two possibilities: (1) the Sur-Nacimiento fault zone connects at
some location to the north with the Palo Colorado-Serra Hill fault
complex, or (2) it proceeds out to the west undetected from the gravity
data. There is a gap in the gravity data in the central portion of the
survey area which may possibly hold the answer to this question.
91
V. FUTURE WORK
It is suggested that additional gravity measurements be made to
the west of Pt. Lobos and in particular on the ridge located between
the two tributaries of the Carmel Canyon. This would perhaps result
in a better determination of the orientation of the gravity isolines in
this area. More measurements are also required in the central
portion of the area to perhaps tie-in the Hurricane Pt. ridging with the
partially enclosed low to the north. Land gravity measurements are
also required on the top and eastern side of Serra Hill to attempt to
explain why the Hurricane Pt. ridging is terminated at the coast.
It is also recommended that a detailed bathymetric survey be
conducted within the entire area. In conjunction with this, either
deep penetration coring or dredging should be accomplished particularly
on the widened shelf area west of Pt. Sur to determine the boundaries
of the Franciscan assemblage and thus the Sur-Nacimiento fault zone.
A correlative magnetic study within the area would be helpful and of
particular use in the two-dimensional profile studies. Finally, sea-
surface gravimetry would be of use in those areas too deep for bottom
gravimetry. Particular attention should be given to the western exten-
sion of the Monterey Canyon located 12 km to the west of Soberanes Pt.
This would be a logical location for the Sur-Nacimiento fault zone if it
does proceed out to the west after leaving the coast.
92
APPENDIX A
DATA REDUCTION CORRECTION VALUES FOR INDIVIDUAL STATIONS
(values in mgal)
STA G CESEfcVEC G THEORETICAL BC FAG EC TC
1 9799C7.24C
2 979906.068
2 9799C6.cec
4 979909. IS4
5 9799C6.1C2
6 9 7 99C2.91C
•7 9799C4. 961
8 9799C<.3C9
9 9799C5.756
1C 9799C4.591
11 9799C7.719
12 9799C2.C13
13 9799C2.369
14 9799C6.C29
15 9799C9.C36
16 979902.465
17 9799C3.C99
18 979699. £53
19 9799C4.C22
2C 979904.027
21 979894.717
22 979901.659
23 979 9 C3. 928
24 979901.628
25 979897.348
26 979897.442
27 979899. C36
2Q 979898.243
29 979892.664
979680.359
6.56
-17.02
-8.47
6 .94
979876.132
9.91
-19.71
-9.80
5 .84
979875.097
7.46
-14.84
-7.38
5.67
979874.580
11.42
-22.73
-11.31
5.41
979874.493
7.27
-14.47
-7.19
5 .44
979874.235
4.3C
-8.54
-4.24
6 .26
979873.976
9. 54
-18.98
-9.44
5.6«=
979873.890
12. CI
-23.90
-11.89
5 .3C
979873.028
6.63
-17.57
-6.74
5.16
979873.286
6. 30
-12.53
-6.23
5.4 9
979E72.338
11.98
-23.84
-11.87
5.5S
979873.200
17. ce
-33.99
-16.91
5.7 3
979873.545
15.33
-30.52
-15.19
5 .69
979872.769
15.27
-30.39
-15. 12
5.62
979872.597
25.87
-51 .50
-25.63
5.9C
979871.993
4.76
-9.47
-4.71
5 ,S 1
979871.907
-30.49
-15.17
5.7C
979871.907
15.93
-31.68
-15.75
5.7 6
979871.476
7.28
-14.49
-7.21
5.75
979871 .390
10. C2
-20. G2
-10.00
5.75
979871.304
18.87
-37.56
-16.69
5.7 1
979870.873
5.5C
-10.94
-5.44
6.29
979870.787
1C.91
-21.78
-10.88
5.66
97987 I. 131
14.25
-28.35
-14.10
5.42
979870.614
14.24
-28 .42
-14.18
5 .54
979870.614
18. 2C
-36.23
-18.03
5.37
979870. 183
3.38
-6.73
-3.35
6 .74
979870.097
12.60
-25.07
-12.47
5.2C
979669.839
16.04
-35.92
-17.88
5 .4<=
93
STA G CESEPVEC G ThECRETICAL
BC
FAG
EC
7C
2C
3L
32
33
24
35
36
37
38
29
40
41
42
43
44
45
46
47
4£
49
50
51
52
53
54
55
56
57
58
59
979899. 12C
979898.626
979892. 4C7
979892.416
979897. 92C
979899. 7C3
979891. 6C1
979897.622
979896.495
979895.472
979895. 2C6
979895. 92C
979891.997
9798 9 4. 399
9 79898.215
979888.067
979892.686
9798 9 2.CC4
97989C.CC2
9 7 9 8 8 7.994
979882.970
979882.677
979887.547
979892. C22
979881.227
979879.915
979882.422
97988 1.895
9 79879.872
979879.272
979869.752
6.64
-17.19
-8.55
5.65
979869. 149
7.92
-15.78
-7.86
6.C2
979869.149
12. 22
-24.35
-12.12
5.29
979869.063
18.95
-37.73
-18.78
5.9 6
9798fc8.460
6.44
-12.83
-6.39
6.C4
979866.632
9.32
-18.57
-9.24
5.51
979868.029
15. 03
-29.93
-14.90
5.76
979868.029
11.64
-23.19
-11.55
6 .C5
979868.029
3.68
-7.34
-3.65
7 .C5
979867.684
7. 12
-14. 19
-7. 08
6.2C
979867.254
3.62
-7.20
-3.59
6 .85
979867. 081
7.45
-14.87
-7.42
6.CC
979867.031
14.35
-28.61
-14.26
5.7 9
979866.476
5.44
-10.84
-5.40
6.42
979866.306
8. 05
-ie.C7
-8. 01
5 .74
979866.048
14.40
-28.70
-14.30
5.5C
979665.875
4.67
-9.20
-4.64
6 .56
979865.273
5.93
-11.82
-5.89
6.27
979865.359
8.33
-16.61
-8.28
5.82
979865.531
11.27
-22.67
-11.30
5.44
979865.273
15.15
-30.18
-15.04
5.28
979864.842
12.28
-26.47
-12.19
5.27
979864.584
11. 12
-22. 15
-11.03
5.54
979864.153
6.23
-12.38
-6. 15
6 .61
979864.41 1
14.29
-28.66
-14.27
5.17
979863.895
1C.19
-20.29
-10.10
5.26
979662.689
6.11
-12.15
-6.04
6.C7
979862.689
11.29
-22.49
-11.20
5.27
979862.948
13.85
-27.59
-13.74
5.C2
979861.570
12.62
-25. 16
-12.53
4.97
94
STA G CESEPVEC G THECRETICAL
EC
FAC
EC
TC
60
979682. £27
979861 .570
8.89
-17.73
-8.84
5.22
61
979884.277
979861.484
5.71
-11.25
-5.65
5.76
62
979363. 186
979860.623
10. 1C
-20. 14
-1C.04
5 .C4
63
979879.234
979860.365
14.68
-29.26
-14.53
4.92
64
97968 1.429
979859.849
1C.82
-21.58
-IC.75
4.9 I
65
979C87.692
979859.676
7.26
-14.47
-7.21
5.24
66
9 7 9 6 8 5 . 7 9 C
979858.644
8.57
-17.07
-8.50
5.C4
67
979862.676
979859.504
3.61
-7.18
-3.57
6 .C8
68
979665.156
979857.525
5.55
-11.06
-5.51
5.16
95
STA 6 OBSERVED G THEORETICAL
BC
FAC
EC
TC
A
979896.381
979875.873
^0.85
2.35
1.50
5.56
B
979893.355
979875.356
-2.73
7.52
4.80
5.41
C
979882. 1C1
979875.528
-7.84
21.63
13 .79
5.97
D
97989C.C88
979874.235
-4.50
12.42
7.9 1
5.9C
E
979884.599
979873.114
-7.67
21.16
13.49
6. 14
F
9796S1.C76
979873.028
-3.58
9.88
6.30
5.95
G
979889.591
979871.321
-3.96
10.91
6.95
6. 1C
H
979893.724
979871.390
-1 .86
5.17
3 .30
6.14
1
979887.258
979870.700
-4.50
12.42
7.91
7.36
J
979889.407
979870.011
-2.83
7.81
4.98
7.78
K
979885.427
979869.322
-4.40
12.13
7.73
6.78
L
979891.470
979869.322
-1.53
4.23
2.70
6.66
M
979889.242
979868.632
-1 .71
4.70
3 .00
7.28
N
979832.151
979867.857
-5.49
15.14
9.65
7.41
O
979891.090
979867.340
-0.03
0.03
C.05
8.74
P
979 891.140
979867.254
-0.03
0.08
•CO 5
8.74
Q
979891. 120
979867.163
-0.03
0.08
0.05
7. 11
R
979891.350
979367.081
0.0
0.0
CO
7.11
S
979891.270
979866.995
0.0
0.0
0.0
7.CC
T
979891.250
979866.323
-0.C3
0.09
C.06
6.8C
U
979885.603
979866.651
-3.27
9.03
5.76
6.46
V
979888.332
979860.651
-1.67
4.61
2.94
6.47
W
979878.993
979865.962
-5.63
15.52
9.89
6.51
X
979878.185
979865.273
-5. 80
15.99
1C.19
7.46
Y
979855.742
979864.756
-18.45
50.89
32.44
11. AC
Z
979878.836
97986-+. 842
-5.12
14.11
8.99
9.15
A'
979873.428
979864.153
-7.64
21.07
13.43
7.91
B1
9 798 36.2 31
979863.464
-0.04
0.11
C.07
7.29
e
979867.251
979862.775
-9.41
25.96
16.55
5.78
96
STA G CESEPVEC G ThECRETICAL
i
ec
FAC
EC
TC
D' 979851. C79
E' 979649.451
F' 979864.466
G' 979669.456
H' 97987C.201
I' 979668.197
J' 979669.655
K' 97967C.745
L' 979855.117
979861.742
979860.795
979859.935
979859.332
979858.988
979857.955
979857.353
979856.665
979857.095
-16.54 45.62 29.08 8.16
-15.07 41.57 26.50 11.62
-7.44 20.51 12.07 7.5C
-4.20 11.85 7.55
-4.71 12.98 8.27
-5.93 16.37 1C.43
-5. 01 13.83 8.81
-4.26 11.76 7.49
6.76
5.79
6.92
6.28
5.78
-12.31 33.96 21.64 10.89
97
APPENDIX B
VARIOUS GRAVITY ANOMALIES AND LOCATIONS
FOR INDIVIDUAL STATIONS
ST/> L/HITICE LONGITUDE DEPTh
(m)
FAA
MF££
se/>
CBA
I
26
31.57
121
57.53
55.2
9.86
14.6
18.4 I
25.35
2
26
31. 1C
121
57.80
63.9
12.22
17.7
22. 12
27.97
Q
36
3C .88
121
57.22
48. 1
16.14
20. 3
23. 60
29.27
4
26
2C .52
121
57.67
73.7
11.84
18.2
23.26
28.67
5
36
3C.5C
121
57.05
46.9
17.14
21 .2
24.42
29.86
6
36
2C.28
121
56.47
27.7
20.13
22.5
24.42
20.69
7
36
2C.12
121
57.08
61.5
12.00
17.2
21.54
27.22
8
36
2C.C5
121
57.72
77.4
11.52
18.2
23.52
28.82
9
36
29 .48
121
57.45
56.9
15.16
20. 1
23 .99
29. 17
10
36
29.67
121
56.83
40.6
16.76
22 .3
25.06
20.57
11
26
29. CC
121
57.50
7 7.2
11.54
16.2
23.51
29.10
12
26
2 9.58
121
58.12
110.2
-5.18
4.3
11 .90
17.62
13
36
29. ac
121
58.68
93.9
-C.70
7.6
14.64
20.52
14
26
29.28
121
57.98
93.5
2.87
11.4
13. 14
23.76
15
26
2 9.15
121
58.87
166.9
-15.06
-C .7
10.8 1
16.71
16
26
2i.ll
121
56.50
30. 7
21.02
2 2.7
25.78
21 .69
17
26
26 .65
121
57.87
98.8
0.70
9.2
16. C2
21.72
18
26
28.72
121
56.80
10 2.7
-3.73
5 .1
12. 20
17.96
19
26
28.27
121
56.53
47. C
18.06
22.1
25 .34
21.09
2C
26
28 .33
121
57.17
t4 .9
12.63
18.2
22.65
28.40
21
36
2 8.22
121
58.65
121.7
-14.14
-2.7
4.72
10.44
22
36
27 .97
121
56. 13
35.4
19.85
22.9
25.24
21.62
22
36
27 .9C
121
56.78
70.6
11.36
17 .4
22,21
27.92
24
36
26.12
121
5 7.67
91.9
2.15
10. 1
16.2 9
21.8 1
25
36
27 .80
121
57.55
92. 1
-1.69
6 .2
12.55
16.09
26
36
27 .78
121
58.63
117.4
-9.40
0.7
8.80
14.17
27
36
27 .48
121
5 5.80
21.8
22. 12
24 .0
25.50
22.24
28
36
27 .40
121
5 7.08
81.2
3.18
10 .2
15.78
21.06
29
36
27.22
121
56.22
116.4
-13.10
-2.1
4.95
10.44
98
ST/i LATITICE LONGITUDE DEPTH
FAA
MF£A
SB£
CSA
3C
36
27,
,18
121
31
36
26 ,
,78
121
32
36
26
.78
121
3 a
36
26 .
.68
121
34
36
26,
,3C
121
35
36
26 ,
,4C
121
36
36
25
,95
121
37
36
25,
.98
121
36
36
25,
.98
121
39
36
25
,72
121
4C
36
25,
,47
121
41
36
25,
,2£
121
42
36
25.
,3C
121
43
36
24.
92
121
44
36
24,
,6C
121
45
36
24,
,6C
121
46
36
24,
,45
121
47
3t
24
,05
121
46
36
24,
,1C
121
49
36
24
.25
121
5C
36
24
,C5
121
51
36
23
.78
121
52
36
22
.63
121
53
36
23.
,26
121
54
36
23
.4 7
121
55
36
23
.ce
121
56
36
22
.75
121
5 7
36
22
,3C
121
58
36
2 2
.43
121
59
36
2 1
.5C
121
56.53
55.7
12.17
17. C
20.61
26.46
56. IC
51. 1
13.70
18.1
21 .63
27.65
56.95
78.9
-1.09
5 .7
11 . 14
16.43
56.22
122.3
-14.37
-3.8
4.5 7
IC . 53
55. 9C
41.6
16.63
2C.2
23. C 7
29.11
56.25
60.2
12.50
17.7
21.83
2 7.34
57. C5
97. C
-6.36
2.C
8.6 7
14.45
56.30
75. 1
6.41
12.9
18. CS
24. IC
55. 4C
23.8
21.13
23 .2
24.81
31.86
55.85
4 6 . C
13.60
17.5
20.7 1
27.01
55. 3C
23.3
20.85
22 .9
24.46
31.31
55.67
48. 2
13.97
18. 1
21 .42
27.42
56.62
92.7
-3.70
4.3
10. tt
16.45
CC -3-2
35.1
17.08
20.1
22.52
28.95
55.82
52. 1
15.94
2C.4
24. CC
2 9.74
56.67
93. C
-6.68
1 .3
7.72
13. 22
55.05
30. 1
18.51
21.1
23. 17
29.73
55. CO
38.3
14.91
18.2
20.84
27. 11
55.63
53.8
8.03
12.7
16. 3£
22.19
56.25
73.5
-C.21
6. 1
11.17
16.6 1
56.83
97.8
-12.49
-4 .1
2.66
7.94
56.3 0
85.8
-8.63
-1.3
4.65
10.0 2
55.72
71.8
C.61
7.C
11 .93
17.47
54.52
40. 1
15.50
19. C
21.73
26.34
56.92
92.9
-11.73
-3 .7
2.66
7.83
55.72
65.7
-4.27
1.4
5.92
11.2-6
54.72
39.4
7.58
11. C
13.69
19.76
>t • Lu
72.9
-3.28
3.C
8.CC
13.27
57.27
69.4
-10.66
-3.C
3. 16
8. 20
5 7. C3
81.5
-7.36
-C.3
5.27
10.24
99
Slfi L^TITLCE LCNGITUCE. DEPTH
FAA
MF££
SE£
CBA
6C
26
2 1.45
121
55.77
57.5
4.54
9.5
13.4 2
18.66
61
26
2 1 .42
121
54.87
36. e
11.44
14.6
17. 14
22.92
62
26
2C.82
121
56.15
65.3
2.42
8 .C
12.52
17.57
63
26
2C .62
121
5 7.75
94.6
-IC.39
-2.2
4.2 9
9.22
64
36
2C.25
121
56.58
69.9
0.01
6 .C
10.64
15.75
65
26
2C.12
121
55. 2C
46.9
13.55
17.6
20.61
26. C5
66
26
19.42
121
55.67
55.3
10.07
14.8
18.64
22.68
67
36
2C .C7
121
54. CC
23.3
16.00
18 .C
19. 6C
25.66
66
36
18.68
121
54.57
35. 8
16.57
19.7
22.12
27.28
100
STA LATITIDE LGNGITUCE
ELEV
FAA
SEA
CBA
A
26
2 1.
48
12L
55.37
7.6
22.86
22. Ci
27.57
B
26
31.
C5
121
56.08
24.4
25.52
22 .6C
28.21
C
26
21.
2C
121
56.72
7C.1
28.21
20.36
26.33
D
36
2C,
"2 "2
121
56.10
40.2
28.27
23.77
29.67
E
26
29,
C -5
121
56.25
68 .6
32.65
24.97
31.11
F
26
29.
4 7
121
56.60
32.0
27.93
24.24
30.29
G
36
28.
,6C
121
56.12
35.4
28.66
24.7 2
30.83
H
36
26,
,32
121
56.12
16.8
27.51
25.62
31.77
1
36
21,
82
121
55.53
40.2
28.97
24.47
31.83
J
36
27.
,25
121
55.33
25.2
27.20
24.27
22.15
K
36
26.
8"?
121
5 5.47
39.3
28.24
23.84
30.62
L
36
26,
9C
121
5 5.63
13.7
26.38
24.6 5
31.51
M
36
26,
4C
121
55.27
15.2
25.31
23.61
30.89
N
36
21,
,82*
121
54.92
49.1
29.44
23.95
31.36
O
36
25.
,5C
121.
54.75
0.2
23.84
23. ec
22.54
P
36
25,
,42
121
54.77
0.3
23.97
23 .94
32.68
Q
36
25,
,28
121
54.78
0.2
24.04
24. CI
31.12
R
36
25
, 2 2
121
54.77
0.0
24.27
24.27
31.38
S
36
25,
,22
121
54.82
CO
24.27
24 .27
31.27
T
36
25
. 12
121
54.83
0.2
24.52
24.49
31.29
U
36
25.
,CC
121
54.72
29.3
27.98
24 .71
31.17
V
36
25
,C2
121
54.83
14.9
26.29
24.62
31.09
w
36
24
,57
121
54.78
50.2
28.55
22.92
29.43
X
36
24
,ce
121
54.33
51.6
28.90
23. 1C
30.56
Y
36
22
.12
121
52.68
164.9
41.87
23.42
34.82
z
36
22
.72
121
54.08
45.7
28. 10
22.99
22.14
A'
26
2 2
.25
121
53.98
68.2
30.34
22. 7C
20.61
B'
36
2 2
,8C
121
54.08
0.4
22.68
22.64
20. 12
C
36
22
.23
121
54.07
84 .1
20.44
21 .02
26.80
101
ST£ LATITUDE LONGITUCE
ELEV
FAA
SEA
CBA
D'
36
21.58
121
53.92
147.8
34.96
18.41
26.57
E'
36
2C.97
121
53.71
134.7
3C.23
15.16
26.79
F'
36
2C.32
121
53.42
66.4
25.06
17.62
25.12
G'
36
15.92
121
53.38
38.4
21.98
17 .68
24.44
H'
36
19.65
121
53.62
42.1
24.29
19.59
25.38
1'
36
16.98
121
53.43
53.0
26.61
20.67
27.60
J'
36
18.57
121
53.10
44.8
26.13
21 .12
27.40
K'
36
ifi.07
121
52.62
38.1
25.84
21.57
27.35
L«
36
16.33
121
53.85
110.0
31.98
19.67
30.56
102
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REFERENCES CITED
1. Andrews, R. S. , 1973. Corrections for Underwater Gravimetry.
Naval Postgraduate School, Department of Oceanography,
Monterey, Ca. (paper submitted for publication)
2. Brooks, R. A. , 1973. A Bottom Gravity Survey of the Shallow
Water Regions of Southern Monterey Bay and Its Geological
Interpretation. M.S. Thesis, Naval Postgraduate School,
Monterey, Ca. 57 p. (unpublished report)
3. Bullard, E. C. , 1936. Gravity Measurements in East Africa.
Phil. Trans. Roy Soc. (London). Ser. A, 235(757): p. 445-531.
4. Cady, J., 1972. Gravity and Magnetics: 2-Dimensional Program.
U. S. Geological Survey, Menlo Park, Ca. (unpublished report)
5. Chapman, R. H. , 1966a. The California Division of Mines and
Geology Gravity Base Station Network (CDMG Spec. Rpt.
No. 90). California Office of State Printing, San Francisco,
Ca. 49 p.
6. Chapman, R. H. , 1966b. The Gravity Field in Northern California,
in Geology of Northern California: E. H. Baily, editor, Bulletin
190, California Division of Mines and Geology, Ferry Building,
San Francisco, Ca. p. 395-405.
7. Colomb, H. P. , 1973. Recent Marine Sediments on the Central
California Continental Shelf Between Point Lobos and Point Sur.
M.S. Thesis, Naval Postgraduate School, Monterey, Ca. 44 p.
(unpublished report)
8. Cronyn, B. C. , 1973. An Underwater Gravity Survey and Inves-
tigation of Northern Monterey Bay. M.S. Thesis, Naval Post-
graduate School, Monterey, Ca. 57 p. (unpublished report)
9. Daly, R. A., G. E. Manger, and S. P. Clark, Jr., 1966.
Density of Rocks, in Handbook of Physical Constants: S. P.
Clark, editor, New York, Geol. Soc. Amer. Memoir 97,
p. 20-26.
10. Dobrin, M. B. , i960. Introduction to Geophysical Prospecting,
2nd. Ed. McGraw-Hill Book Co. , Inc., New York. 446 p.
106
11. Dohrewend, J. C. , 1971. Marine Geology of the Continental
Shelf Between Point Lobos and Point Sur, California:
A Reconnaissance. M.S. Thesis, Stanford University, Stanford,
Ca. 60 p. (unpublished report)
12. Ellsworth, W. L. , 1971. Geology of the Continental Shelf, Point
Lobos to Point Sur, California. M.S. Thesis, Stanford Uni-
versity, Stanford, Ca. 33 p. (unpublished report)
13. Gilbert, W. G. , 1971. Sur Fault Zone, Monterey County,
California. Ph. D. Thesis, Stanford University, Stanford, Ca.
80 p. (unpublished report)
14. Grant, F. S. , and G. F. West, 1965. Interpretation Theory in
Applied Geophysics. McGraw-Hill Book Co. , Inc. , San
Francisco, Ca. 583 p.
15. Greene, H. G. , 1970. Geology of Southern Monterey Bay and
Its Relationship to the Ground Water Basin and Salt Water
Intrusion. Open-file Report, U. S. Geological Survey, Menlo
Park, Ca. 50 p. (unpublished report)
16. Greene, H. G. , W. H. K. Lee, D. S. McCulloch, and E. E.
Brabb, 1973. Faults and Earthquakes in the Monterey Bay
Region, California. Misc. Field Studies Map MF-518, U. S.
Geological Survey, Menlo Park, Ca. 14 p.
17. Hayford, J. F., and W. Bowie, 1912. The Effect of Topography
and Isostatic Compensation Upon the Intensity of Gravity
(U.S.C. & G.S. Spec. Ptib. No. 10). U. S. Government
Printing Office, Washington, D. C. , 132 p.
18. LaCoste, L. J. B. , 1967. Measurement of Gravity at Sea and in
the Air. Rev. Geophys., v. 5, p. 477-526.
19. LaCoste and Romberg, Inc. 1970. Operating and Repair Manual,
Models HD and HG Underwater Gravimeters. LaCoste and
Romberg, Inc. , Austin, Tex. (unpublished report)
20. MacDonald, G. J. F. , 1966. Geodetic Data, in Handbook of
Physical Constants. S. P. Clark, editor, New York, Geol.
Soc. Amer. Memoir 97, p. 219-221.
21. Martin, B. D. , 1964. Monterey Submarine Canyon, California:
Genesis and Relationship to Continental Geology. Ph.D. Thesis,
University of Southern California, Los Angeles, Ca. 249 p.
(unpublished report)
107
22. Martin, B. D. , and Emery, K. O. , 1967. Geology of Monterey
Canyon, California. American Assoc. Petroleum Geologists
Bull., v. 51, p. 2231-2304.
23. Nettleton, L. L. , 1971. Elementary Gravity and Magnetics for
Geologists and Seismoligists. Society of Exploration Geo-
physists, Tulsa, Okla. 121 p.
24. Page, B. M. , 1970. Sur-Nacimiento Fault Zone of California:
Continental Margin Tectonics. Geol. Soc. America Bull. ,
v. 81, p. 667-690.
25. Phifer, D. W. , 1972. Recent Marine and Thrust Faulting in the
Santa Lucia Mountains of Soberanes Point Quadrangle Monterey
County, California. Student Report, Naval Postgraduate School,
Monterey, Ca. 35 p. (unpublished report)
26. Shepard, F. P. , 1948. Investigation of the Head of Monterey
Submarine Canyon. Scripps Institute of Oceanography.
Submarine Geology Report 1. 15 p.
27. Souto, A. P. D. , 1973. A Bottom Gravity Survey of Carmel Bay,
California. M.S. Thesis, Naval Postgraduate School, Monterey,
Ca. 57 p. (unptiblished report)
2 8. Swick, D. H. , 1942. Pendulum Gravity Measurements and Iso-
static Reductions (U.S.C. k G.S. Spec. Pub. No. 232). U. S.
Government Printing Office, Washington, D. C. 82 p.
29. Thompson, G. A. , and M. Talwani, 1964. Crustal Structure
from Pacific Basin to Central Nevada. Jour. Geophys.
Research, v. 69, no. 22, p. 4813-4837.
30. Trask, P. D. , 1926. Geology of Point Sur Quadrangle, California.
University of California Publications Bulletin of the Department
of Geological Sciences, v. 16, no. 6, 186 p.
31. U. S. Department of Commerce. 1973. Tide Tables, High and
Low Water Predictions, 1973, West Coast of North and South
America Including The Hawaiian Islands. U. S. Government
Printing Office, Washington, D. C. 226 p.
32. Wollard, G. P. and J. C. Rose, 1963. Internation Gravity
Measurements. George Banta Co. , Inc. , Menasha, Wise.
518 p.
108
INITIAL DISTRIBUTION LIST
No. Copies
1. Defense Documentation Center 2
Cameron Station
Alexandria, Virginia 22314
2. Library, Code 0212 2
Naval Postgraduate School
Monterey, California 93940
3. Department of Oceanography 3
Naval Postgraduate School
Monterey, California 93940
4. Oceanographer of the Navy 1
Hoffman Bldg #2
2461 Eisenhower Ave.
Alexandria, Virginia 22314
5. Office of Naval Research 1
Code 480-D
Arlington, Virginia 22217
6. Professor Robert S. Andrews 10
Department of Oceanography, Code 58Ad
Naval Postgraduate School
Monterey, California 93940
7. Professor Joseph J. von Schwind 3
Department of Oceanography, Code 58Vs
Naval Postgraduate School
Monterey, California
8. Lieutenant Walter B. Woodson, USN 3
USS Gallant (MSO-489)
Fleet Post Office
San Francisco, California
9. Dr. Howard Oliver 1
United States Geological Survey
345 Middlefield Road
Menlo Park, California 94025
109
10. Dr. S. L. Robbins
United States Geological Survey
345 Middlefield Road
Menlo Park, California 94025
11. Mr. H. Gary Greene
United States Geological Survey
345 Middlefield Road
Menlo Park, California 94025
12. Gravity Section
Naval Oceanographic Office
Washington, D. C. 20390
13. Mr. H. B. Parks
LaCoste and Romberg, Inc.
6606 North Lamar
Austin, Texas 78752
14. Master R/V ACANIA
Department of Oceanography
Naval Postgraduate School
Monterey, California 93940
15. Lieutenant Clayton H. Spikes
3303 Sycamore Place
Carmel, California 93921
16. Dr. Robert E. Stevenson
Scientific Liaison Office of ONR
Scripps Institution of Oceanography
La Jolla, California 92037
17. Captain W. B. Woodson, Jr., USN (Ret)
Paradise Avenue
Middletown, Rhode Island
18. Library, Code 3330
Naval Oceanographic Office
Washington, D. C. 20370
19. Dr. Gary Griggs
University of California, Santa Cruz
Division of Natural Sciences
Santa Cruz, California 95060
110
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SECURITY CL ASMFIC ATION OF THIS PAGE (When Data Entered)
REPORT DOCUMENTATION PAGE
READ INSTRUCTIONS
BEFORE COMPLETING FORM
1. REPORT NUMBER
2. GOVT ACCESSION NO
3. RECIPIENT'S CATALOG NUMBER
4. TITLE (and Subtitle)
A Bottom Gravity Survey of the Continental
Shelf Between Point Lobos and Point Sur,
California
5. TYPE OF REPORT ft PERIOD COVERED
Master's Thesis
September 197 3
6. PERFORMING ORG. REPORT NUMBER
7. AUTHORf*,)
Walter Browne Woodson, III
8. CONTRACT OR GRANT NUMBERft;
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Naval Postgraduate School
Monterey, California 93940
10. PROGRAM ELEMENT. PROJECT, TASK
AREA 4 WORK UNIT NUMBERS
II. CONTROLLING OFFICE NAME AND ADDRESS
Naval Postgraduate School
Monterey, California 93940
12. REPORT DATE
September 1Q73
13. NUMBER OF PAGES
112
14. MONITORING AGENCY NAME ft ADDRESSf// dttterent from Controlling Otllce)
15. SECURITY CLASS, (ot thia report)
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16. DISTRIBUTION STATEMENT (ol this Report)
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18. SUPPLEMENTARY NOTES
19. KEY WORDS (Continue on reverie aide If neceeaary and Identify by block number)
20. ABSTRACT (Continue on reveree tide If necaaaary and Identity by block number)
From an occupation of 68 ocean bottom and 38 land gravity stations between
Pt. Lobos and Pt. Sur, California, a complete Bouguer anomaly map was
produced and analyzed. The steps in data reduction leading to the complete
Bouguer anomaly field is presented, unique features of which are associated
with bottom gravimetry. The geological interpretation of the gravity data
shows excellent correlation with earlier seismic records of the proposed
offshore extension of the Serra Hill fault, a structure long associated with
DD .^s 1473
(Page 1)
EDITION OF 1 NOV 65 IS OBSOLETE
S/N 0102-014- 6601 |
111
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SECURITY CLASSIFICATION OF THIS PAGE (When Data Entered)
Unclassified
i'UCU^ITY CLASSIFICATION OF THIS PAGEfH'hen Data Ente/sd)
20.
the Sur-Nacimiento fault zone. Two dimensional models of gravity anomaly
profiles were constructed across this fault and another fault located several
kilometers to the northwest and extending into the western tributary of the
Carmel Canyon. The results indicate a minimum vertical displacement of
the basement of approximately 2 km on the southwest sides. It was con-
cluded that these two faults are one in the same. Evidence is presented
which indicates that the Palo Colorado fault zone, located approximately
2 km to the east, parallels the Serra Hill fault and subsequently leads into
the eastern tributary of the Carmel Canyon.
DD Form 1473
, 1 Jan 73
S/N 0102-014-6601
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SECURITY CLASSIFICATION OF THIS PAGE(Wi»n Dae Enffd)
112
146318
Thesis
W8436 Woodson
c i A bottom gravity sur-
vey of the continental
shelf between Point Lo-
bos and Point Sur, Cal-
ifornia.
Thesis 146318
V/8436 Woodson
c,l A bottom gravity sur-
vey of the continental
shelf between Point Lo-
bos and Point Sur, Cal-
i fornia.
thesW8436
A bottom gravity survey of the continent
3 2768 001 90624 1
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