TECHNICAL REPORT
A MARINE MAGNETIC SURVEY OFF THE EAST COAST OF THE UNITED STATES
PROJECT N-20
Geomagnetics Branch Marine Surveys Division
SEPTEMBER 1962
U. S. NAVAL OCEANOGRAPHIC OFFICE WASHINGTON, D. C. Price 40 cents
Ca
FOREWORD
The results of the survey described in this report are
considered to be of significance to both the Navy and the
scientific community. The region investigated is a transition
zone between a continental mass and a true oceanic basin. Geo-
physical investigations of such regions may lead to a better understanding of the earth's major crustal features and their origin. The use of geophysical exploration techniques such as
described here provide a means of deducing information about
the earth's deeper structures lying beyond the limits of
direct measurement.
. C, STEPHAN Rear Admiral, S. Navy Commander
ACTA
O 0301 0044491 oO
iii
HOON
CONTENTS
Ibo INTRODUCTION A. Purpose B. Background C. Survey Area II, SURVEY OPERATIONS A. Conduct of Survey B. Control C. Instrumentation D. Personnel III, DATA PROCESSING A. Preliminary Data Processing B. Magnetic Temporal Variations C. Total Magnetic Intensity Contour Chart D. Residual Magnetic Intensity Contour Chart E. Profiles IV. SURVEY RESULTS A. General B. Discussion of Data Wo SUMMARY OF FINDINGS REFERENCES
APPENDIX - Fredericksburg Magnetograms, July 18-24, 1961
Page
24
25
FIGURES
Track Chart and Profile Index
Total Magnetic Intensity Contour Chart
Residual Magnetic Magnetic Magnetic Magnetic Magnetic Magnetic Magnetic Magnetic
Magnetic
Magnetic Intensity Contour Chart and Bathymetric Profiles A-A' and Bathymetric Profiles B-B' and Bathymetric Profiles C-C' and Bathymetric Profiles D-D' and Bathymetric Profiles E-E' and Bathymetric Profiles F-F' and Bathymetric Profiles G-G' and Bathymetric Profiles H-H'
and Bathymetric Profiles I-L'
vi
Page 12
13 14 15 16 17 18 19 20 21 22
23
I. INTRODUCTION
A, Purpose
In July 1961, the USS PREVAIL (AGS-20) conducted total magnetic intensity, bathymetric, and bathythermal survey operations for the U. S. Naval Oceanographic Office.* The purpose of the magnetics phase of this survey was to define more precisely the characteristics of the earth's magnetic field over the continental shelf, slope, and adjacent deep-water area off the east coast of the United States.
Coincident with the geomagnetic and bathymetric measurements, bathythermograph observations and continuous recordings of sea surface and injection temperatures were taken. These observations were part of a project to develop a method for predicting the ocean's thermal structure. Thermal structure data are reported in U. S. Naval Oceanographic Office Technical Report 113 (in prepa- ration) and are not included in this report.
B. Background
Several airborne magnetometer survey suaeus had been flown by Oceanographic Office Project MAGNET survey aircraft over the east coast of the United States and the adjacent ocean area. Analysis of the data recorded along these tracks indicated that a magnetic anomaly is characteristically present at or Me: the location of the continental slope. Distinct magnetic anomalies of about twenty
‘
miles in horizontal extent and with various shapes and amplitudes
*In accordance with Public Law 87-533 effective 10 July 1962, the U. S. Navy Hydrographic Office was redesignated as the U. S. Naval
Oceanographic Office.
always appeared on magnetic profiles flown transverse to the slope. In this area, seismic investigations by others indicated the presence of a ridge in the crystalline basement rocks. However, it was not known whether this seismic ridge was the source of the magnetic anomalies. Likewise, neither the detailed configuration of the magnetic anomalies nor the exact positional relationship of the anomalies and the continental slope were known. C. Survey Area
The survey was conducted in an irregularly shaped area lying between latitudes 35°N and 40°N and longitudes 70°W and 76°W. The survey track lines were run approximately perpendicular to the con-
tinental slope. Specific survey tracks are shown in Figure 1.
II. SURVEY OPERATIONS
A. Conduct of Survey
The PREVAIL departed New York on 17 July and arrived in Washington, D. C., on 25 July after completing almost 2500 miles of survey track. As shown in Figure 1, average track spacing was approximately 30 miles with the tracks trending north- west and southeast. This particular survey track configuration was established to best meet both magnetic and bathythermal survey requirements. The average speed of advance over the survey track was 12.5 knots. B. Control
Survey control was by Loran-A with additional position deter- minations by radar where possible. The position of the ship was determined every fifteen minutes and then plotted on Mercator Plotting Sheets (H.0O. 3000 series). On the shoreward side of the survey area, both Loran-A and radar were used. Here, the probable position accuracy is estimated as being within + 1 mile. On the seaward side of the survey area, radar fixes were not available, and only Loran-A fixes were taken. Here, the probable position accuracy is estimated as being within + 2 miles. C. Instrumentation
A Varian nuclear resonance magnetometer, Model XN~4901, was used to measure the earth's total magnetic field intensity. With this instrument, the total field intensity can be measured to an
accuracy of about + 1 gamma (0.00001 oersted). Magnetometer equip-
3
ment consisted of a power supply, preamplifier, counting circuits, analog recorder, and towed sensing unit. The sensing unit, a Varian Model X-49-813 using 700 feet of Simplex #310 two-conductor cable, was towed 400 feet astern. This sensing unit was streamed and recovered manually. Console electronic equipment was installed in the drafting room on the after part of the ship. Data were recorded in analog form on a Varian G-11 recorder in units of "mag- netometer counts". These units, an inherent property of the magne= tometer design, are an inverse function of the total magnetic field intensity. In the survey area, one magnetometer count is equal to approximately 1.3 gammas.
Bathymetric instrumentation aboard the PREVAIL consisted of an Edo AN-UQN-1B sonar receiver-transmitter, the output of which was recorded directly in fathoms on a Mark V Precision Depth Recorder (PDR). This type of recorder can be read to the nearest one fathom. The bathymetric recording instrumentation was located in the ship's drafting room. D. Personnel
Two geophysicists from the Geomagnetics Branch, U. S. Naval Oceanographic Office installed and operated the magnetometer system.
PREVAIL personnel operated the bathymetric instrumentation.
III, DATA PROCESSING
A, Preliminary Data Processing
The magnetometer recorder traces were scaled at time intervals of fifteen minutes and also wherever maximum and minimum magnetic intensity values were recorded. These values were converted from Magnetometer counts to gammas and plotted on the smooth plot of the survey track. The Precision Depth Recorder traces were scaled in a similar manner.
B, Magnetic Temporal Variations
No attempt was made to remove temporal variations of the earth's magnetic field from the data. Records of the Fredericksburg, Virginia, Magnetic Observatory indicate that no severe disturb- ances occurred during the time of the survey. Magnetograms and calibration data are reproduced in the Appendix.
The Fredericksburg observatory is approximately 150 miles from the shoreward side of the survey area and about 500 miles from the seaward side. Because of these distances, it is not pos- sible to determine accurately the magnitude of the errors intro- duced by the temporal variations. Nevertheless, the magnetograms should indicate times when the data cannot be considered completely reliable. Variations that occurred will introduce small errors in the location of contour lines, particularly in the areas of shallow magnetic relief. However, it is considered that they had little effect on the magnitude and position of the most signifi-
cant anomalies.
C. Total Magnetic Intensity Contour Chart (Figure 2)
The total intensity values plotted on the Smooth Track Chart were contoured at 50-gamma intervals. The contours are shown in Figure 2.
Dashed contours represent extrapolated data.
D. Residual Magnetic Intensity Contour Chart (Figure 3)
In order to more clearly define the anomalies, the regional gradient of the total magnetic intensity was removed from the original values. To accomplish this, the total intensity contours from H. 0. Chart No. 1703, The Total Intensity of the Earth's Magnetic Force (for the year 1955) were corrected to the year 1961 and then inter- polated at 50-gamma intervals. These interpolated contours were then reproduced on the total intensity contour sheet of the survey area. At each point on the sheet where survey plot contour lines intersected charted contour lines taken from H. 0. 1703, the differ- ence was computed. If the survey contour value was greater than the charted contour value, a plus (+) value was assigned to the differ- ence; if smaller, a minus (-) value was assigned. An overlay was placed over these two contour representations, and the differences at contour intersections were plotted and contoured at 50-gamma intervals. The Residual Magnetic Intensity Contour Chart for the survey area is shown in Figure 3.
E. Profiles
Profiles of the total magnetic intensity and the measured bathymetric depth along each track are Se sccar ee in Figures 4 through 12. These profiles were constructed using the smooth-plotted survey tracks as base lines. An index to the geographical location
of each profile is shown in Figure 1. 6
IV. SURVEY RESULTS
A. General
A significant advantage of a shipborne magnetic survey is that bathymetric measurements can be taken simultaneously with the magnetic measurements. Direct comparison of magnetic and bathymetric data relative to each other is thus possible, irrespective of the certainty of the ship's true position.
The data contained in this report provide useful infor- mation relating to the geologic structure pattern jn this area. These data can be correlated with similar information from adjacent regions. Such correlation may yield clues leading to a better understanding of the relationship between conti- nents and ocean basins.
B. Discussion of Data
The magnetic field contour pattern in the survey area (see Figure 2) contains a large, elongate, magnetic anomaly on the western side. This anomaly has lineations corresponding closely to those of the continental slope. On the eastern side, the increasing complexity of the contour pattern suggests the existence of a magnetic feature lying just outside the survey area. Between these two features is a broad area void of magnetic anomalies.
In the survey area, the bathymetric data indicate that
the sea bottom has no topographic features capable of accounting
for the observed magnetic anomalies. Figure 1 shows that a portion of the survey track connecting points C and D passed directly over Baltimore Canyon. Similarly, the track connect- ing points E and F passed directly over Norfolk Canyon. In neither case was there any magnetic field change to correspond with these prominent topographic features.
Using the data shown on Profiles G-G', H-H', and I-I', depths to the source of the large magnetic anomalies that were found near the continental slope were estimated. These depth estimates were made in accordance with empirical slope methods of Vacquier et al (1951). The average depth estimates to
magnetic sources for these profiles are as follows:
Profile G-G' 19000 feet Profile H-H' 18900 feet Profile I-L' 19980 feet
Depth estimates made from magnetic data from a single survey track are at best only approximate. It was found,
"magnetic depths'’ estimated above are in
however, that the reasonable agreement with the depth to the crystalline basement complex, as determined from seismic and drilling data by Ewing et al (1950). It appears that the top of the magnetic source
is probably closely coincident with the basement surface.
North of this survey area, seismic data (Ewing et al, 1950) indicate the existence of a ridge on the surface of the erystalline basement. The possibility has been considered that this ridge may extend into the survey area and may be the source of the magnetic anomalies found in the vicinity of the continental slope. However, King et al (1961) computed values of the magnetic polarization intensity that this ridge would be required to have in order to produce the magnetic anomalies observed over it. These computed values were too large to be plausible.
In the southern part of the survey area, the magnetic anomalies peak more sharply. This phenomenon may indicate a shallowing of the basement in that region.
It appears that the most probable general explanation for the continental slope magnetic anomaly is that advanced by King et al (1961). These investigators suggest that although basement topography probably contributes to the magnetic pro- file, the continental slope magnetic anomaly may be partly the expression of a large mass or series of masses of more highly magnetic rocks within the basement.
Another significant feature is the relative position of the peak of the magnetic anomaly. Profiles H-H' and I-I' (Figures 11 and 12) are representative of the southern part of
the survey area. These profiles show the peak of the anomaly
to lie seaward from the break between the continental shelf and the continental slope. In the northern part, profiles A-A' and B-B' do not show any peak lying seaward but indicate that the peak lies shoreward from this break. This difference in trend sug- gests that the lineation of the magnetic anomaly is not directly related to that of the continental slope. Instead, the lineations of both are probably related to a subsurface structural trend.
The small, broad anomalies occurring about 60-80 miles sea- ward from the continental shelf have been reported previously by Keller et al (1954). They were noted as occurring in approxi- mately the same location as an increase in isostatic gravity anomalies. Bathymetric data reveaied no topographic features to account for the anomalies. Consequently, they may be reflections of some type of deep-seated lithologic contrast. In profile view, these anomalies are best seen on Profiles A-A', D-D', and F-F'
on Figures 4, 7, and 9 respectively.
10
V. SUMMARY OF FINDINGS
Magnetic measurements across the continental slope and adjacent deepwater area off the east coast of the United States revealed the presence of an elongate anomaly of a few hundred gammas amplitude. This anomaly hasa lineation corresponding closely, but not exactly, with that of the continental slope. Depth estimates made on this anomaly are in reasonable agree- ment with seismic depths to crystalline rocks. This agreement suggests that the anomaly is caused by contrasts in intensity of magnetic polarization within the basement.
The center of the survey area is void of magnetic features. However, small, broad anomalies occur about 60-80 miles east of the continental slope. Bathymetric data revealed no topographic features capable of accounting for these anomalies. Consequently, these anomalies may be reflections of some type
of deep=seated lithologic contrast.
11
U.S. NAVY HYDROGRAPHIC OFFICE
PROJECT N-20
MARINE MAGNETIC SURVEY
TRACK CHART AND PROFILE INDEX
vo ef N \ NX \ < ya ! / f = Av Ie x Ba oa \ x EN iN A AY \ \ x a » ‘ ~~ A a SS NS aN L PS ~ 4. eon SN 7 ba \ aN er . > & ah ; \ WA me l LG \ .V, BS iene Ni oe Aes ce Ne ic: SI ——— 54 IN Ny,
12
U.S, NAVY HYDROGRAPHIC OFFICE
PROJECT N-20 MARINE MAGNETIC SURVEY RESIDUAL MAGNETIC INTENSITY CONTOUR CHART
(Magnetic regional gradient removed) Mercator poreion
74 73° UEP 7° 70°
e Ne a
ee
+250
i) v
. Le \)
: 8 f
’
ee Zz SN = > Xp, -
FIGURE 3
14
SVWWY9S NI ALISNSLNI SILANDVW AVLOL
WW SATIAONd DIYLANAHLVE GNV SILANSOVW
o0o8rs
oooss |
oozss | oovss al I a
iW
Fi
ooess =
coors —|
0079S a
Vv AYNSIS
Oe NOILVES99VXE WOILYSA
TED RE HL Re p
000z
SWOH1V4 NI Hidad
15
of fe} 4 > [P = > @ Zz m al (9) Zz = m C4 @ = < Zz i) > = = > oO
=|
(2-8 SAMNIAOYNd OIYLANAHLVE GNV DILANSVWN
S 4gndSla4
Loe NOILVYS99OVXA TWOILYSA
SWOHLV4 NI H1d3ad
16
4 fo) 4 > r = > @ fi = (9) Zz 4 m Zz @ =| < Fd 9) > ES = > 7)
/O-D SAMNAOYd DINLAWAHLVE GNV SILANSVW
9 4aYyNSIA
VWHADOVXA 1V Ya
SWOHLV4 NI Hid3d
17
,0-G SAMNsAOYd DIMNLANAHLVE GNV SILANSVW
LOE NOILVYSD9OVX]S IVOILYSA
SVWWYVS NI ALISNALNI SILANOVW TWvLoL
. SWOHLV4 NI HidaG
18
4 fo) 4 > cr z= > in} Zz m al a z 4 m Zz @ =| < Zz o > = = > 0}
4-3 SAMAONd DIYNLANAHLVE GNV SILANSDVWN
8 4yndl4
FOE NOILVYSD9VXaS AVOILYSA |
VAAAT WAS
SWOH1V4 NI Hidad
19
SVWWYS NI ALISNSLNI SILANOSVW WWLOL
oorys —
oo9vs —_
oooss —
oozss
oo9ss
,4A-a SATIAONd DIYNLAWAHLVE GNV DILANSVW
6 3yndIl4
FOE NOILVYSS9OVXS WOILYSA
oo9 1
SWOHLVW4 NI Hidad
20
DEPTH IN FATHOMS
2400 —4
MAGNETIC AND BATHYMETRIC PROFILES G-G’
VERTICAL EXAGGERATION 30:1
FIGURE 10
21
54600
TOTAL MAGNETIC INTENSITY IN GAMMAS
f— 55200
-— 55000
-— 54800
-— 54600
f— 54400
TOTAL MAGNETIC INTENSITY IN GAMMAS
Fe fa dee eee No Ua SEAIWEVENO aie peel Bit
54200
400 —
800 —
1200 —
DEPTH IN FATHOMS
2000 —
2400 —
VERTICAL EXAGGERATION 30:1
FIGURE 11
MAGNETIC AND BATHYMETRIC PROFILES H-H’ 22
I— 55000
}— 54800
t— 54600
TOTAL MAGNETIC INTENSITY IN GAMMAS
54200
800
wo
=
°
=
&
iS 1200 —}
z
I
FE
Oo
a 1600 —}
VERTICAL EXAGGERATION 30:1
FIGURE 12
MAGNETIC AND BATHYMETRIC PROFILES I-I’ 23
REFERENCES
Ewing, M., Worzel, J. L., Steenland, N. C., and Press, F., "Geophysical Investigations in the Emerged and Submerged Atlantic Coastal Plain, Part V, Woods Hole, New York, and Cape May Sections." Bulletin of the Geological Society of America, Vol. 61, No. 9 (Sept. 1950), pp. 877-892.
Keller, F., Meuschke, J. L., and Alldredge, L. R., "Aeromagnetic Surveys in the Aleutian, Marshall, and
Bermuda Islands.'' Transactions of the American
Geophysical Union, Vol. 35, No. 4 (Aug. 1954), pp. 558-572.
King, E. R., Zietz, I., and Dempsey, W. J., The Significance of a Group of Aeromagnetic Profiles Off the Eastern Coast of North America. U. S. Geological Survey, Professional Paper 424=-D. 1961.
Vacquier, V., Steenland, N. C., Henderson, R. G.,
and Zietz, I., "Interpretation of Aeromagnetic Maps."
The Geological Society of America Memoir 4/7. 1951.
24
APPENDIX
FREDERICKSBURG MAGNETOGRAMS, JULY 18-24, 1961
25
Table of Base-line and Scale Values for Full-size Magnetograms
Fredericksburg Magnetic Observatory Standard Magnetograph
PRELIMINARY VALUES
Declination Horizontal Intensity Vertical (D) (H) Intensity (Z) Interval Base-line Scale Base-line Scale Base-line Scale value value value value value value ° ! ' /mm x & /mm x 7 /mm Jul 18-24 6 22 0.49 19165 73.65) 53055 3.0
1961
Base-line separation distance on original magnetograms Z-H 111 mm.
D=B+5S d H=B+S_h Z = BL+S 2 ip) 1D) wee H 4H mm Z 2 mm
D (gamma) scale value = 2.7 %/mm
Directions of increase on magnetograms: D (W) up: H up: Z up
26
Ont
OT
Zr “LIN AGL 8
Zz
T96T}6T AINE VA hness 1430344
1 ih v, i H 7 Hd : H U WSZ 8 a = Z vr eu AW Z T96T |8T AINE
VA ‘SYNgSyyIN3IG3Ru4
27
mn i eal sen H H+ = eS el ° : o a fC : =| a Be a Al eae |e A fe pees ——— Sale & i Gaia z =| (ie (ps eae Cel paw anima ot =e 1961| 12 ATNr wie 'VA ‘SHNESwoIWgaG3NI I+
1 D H sit re sit H O1+— sti+ ° | “LW WSZ 8 a sas a _| . Sway | S ie) Sie oct Lae 8 on eae Z Feed 1961} 02 AINE
VA ‘9H eee
28
T96T| €2 AINE
VA belies ewan
MWA!
Zz
T96T} 22 AINE MNESNDWI0IN4s
29
ae ae NG Ses Haga | <A) Coos Es t ir, By f al Esta maat tf mwa a
As
a= ee
ee iti ihe ad ae Peeler emeniteraty ye 2 ry we ~_
Mis adnan! Siar vivebcehe et Seale
€€L-aL
se7eqg pejtug ey7 jo aseop 4seq e4I FFOQ APAINgG OFJouseW euTAeW YW :2T9TF
ysn ‘ado[g [BqUeUTJUOD 4seOD ASeY-TeTl3sertey, ‘uspTRousey
C€l-aL
se7ejg peqtug ey7 jo 48e0D seq ey FJQ ADAING OFAouseW eUTIeW y :eTITI
yvsn ‘adotg [eqUeUTJUOD 7yseOD 1Seq-[epAqseazey, ‘mspJeusey
“TT
*eqep OfUstTes eTqeTpTeae
pue oTr}JemAYIeG OF UOTIeTeA S7T
pue se7eqg perlzug e472 Jo Aseoo JAsvo
24} JJO eee BATSUSERXD Ue JO 192 -oPLeYyO ITJouseUIOeS |} seqytrIseq
xTpueddy soousteyoy
* (€€T-UL) “SST ZI surpnpout *d 6Z °7961 tequeqdag ‘SHLVLS CHLINA FHL JO LSyoo LSva
SHL F410 AMAUNS OLLANOVN ANIYVW V *20TFFO OTYdez3ours09 TRPAeN *S *f
‘ejep oTWstes oTqe,peae
pue oTazJewAyIeq OF UOTFeTEA SAT
pue se7e}g paytug aya Jo yseoo yseo
ay} JJO Pele sATSUSeRKe Ue Jo 199 -oPIPYO OTJoOUSseUOSS dy saqzTAOSeq
xTpueddy saoustN yoy
* (€€T-UL) *s8TF ZT Surpnqour *d 67 *796T tequeqdag ‘SHLVLS GALINA FHL JO LSyoo Lsva
WHL FO ATAYNS OLLANOWN ANIYVW V *S0TFIO OTYderZouPs0g TRPARN *S °n
C€l-aL
s23e4g peytug e427 Fo 3se0p 3sey ey FIO AodAing OFIousey eUTAeW VW :°7973
ysn ‘adotg [ejUeUT UCD 4seOD
Jseq-TeFaqseatey, ‘wspTJouse_
€€T-aL
seqeqg peytug eyq jo aseop Asey 24 JFJOQ AsvAing DFReuseW eUTAeW W :2e79TI
Ysn Sedotg [equeutqUoOg AseoD
JSEY-TBTIIserzey ‘us TJousey
*eJep oyWSTes aTqeTpTeae
pue ofr}ZewAYIeq OF UOTIeTEA SAT
pue soq7ejg peyzug 242 Fo 4yseoo 4seo 247 FJO PSAP BATSUSIXd Ue Jo Jaq
OneyE -OPLeYO OTJOUZEWOSS |Yy} Saq;tosaq XP pueddy sadusteyoy “ft * (€€T-UL)
“s8TZ ZI Surpnptour -d 6Z *Z96T Tequeqdag “SHLVLS GHLINA aHL 40 LSvood Lsva
aE SHEL AHO AHAUNS OLLANDSVW ANTYVW V “29TFJO WTYderZoueecg [eAeN *s *p
‘Beep IWSsTes aTqeTTeae
pue oTrt}zemAY}eq OF UOTIeTEI sqTt
pue se7eqsg peytuQ 24 Fo 4Jseoo yseo ey} JJO Bee BATSUSeqXe Ue Fo aq
OTR -oeleYD DTJousewoss ay} seqTrzoseq xTpuoddy soousrteyzoy °F * (€ET-UL)
“sty ZT Suppnypout *d 6Z *796T tequejdas ‘SHLVLS GHLINA SHL JO LSyoo Lsva
“yh SHE AHO ATAYNS OLLENSVW ANINVN V *S0TJJO WTYyderzZourecg TeaeN “Ss “DQ