A BOTTOM GRAVITY SURVEY OF THE SHALLOW WATER REGIONS OF SOUTHERN MONTEREY BAY AND ITS GEOLOGICAL INTERPRETATION Robert Andrew Brooks Library Naval Postgraduate School Monterey, California 93940 i) i ' * :'". .1 wonierey, alifornia ii->* &*t\ B-ian r< A BOTTOM SHALLOW WATER BAY AND ITS GRAVITY SURVEY OF THE REGIONS OF SOUTHERN MONTEREY GEOLOGICAL INTERPRETATION _, by Robert Andrew Brooks , Sr . Advisors : Robert S. J . J . von Andrews Schwind March 19 7 3 i KppKoved &o-l public h.eJ..zo.bz; dUtnJJoation unlimited. A Bottom Gravity Survey of the Shallow Water Regions of Southern Monterey Bay and Its Geological Interpretation by Robert Andrew Brooks , Sr. Lieutenant, United States Navy B.S., United States Naval Academy, 1966 Submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE IN OCEANOGRAPHY from the NAVAL POSTGRADUATE SCHOOL March 19 7 3 Library rnia 9394Q ABSTRACT Eighty-two ocean bottom gravity stations in southern Monterey Bay were occupied in the summer and fall of 1972 from the R/V ACANIA. A land gravity survey of ten stations about the perimeter of the Bay was conducted in the spring of 19 72. Gravimeters employed were LaCoste and Romberg Models H6G and G-17B, respectively. Conventional steps in data reduction are discussed, and a terrain correction theory unique to ocean bottom gravimetry is presented. The complete Bouguer anomaly (CBA) field for bottom and shoreline surveys is included. The geological interpretation of the gravity data is discussed briefly. Sub-bottom structure of southern Monterey Bay as determined by seismic reflection is verified by the CBA field, and a calculated density contrast between the basement granodiorite and overlying sedimentary strata is found to be realistic. The data supports the existence of a fault oriented beneath the Monterey Submarine Canyon. TABLE OF CONTENTS I. INTRODUCTION 11 A. SURVEY PURPOSE 11 B. AREA DESCRIPTION AND GEOLOGICAL SETTING 13 C. PREVIOUS WORK 17 II. THEORY OF GRAVITY MEASUREMENTS 18 A. INSTRUMENTS 18 1. Stable Gravimeters 18 2. Unstable Gravimeters 19 B. OCEANIC VERSUS LAND GRAVIMETRY 19 III. EQUIPMENT 23 A. GRAVIMETERS 23 1. LaCoste and Romberg Model G-17B Geodetic Land Gravi meter 25 2. LaCoste and Romberg Model 116 G Underwater Gravimeter 25 B. AUXILIARY EQUIPMENT 25 IV. PROCEDURE ■ 31 A. GRAVIMETER CALIBRATION 31 B. SHORELINE SURVEY 32 C. SHIPBOARD INSTALLATION AND SEA TRIALS 33 D. SURVEY OPERATIONS 36 1. Navigation 36 2. Measurements 39 3. Environmental Effects 41 V. DATA REDUCTION AND PRESENTATION 4 3 A. OBSERVED GRAVITY 43 3 B. LATITUDE CORRECTION 44 C. EARTH TIDE CORRECTION 46 D. DRIFT CORRECTION 46 E. FREE-AIR CORRECTION 47 F. BOUGUER CORRECTION 49 G. TERRAIN CORRECTION 53 H. CURVATURE CORRECTION 60 I. SHORELINE SURVEY 61 J. DATA PRESENTATION 62 VI. DATA ANALYSIS AND GEOLOGICAL INTERPRETATION 6 8 VII. FUTURE WORK 79 APPENDIX A: SUPPLEMENTARY STATION INFORMATION 80 COMPUTER PROGRAMS 85 REFERENCES CITED 87 INITIAL DISTRIBUTION LIST 89 FORM DD 1473 91 LIST OF FIGURES Page 1. U. S. Naval Postgraduate School's Oceanographic Research Vessel R/V ACANIA 12 2. Survey Area 15 3. Geological Map of the Survey Area 16 4. Schematic of a Stable Gravimeter: the Hartley Model 20 5. Schematic of an Unstable Gravimeter: the Thyssen Model 20 6. Simplified Diagram of the LaCoste and Romberg Gravimeter 24 7. Model H6G Gravimeter Ready for Use 26 8. Internal View of the Model H6G Gravimeter 27 9. Schematic Diagram of the Auxiliary Equipment 30 10. Station Locations 34 11. Auxiliary Equipment Installed Aboard the R/V ACANIA 35 12. Schematic Representation of the Free-Air and Bouguer Corrections J^ 13. Schematic Representation of the Terrain Correction for a Gravity Station on the Sea Floor 56 14. "Mass-Adjusted" Free-Air Anomaly Map of Southern Monterey Bay . 15. Complete Bouguer Anomaly Map of Southern Monterey Bay 65 16. Comparison of CBA and Depth of Granite Sub- structure as Determined by Seismic Reflection for Profile A-B 71 17. Comparison of CBA and Depth of Granite Sub- structure as Determined by Seismic Reflection 72 for Profile C-D Page 18. Composite Gravity Map of Monterey Bay 75 19. Two Possible Structural Explanations for the Inferred CBA Field in the Vicinity of the Monterey Submarine Canyon 77 LIST OF TABLES TABLE PAGE I. Features of the LaCoste and Romberg Model H6G Underwater Gravimeter 28 II. Summary of Terrain Correction Procedure for a Gravity Station on the Sea Floor 58 III. Data Presentation 66 IV. CBA Error Magnitude Estimate 69 LIST OF SYMBOLS AND ABBREVIATIONS BC - Total Bouguer Correction BCi - Attractive Force per Unit Mass of Overlying Water BC2 - Attractive Force per Unit Mass of Underlying Water BC3 - Attractive Force per Unit Mass due to Rock vice Water in Underlying Layer CBA - Complete Bouguer Anomaly CC - Curvature Correction CV - Observed Counter 0 „ i Value D - Drift Correction ^9n - Vertical Gradient dz of Newtonian Gravity ET - Earth Tide Correction FAA - Free-Air Anomaly FAA' - "Mass-Adjusted" Free- Air Anomaly FAC - Free-Air Correction Jn % 9t H L M R 9 9c SBA TC Z Zt Universal Gravitational Constant Newtonian Gravity Observed Gravity Theoretical Gravity Elevation above Sea Level Layer Thickness of a Geological Unit Latitude Mass of the Earth Radius of the Earth Density Density Contrast Be- tween Granodiorite and Monterey Formation Density of Crustal Material Density of Sea Water Simple Bouguer Anomaly Terrain Correction Distance from Sea Sur- face to the Ocean Floor Distance from Sea Surface to Mean Sea Level ACKNOWLEDGEMENTS The author wishes to express his appreciation for the help- ful guidance provided by his advisors, Dr. Robert S. Andrews and Dr. Joseph J. von Schwind of the Naval Postgraduate School (NPS) Department of Oceanography. Dr. Howard Oliver of the U. S. Geological Survey (USGS) , Menlo Park, California, loaned the land gravimeter for the shoreline portion of the survey. Dr. S. L. Robbins , also of USGS, provided modifications of the Swick tables for terrain corrections. Mr. Cliff Gray of the Naval Oceanographic Office's gravity section was instrumental in making the underwater gravimeter and auxiliary equipment available. Dr. Rodger Chapman of the California State Division of Mines and Geology provided the most recent Bouguer gravity maps of the Central California area for data analysis. Mr. H. Gary Greene, USGS marine geologist, was most helpful in direct- ing the author's examination of the related geology in southern Monterey Bay. Dr. Warren Thompson, Professor of Oceanography, supplied the necessary ocean tidal records. The author is greatly indebted to Captain Woodrow Reynolds and the crew of the R/V ACANIA for their cheerful and professional help in all aspects of the survey. Mr. Dana Mayberry, the NPS Oceanography Department's electronics technician, assisted the author time and again in the many problems of meter circuitry. Several members of the NPS Public Works Department lended tangible logistic support in many ways, and thanks go to Mr. Pete Wisler, Mr. Herschel Crosby, and Mr. Jim Levine. Finally, the survey would not have been possible without the assistance of co- workers Lt. Brian Cronyn , USN , and Lcdr. Antonio Souto of the Portuguese Navy. 10 I. INTRODUCTION Geological oceanographers have three principal means of investigating structures below the ocean floor: seismic, magnetic, and gravimetric surveys, the latter of which is the subject of this writing. The relatively small variations in the acceleration of gravity over the surface of the earth are largely due to such obvious factors as position, elevation, crustal density, and local terrain; once these effects which may be unrelated to substructure irregularities are removed, however, the investigator is left with a meaningful residual in the form of the complete Bouguer anomaly (CBA) . This mea- sure of anomalous attractive force per unit mass then consti- tutes a basis for the interpretation of the upper few hundreds or thousands of feet of the earth's heterogeneous crust. This study is based upon measurements made at 10 land stations and 82 underwater stations which were occupied in the spring and summer of 1972. The bottom gravity data collected was obtained aboard the Naval Postgraduate School's (NPS) re- search vessel R/V ACANIA (Fig. 1) . A. SURVEY PURPOSE The main purpose for conducting this survey was to learn more about the structural geology of southern Monterey Bay by obtaining gravity data in areas too shallow for larger vessels capable of surface gravimetry. Such vessels must stabilize their course and speed for several minutes after each turn 11 iy*8&bfe&Bi o u to CD co CD u •H tc ^ en o c rO CD O o CO O o u to CD -U rd 3 rO S-i • -p H co o a u H rd > > \ ro Pi w CD CO CO CD > CD U -H Cm 12 before useful data can be obtained; this usually places pro- hibitive restrictions on how close to shore the vessel may work, depending upon local bathymetry. But such geographical restrictions do not apply to bottom gravimetry, since the research vessel can slowly decelerate to even very shallow stations and stop for meter lowering and measurement. The R/V ACANIA was able to survey within 100 yards of the beach in water less than 50 ft deep. Additionally, bottom gravimetry is generally more accurate than sea surface gravimetry due to the stability of the measuring platform, the sea floor. An- other advantage of bottom gravimetry is its closer proximity to the density contrasts that produce a gravity anomaly; sur- face ship measurements must contend with the attenuating effect of vertical distance from the meter to the substructure (Beyer, et al . , 1966). This means that the deeper the station, the greater the accuracy of bottom gravimetry over surface gravi- metry (assuming equal meter precision). After a brief area orientation, this report will describe some basic theory of gravity measurements, specific details on equipment used, and various procedures employed during the survey itself. This will be followed by a section explaining each step in the reduction of the raw data, and an analysis of the results in terms of geological significance will conclude the report. B. AREA DESCRIPTION AND GEOLOGICAL SETTING Monterey Bay is located about 70 miles south-southeast of San Francisco, California. It is characterized by gently 13 sloping sandy bottoms in the north and south , divided by one of the largest submarine canyons in the world. The area encompassed by the author's survey includes that portion of southern Monterey Bay within the 50- fm depth contour from Pt . Pinos to Moss Landing (Fig. 2) . With the exception of the rocky bottom in the extreme southwestern portion of the survey area and the predominance of green muds near the Monterey Submarine Canyon, a sandy ocean floor exists in southern Monterey Bay. Cretaceous Santa Lucia granodiorite , readily visible at Pt. Pinos and constituting much of the adjacent sea floor, is buried to in- creasing depths beneath less dense sediments and sedimentary rocks as one proceeds toward the Salinas River mouth-Moss Landing area (Fig. 3) . Much of the overlying material to the north and east is the Monterey Formation, a Miocene rock consisting largely of silicious mudstone , diatomite, and marine shale, interbedded with beds of opaline chert. Above the Monterey Formation are thin layers of the Pliocene Paso-Robles Formation and Pleistocene Aromas red sands. Thinner layers of deltaic deposits from the Salinas River have been laid down above these formations during the Holocene period. Additionally, the Pliocene Purisima Formation, abundant in northern Monterey Bay, has recently been found along the south wall of the Monterey Submarine Canyon (H. G. Greene, personal communication, 1973) . 14 to CD u < CD > U D c/2 CN 0) a •H &4 15 Figure 3. Geological Map of the Survey Area (after Greene, 1970). 16 C. PREVIOUS WORK Past geological investigations of Monterey Bay have mainly concerned themselves with the origin and present structure of the Monterey Submarine Canyon, as seen in the studies by Shepard (1948) , Martin (1964) , and Martin and Emery (1967) . Geological research in the shallow water regions of the Bay has been conducted, but most of these efforts investigated specific geological properties, such as bottom sediment type and distribution. The first extensive and detailed report of the geology of southern Monterey Bay was recently presented by Greene (1970). This work reviewed the results of previous bathymetric data, rock dredges, and grab and core samples, as well as the author's own seismic reflection survey results, in order to make inferences con- cerning the salt water intrusion problem in the aquifers of the lower Salinas Valley. While gravity data has heretofore been totally absent in the shallow off-shore regions , various investigators have surveyed adjacent land areas. Fairborn (1963) , Sieck (1964) , and Ivey (1969) have surveyed the local Monterey-Salinas-Ft . Ord region, and Bishop and Chapman (196 7) compiled land gravity data from many sources in preparing their Bouguer gravity map of the area. 17 I I . THEORY OF GRAVITY MEASUREMENTS A. INSTRUMENTS When the variation of a quantity is extremely small com- pared to the value of the quantity itself, measurement of that variation requires some method of exaggeration in order to achieve desired sensitivity. Such is the case at hand: gravity values at the equator and the poles are approximately 978 and 9 83 gal, respectively (owing mainly to the poleward decreases of the local radius of the planet and the centrif- ugal force due to its rotation) ; hence, the entire range of possible variation relative to the mean value is about 0.5%. Additionally, this small total range of values is about a hundred times greater than the range observed in Monterey Bay, so the requirement for precise instrumentation is clear. Most of the numerous different gravimeter designs that have become operational over the years illustrate this idea. Depending upon operating principle, they fall under one of two general categories: stable and unstable systems. 1 . Stable Gravimeters Stable gravimeters use a spring-balance system wherein the attractive force on a known mass suspended from the end of a spring is linearly proportional to the resultant elongation. In the case of the Hartley gravimeter, the minute elongation is made more measurable by passing a small , horizontally- hinged beam through the spring-mass system and attaching a 18 mirror close to the unhinged end, where beam displacement is greatest (Fig. 4) . Light from a simple remote source is re- flected onto a graduated scale for the observation of relative differences. A micrometer screw attached to an auxiliary spring is located near the hinged end of the beam so that the light beam may be brought Lack to some fixed reference position The amount the micrometer screw is turned to accomplish this zeroing process is thus a measure of the difference between the local gravity value and the value associated with the re- ference position. The system is termed stable because the only force acting to counter gravity is the spring restoring force. 2 . Unstable Gravimeters For deflections of the mass in the unstable (or astatic- balance) gravimeter, the force of gravity is supplemented in increasing the spring elongation's restoring force by a third force which acts in the same sense as that of gravity itself. The Thyssen gravimeter is a typical example (Fig. 5) . The clockwise moment of an auxiliary mass above the pivot point results in additional beam displacement which can be kept linearly proportional to gravity. Thus, greater sensitivity is the chief advantage of the unstable gravimeter; for a given change in gravity, the unstable device will undergo greater beam displacement than that of a comparable stable gravimeter. B. OCEANIC VERSUS LAND GRAVII1ETRY In the case of land gravity meters, systems like those described above are enclosed in an instrument roughly the size 19 / M • ^ / Micromefer J>? / / / / / / / / / :> / / / Screw * i^ Auxiliary Spring zzzzz ight Mass To Graduated Scale I I L I Figure 4. Schematic of a Stable Gravimeter: the Hartley Model (after Dobrin, 1960). U / ////// / /~TT7 / / / / / ZZ / / LAA/JEZZ /J , ._. ^ ft c 4-> 4-1 G 0) •H > QJ +J C 0) ■H 27 Table I. Features of the LaCoste and Romberg Model 116 G Underwater Gravimeter Ba se Size (with legs) 39-inch equilateral triangle Height 29 inches Weight 350 lb , including enough weight to insure acceptable sink rate Range World wide (7000 mgal) Optimum Accuracy +0.0 2 mgal Realistic /vccuracy +0 . 10 mgal Power Source 115 vac, 60 Hz Routine Maximum Operating Depth 600 ft Maximum Operating Depth Tested 2900 ft Maximum Bottom Slope Permissible 15° Depth Indication Pressure sensor (0.5% accuracy) connected to casing port Leveling System Optically-controlled servo motors driving rack and pinion assemblies Flood Indication Open circuit, 1/4-inch gap, inside bottom of lower hemisphere 28 summarized as follows: a gasoline engine provides mechanical energy to a hydraulic pump transforming the rotary motion into hydraulic pressure (Fig. 9). This hydraulic pressure is used to operate both the winch and A-frame using two, two-way con- trol valves. One valve controls a hydraulic motor which con- verts the fluid pressure back into rotary motion to drive a heavy-duty chain link connected to the winch reel's shaft. The A-frame is hinged at deck level and actuated from above by two hydraulic pistons controlled by the other valve. With the gravimeter connected electrically to the termina- tion at one end of the cable, the other end is routed inside the hollow shaft of the winch and out the side to a slip-ring assembly. The outer side of the slip-ring assembly is wired to a cannon plug connection on the control box, which in turn is connected through a rectifier and an isolation transformer to one of the ship's 115 vac outlets, the basic power source for the entire electrical system. 29 TO GRAVIAETER Figure 9. Schematic Diagram of the Auxiliary Equipment 30 IV. PROCEDURE This survey was part of a larger effort to investigate off-shore gravity values over a more extensive area: two co- workers (also NPS students) recorded data in northern Monterey Bay and Carmel Bay. Since bottom gravity studies are by no means a one-man operation, they were assisted by the author and provided similar assistance to him during his survey. A. GRAVIMETER CALIBRATION Prior to beginning the shoreline survey on 26 April 1972, the land meter was calibrated by the author and co-workers under supervision of U. S. Geological Survey (USGS) personnel. Readings were taken at several benchmarks from USGS head- quarters in Menlo Park (USGS 1 JD) to Skeggs Point Scenic Viev; (U. S. Coast and Geodetic Survey Station B-388) , a frequently used calibration route spanning a range of 137.20 mgal (Chapman, 1966) . Subsequent reduction of counter values to gravity values resulted in a difference of 137.15 mgal over the entire range. Smaller differences were found at the intermediate benchmarks. The underwater gravimeter was initially calibrated by the manufacturer prior to shipment to Monterey in the spring of 19 72. After the survey was completed, another calibration check of the meter was in order. Early in 19 7 3 the author occupied CA-2 59 (Woollard Airport Base WA-84) at the Monterey County Airport and a benchmark located at the base of the steel tower on the Monterey Coast Guard pier, designated WH-29 by 31 Woollard and Rose (1963) . The meter readings resulted in a gravity difference less than 0.5 mgal greater than the 22.5 mgal established value (Chapman, 1966) . The difference was probably due to the recent construction of a second terminal building next to CA-259 . Since the airport value is lower than the pier value, and since the effect of the new building is an additional upward attraction, the difference between the two reference stations has probably increased due to the new construction. In view of this consideration, as well as the fact that the published location of the CA-259 benchmark could not be found exactly (due to the construction) , it is estimated that the observed gravity difference between the airport and pier stations was within 0.1 mgal of the true value. B. SHORELINE SURVEY A preliminary gravity survey of the perimeter of southern Monterey Bay was undertaken in late April, 1972, with the land gravimeter discussed above. The information from this shore- line survey of 10 stations was later used to verify the near- by CBA values and isoline trends of the underwater survey, since land gravimetry is an order of magnitude more accurate than oceanic gravimetry. (While land gravimeters are read to the nearest 0.01 mgal and subsequent corrections' accuracies can lead to CBA values of 0.1 mgal accuracy, remote-controlled bottom gravimeters, read to the nearest 0.1 mgal, can result in a CBA accuracy of at best 1 mgal.) In addition to verifying the bottom survey results, these land stations also served to 32 tie in the combined survey with previously surveyed areas in- land. All 10 land stations were located (vertically) within a few feet of sea level so that the small magnitude of the subsequent gravity corrections would yield better CBA accuracies. The stations were fairly evenly spaced from Pt . Joe to Moss Landing (Fig. 10) . Visual estimates of station level relative to the average existing sea surface elevation were recorded at each station, so that station elevations rel- ative to mean sea level could be calculated later from tidal information. Exact locations and depths/elevations of both underwater and shoreline stations are included in Appendix A. C. SHIPBOARD INSTALLATION AND SEA TRIALS When the auxiliary equipment arrived in Monterey in late June, 19 72, preliminary inspection indicated that it had been some time since the system was used. A re-termination of the cable and an engine overhaul were required before the survey could begin. An initial concern was the question of location of the equipment aboard the ship. It was decided that the after section of the upper level would be most suitable in view of the physical requirements associated with meter lowering and the intended length of stay of the equipment aboard. The A- f rame , supporting plate, and gravimeter were stationed on the starboard side so that the data recorder in the dry lab directly below would be able to observe cable speed and direction of motion (Fig. 11). Also, the crane's location in close proximity 33 2 1 Nautical Miles • Underwater Stations Seaside □ Land Stations Monterey Figure 10. Station Locations 34 35 on the starboard side seemed compatible with the plan to bring the meter to and from the main deck for each station. The winch was placed abeam the A-frame area but on the port side, in an attempt to spread out the 4 ton weight of the equipment. The engine and hydraulics stand were located on the ship's centerline and slightly forward of the other components. A brief sea trial period was arranged prior to the start of the actual survey in order to acquire a familiarity with conditions to be encountered. A trial station was occupied west of Seaside, California, in about 100 ft of water. SCUBA divers accompanied the meter to the bottom to observe the sink rate, take some underwater photographs, and determine if excessive cable would foul the feet of the gravimeter. How- ever, a strong wind and too little excess cable resulted in dragging the meter across the sandy bottom while the meter was in the undamped mode. It was decided to return to the ACANIA's mooring to check for possible damage to the mass suspension system. Fortunately the counter reading reproduced the value obtained just prior to departure for the station, a sign that no damage had been sustained. This preliminary venture into the Bay showed that the stations would have to be occupied with the ship's bow into the wind so that the major wind-induced position corrections could be accomplished with ahead or astern engine commands. Since the ACANIA has no bow-thruster , the bow could be kept into the wind with alternate use of one or both of the engines and rudder. Also, a feel for the amount of cable payed out as 36 a function of cable angle and tension was obtained, since the divers reported that cable fouling about the base of the meter was a remote possibility (owing mainly to the cable's large diameter and resultant large radius of curvature) . D. SURVEY OPERATIONS Prior to the onset of the survey, selection of station locations was governed mainly by the requirement for a fairly evenly-spaced grid from which an accurate scalar field of the CBA could be extracted. In view of the size of the survey area and the expected ACANIA availability periods, it was felt that a minimum of 75 stations would be needed to satisfy this requirement. A chart showing the spacing of about 100 intended stations similar to Figure 10 was provided to the bridge for course information. Whether the actual stations occupied coin- cided with these intended positions was not of great concern; the important thing was to determine the position of each station as accurately as possible during and immediately after its occupation in order to adjust the planned location of the next station for a possible gap in coverage. Stations were numbered in order of occupation (except for those on land) . Upon completion of the survey, the 92 stations occupied covered an area of 50 sq n mile, representing a station density of almost 2 per sq n mile (Fig. 10) . 1. Navigation With the exception of three stations occupied during morning fog (and consequently requiring radar positioning) , 37 all of the bottom station positions were determined by visual lines of bearing from prominent landmarks during hours of both daylight and darkness. Three or four such bearings were taken at each station, and these were often supplemented by radar distances from coastline features. Standard navigational procedures were used to estimate the true location of the ship's position in the small area where lines and arcs of position converged. Most of the three-line position triangles were on the order of 0.1 n mile or less on a side, and a good portion of the plots were point fixes. Bearing-taking was begun as soon as the ACANIA became dead-in-the-water (before the meter was lowered to the bottom) and was always completed well before the meter was raised after the measurement. VJhen it appeared the vessel was drifting, a second set of bearings was taken. Differences in meter position and ship's position at each station were thus small enough to safely neglect consideration. In view of these factors, it is stated with a high degree of reliability that station position errors at sea were at worst 0.1 n mile. If such an error was in a north-south direction the value of theoretical gravity and final CBA would differ by only 0.14 mgal . Land station navigation was less complex and far more accurate. Six of the ten stations were selected for occupation because their exact location was clearly shown on the charts used. While the other four station locations were transferred to the chart from hand drawn maps , it is certain that their position accuracy was at least twice as good as that of the oceanic stations. 38 2 . Measurements During the survey the meter was first lowered to the sea surface and a depth counter value was obtained by nulling the appropriate galvanometer; this was repeated as soon as the meter reached the sea floor. Next the leveling mode was selected and required flood and tilt checks were made. (Only at 2 of the 82 bottom stations did the greater-than-15° bottom slope indication appear, requiring repositioning of the meter.) After the high-speed leveling switch was activated to make coarse leveling adjustments, the meter was put into the read mode. In this mode, while fine leveling adjustments continued automatically, the beam position and gravity counter switches were alternately used to stop the motion of the beam position galvanometer needle in the null position. During this process the correct counter value was always approached from the same direction (namely, from too low a counter value) to avoid hysteresis errors. When what was determined to be the correct counter value was obtained, an over-adjustment of 0.1 counter units was introduced to check for a reversal in the direction of beam position motion. This method of obtaining the gravity counter value is not only much faster than the slope method used with the strip chart record of time versus beam speed, but also about as reliable (H. B. Parks, LaCoste & Romberg, Inc., personal communication, 1972). After the gravity counter value was recorded the mass was again clamped in the deck mode and the meter raised. 39 The meter was normally on the bottom for about 4 or 5 min, 2 or 3 min of which were spent in the read mode. In- cluding transit time between stations (but excluding transit time to and from the first and last stations of each track) , the 82 bottom stations were occupied in a total of 25 hours, yielding an average rate of better than 3 stations per hour, or about 18 min per station. The deepest stations usually required the most time. At the shoreline stations the land gravimeter was leveled visually with two bubble glasses and manually operated screws attached to the frame. The scale was then illuminated with a light switch and the mass was manually undamped. Cor- rect counter values were obtained by simultaneously observing the beam position through the microscope eyepiece and manually adjusting the measuring screw to null it in the reference position. Fortunately, snycronization between the counter read- ings of the bottom gravimeter and the control box was maintained throughout the author's survey. Between the August and Septem- ber cruises, however, loss of snycronization did occur once. This was evident when a routine counter reading at Station 1 was several milligals from its standard value, and the loss required opening the inner gravimeter case to read its counter value so that the control box counter value could be reset to agree . An important set of measurements midway through the survey, accomplished by repetitive occupation, established a 40 3.20 mgal difference between Station 1 on the floor of Monterey Harbor and Station E, the USCG pier benchmark. In addition to tieing in all bottom gravity readings with a reference benchmark value, this result also related values recorded by the two different meters with a station common to both surveys. 3 . Environmental Effects From a climatological viewpoint, the summer months are best suited, due to wind conditions, to gravity operations in southern Monterey Bay. By observing surface bubble relative motion as well as cable angle, the ship's master was usually able to maneuver the R/V ACANIA to avoid being blown downwind of the desired position directly above the meter. The wind- induced sea and swell presented more of a problem to survey operations by causing noticeable pitch and roll. This occa- sionally resulted in the gravimeter hitting the extended A- f rame during lowering and raising evolutions . Readings taken in shallow depths were often charac- terized by a slowly fluctuating beam position in addition to the normal motion of the beam position prior to obtaining the final reading. This required observation of the rate of change of the position of the fluctuation's mean so that the gravity counter switch could be used to eventually hold this mean in a fixed position. Since the automatic averaging capability of the control box was inoperative, the averaging process had to be done by eye, thereby introducing an esti- mated reading error of 0.1 counter units into the final counter values . 41 The fluctuating beam position was, in part, the visual indication of a form of micro-seismic bottom motion caused by surface swell and wave activity, its influence on beam position being a function of depth and type of bottom sediment (LaCoste, 1967) . Additionally, a swaying of the cable was probably felt by the meter through the rigidly-attached cable termination. The effect of this problem on survey operations was the need for more time in the read mode to obtain the correct counter value, and, hence, the occupation of fewer stations per hour. Fog conditions often prevail in the east central region of Monterey Bay during the early morning hours of spring and summer. Such was the case during the occupation of Stations 36, 37, and 38. Its only effect upon the survey was a slight deterioration of positioning accuracy; in the absence of visual contact with distant landmarks , radar navigation was utilized. 42 V. DATA REDUCTION AND PRESENTATION The method whereby the raw data collected during the sur- vey is transformed into a meaningful CBA picture is discussed below. Although much of the theory involved in data reduction applies to both land and ocean stations, there are important differences. Some of the individual corrections discussed apply only to bottom gravity data; a summary of the reduction methods applicable to land stations follows in a separate section . The majority of the required calculations were accomplished through use of the NPS IBM-360 computer. Programs were written by the author in Fortran language and numerical significance was double precision. Copies of these programs are included at the end of this report. A. OBSERVED GRAVITY As pointed out earlier, gravimeters measure gravity differ- ences rather than absolute gravity values. Since gravity counter units as recorded from the gravimeter control box are numerically close to, but not exactly equivalent to, milligals, they must first be converted to milligals before addition to or subtraction from a reference gravity value. The absolute reference used for this survey was 979 891.7 mgal , the gravity value at WH-29 on the Monterey Coast Guard pier. In practice a more convenient intermediate reference station at the mooring buoys (Station 1) was used to tie in all of the other station readings and to monitor meter drift. 43 The control box counter reading at the pier benchmark was 3323.05 units; hence, the formula used for computing observed gravity at each bottom station was : gQ = 979891.7 + [( CVQ - 3323.05 ) x ( 1.03985 )] (1) where gQ is in milligals and CV0 represents the observed gravity counter value at the station in question. The 1.0 39 85 factor converting counter unit differences to milligals was obtained from a table provided by the manufacturer listing (a) a milli- gal value associated with each even hundred counter units as well as (b) conversion factors for values between the counter units listed. Since all of the stations1 observed counter values fell between 3300 and 3400, only one such conversion factor from the listing was required. B. LATITUDE CORRECTION The first correction (although the order of various cor- rections is not defined in any mandatory way) applied to the observed value of gravity is normally the subtraction of the theoretical value of gravity based on geographical position. The determination of theoretical gravity obviously requires a precise knowledge of the shape of the earth so that the dis- tance from the surface to the earth's center and the centrif- ugal force are known for any location. The actual geometry has been determined to be very nearly a triaxial spheroid, or ellipsoid of revolution having depressions along the two 45° latitude lines and a flattening and bulging of the equator (Dobrin, 1960). Years of geodetic investigation have yielded numerous revised coefficients for the equation of this 44 reference spheriod to more accurately approximate the earth geoid. The author's selection of a particular equation was governed (as were all aspects of data reduction) by a desire to conform with the procedures of USGS and the California State Division of Mines and Geology in an effort to render the final CBA map compatible with existing land gravity maps. The coefficients used were those of the 19 30 International Spheroid, the theoretical value of gravity being a function of latitude only (Dobrin, 1960) : gt = 978049.0 (1 + 0.0052884 sin2L - 0.0000059 sin22L) (2) where L is the latitude. Equation (2) results in a predicted south-to-north increase in the value of local theoretical gravity of 1.44 mgal/n mile. The assumption that the geoid and the reference spheroid coincide would appear erroneous at first glance, since, in fact, they can differ vertically by as much as 50 m in some regions of the world. However, a more meaningful measure of the validity of the effect of this assumption is the change in this difference over the area of interest. In the small domain of Monterey Bay, the change in the distance from the equipotential surface of the geoid to that of the reference spheroid is on the order of a few centimeters. It may there- fore be concluded that final CBA values will still result in an accurate representation of small-to-intermediate scale geological features of the upper crust (Grant and West, 1965) . 45 C. EARTH TIDE CORRECTION In that the interior of the earth possesses finite elasticity, the attractive forces of heavenly bodies (partic- ularly the moon and sun) continually act to deform its shape. As is the case with the ocean surface, the pertinent manifes- tation of this phenomenon is a small scale vertical fluctuation of the earth's crust which can affect observed gravity values to a measurable extent. The true distance from a point on the surface to the earth's center can change by as much as 1 ft in a matter of hours, representing a 0.1 mgal change in gravity; thus, the effect of these earth tides on gravity measurements made over such an interval of time must be considered. The USGS earth tide Fortran program was utilized to obtain the required corrections. Dates and times of station occu- pation along with a geographically-central location of 36° 42' N, 121° 52* W were the input parameters. The output values were tabulated at 20 min intervals. A copy of this program may be obtained from the NPS Department of Oceanography, Code 58Ad. D. DRIFT CORRECTION Readings taken at Station 1 before and after each track were, more often than not, slightly different. These differ- ences were attributable to three causes: (a) earth tidal variation during the time interval between base station occupations; (b) ocean tidal variation during the same period; (c) meter drift. 46 Since the ACANIA was secured to fixed mooring buoys in an east-west orientation, possible north-south variation in the actual location of Station 1 before and after each track was negligible. To determine true meter drift it is only neces- sary to remove the effects of (a) and (b) above from the total "apparent drift" actually measured. Recalling that counter unit differences are very nearly equivalent to milligal dif- ferences, earth tide variation was removed directly from counter readings. Similarly, the changes in attraction due to differ- ent sea surface tidal heights above the meter were removed. (Each 1 ft of water above the meter decreased observed gravity by 0.0127 mgal . ) The resultant drift values for each track were then applied linearly over the range of stations. The greatest drift rate was 0.0 36 mgal/hour. E. FREE-AIR CORRECTION The formula for the value of the Newtonian portion of gravity (in milligals) for a unit mass at sea level (gn) is: G M 9n = "^2 (3) where G is the universal gravitational constant (6.670 x 10 cm g~lsec ) , M is the mass of the earth, and R is the distance to the earth's center. The free-air correction (FAC) to gravity data accounts for the fact that the measure- ment was made at some elevation other than mean sea level, the assumed height of the surface of the reference spheroid. (A gravity station located exactly at mean sea level, then, would not require this correction.) Disregarding the mass between 47 the station and sea level for the time being, the free-air correction is obtained by differentiation of (3) (Heiskanen and Vening Meinesz, 19 5 8) : d3n FAC - ~^- x (H) = (0.09406 mgal/ft) x (H) (4) where H is the elevation of the station above mean sea level . A simple modification of this equation yields a formula which applies to bottom gravimetry: FAC = (0.09406 mgal/ft) x (Water depth - Tide height) (5) Since equation (3) implies decreasing values with increasing distance from the center of the earth, the free-air correction is negative for underwater stations. A first-order gravity anomaly, the free-air anamoly (FAA) , is obtained with the inclusion of this correction along with others mentioned thus far: FAA - g0 - gt + ET + D - FAC (6) where g0, g^ / and FAC are as defined by equations (1) , (2) , and (5) , respectively, ET being the earth tide correction, and D the drift correction. The free-air correction effectively refers station level to sea level, and is easily the largest single correction to be applied. It is therefore clear from equation (5) that accurate values of station depth and tide level are critically important if an acceptable degree of precision is to be achieved. The pressure sensor in the gravimeter chamber pro- vides depth counter indications from the sea surface to the sea bottom; the difference in the surface and bottom readings 48 is converted to depth in feet by means of a linear relation extracted from sensor calibration graphs provided by LaCoste and Romberg. All tide heights used were based on the assumption that the existing tide level at each station in the survey area was identical with that simultaneously recorded in Monterey Harbor. Levels were recorded by the NPS tide recording de- vice located in the assistant harbormaster's office on Municipal Wharf No. 2 in Monterey Harbor. A reference line was drawn on these records at 5.5 ft above staff zero. Mean sea level is 5.9 ft above Monterey's staff zero (H. V. Maixner, personal communication, 19 73) . Thus, tide height distances were measured relative to 0 . 4 ft above the reference line. These tide values were compared with appropriate values from the published 19 72 tide prediction tables (U. S. Department of Commerce, 19 72) ; agreement was very good. In order to make this comparison it v/as necessary to determine the vertical distance from mean lower low water to mean sea level for Monterey, since all tide table values are referenced to mean lower low water. This difference was found to be 2.7 ft. F. BOUGUER CORRECTION While the free-air correction is concerned with changes in the denominator of the right-hand side of equation (3) , the Bouguer correction (BC) addresses changes in the numerator. This correction considers how the attraction between the mass within the gravimeter and the earth's center of mass is 49 affected by (a) the presence of material between the station's elevation (or depth) and sea level, and (b) the density of this material. The method of the Bouguer correction first assumes that the measurement was made on an infinitely flat ocean bottom, so local terrain irregularities are temporarily neglected. Removing the upward attraction of an infinite slab of over- lying water whose thickness equals the station depth would effectively replace that slab of water with air and result in a higher value of observed gravity. Since the formula for the attraction of such an infinite slab is 2 Tr G 9 h , where 9 is the density and h is the thickness (Dobrin, 1960) , the first part of the Bouguer correction (BCi ) , the removal of the sea water's attraction, is: BC-l = 2TTG 9W Z (7) where Z is the pressure sensor depth and 9 is the density of sea water. The second part of the Bouguer correction (BC2) accounts for the increased downward attraction for the sea water that would have existed had the measurement been made at mean sea level (thus, the previous inclusion of the free-air correction is assumed) : BC2 = 2TTG 9W (Z-Zt) (8) where Zt is the tide height relative to mean sea level. The third and final part of the Bouguer correction (BC3) considers the additional downward attraction that would have existed had the material between sea level and the ocean bottom been an infinite slab of crustal material instead of water: 50 BC3 = 2TTG ( 9R - 9W ) (Z-Zt) (9) where pR is the density of average crustal material. Figure 12 summarizes these considerations schematically. The total Bouguer correction is then: BC = 2> G pwZ + 2 YTG 9w (Z-Zt) + 2 VrG (pR -pw) ( Z-Zfc) (10) or, combining terms, BC = BCjl + BC2 + BC3 = 2 FT G pw Z + 2TTG 9r (Z-Zfc) . (11) The density of sea water was taken as 1.02 7 g/cirP , and an aver- age crustal density of 2.6 70 g/cm was used for pR . it is clear from Figure 12 that the Bouguer correction is positive for under- water stations. Once it is included with the other corrections, the simple Bouguer anomaly (SBA) emerges: SBA = gQ - gt + ET + D - FAC + BC (12) Or, from equation (6) , SBA = FAA + BC. (13) Just as bottom gravity data is reduced to the complete Bouguer anomaly in order to compare it with land gravity anomalies , its reduction to a gravity anomaly intermediate between the free-air and simple Bouguer anomalies will permit comparison with FAA values as determined by a surface ship. By only adding BC-. and BC2 to the free-air anomaly of the bottom data, such a value is obtained (Fig. 12-4 applies) . This may be referred to as a "mass-adjusted" free-air anomaly (FAA') . Since the free-air and Bouguer corrections are both func- tions of station depth, pressure sensor errors must be exam- ined to determine their effect on these corrections. A 1-ft depth error produces a combined FAC/BC error of 0.0 5 mgal 51 SITUATION PRIOR TO FAC AND BC ?~0 -J- tv sea wafer P =1.027 v^l rocU^- 2.670 Station \ gob U REMOVAL OF OVERLYING WATER ATTRACTION ELEVATION CORRECTION air -msl V £ = 2.670 gob+ BC1 air Si e=0 — — — — — rrtsl Imaginary Station "ob+BC,- FAC C= 2.670 INCLUSION OF V/ATER ATTRACTION BELOW INCLUSION OF ROCK ATTRACTION BELOW 7K air "e^~o msl C = 1-027 TQob + BC! - FAC +BC2 £=2.670 € =2.670 9ob + BC, - FAC + BC0 + BC Figure 12. Schematic Representation of the Free-Air and Bouguer Corrections. (All densities are in g/cm^.) 52 (the two corrections are of opposite sign) . Although the manufacturer's claim of pressure sensor accuracy is 0.5% (this would correspond to + 2 . 3 ft for the deepest station) , a more conservative assessment was used. It was assumed that the depth determination was in error by 4 ft at most, so the combined FAC/BC error is +0.20 mgal or less. G. TERRAIN CORRECTION Easily the most time-consuming aspect of gravity data re- duction is the topographic, or terrain correction (TC) . Often- times this correction may be ignored altogether (J. D. Rietman, personal communication, 19 72) , and this has been generally true for previous underwater gravity work (much of which seems to have been done in the Gulf of Mexico) . However, due to the close proximity to Monterey Bay of very deep abyssal plains in the Pacific, as well as such intermediate-scale features as the Monterey Submarine Canyon and various coastal mountain ranges, this sort of simplification would be unreal- istic at best. (In .the final result, quite noticeable differ- ences were found in the positions of SBA and CBA isolines spaced at an interval of 5 mgal.) Consider a gravity station on the ocean floor located on the upper rim of a deep trench. The fact that the trench is filled with sea water produces a smaller value of observed gravity than would be measured if the trench were filled in with solid crustal material. The upward attraction of a nearby guyot would also act to decrease observed gravity. Thus any terrain deviation above or below station level reduces observed gravity relative to its flat- 53 bottom value, both on land and under the sea surface. This correction is therefore always positive. The generally accepted method for calculating the gravitational attraction of topographic irregularities was first developed by Hay ford and Bowie in 1912. It consists of approximating the volume of excess or deficit terrain surround- ing the station with a series of concentric cylindrical shells of varying height (depending on the local elevation or depth) . In plan view this appears as a set of circular zones; the zones are lettered alphabetically from the station outward and divided into many compartments. This bulls-eye representation is then put on glass or acetate templates scaled to conform with appropriate topographic or bathymetric charts. By centering such a template on the station, the average elevation or depth of each compartment is estimated to determine its dis- tance above or below station level. This difference between compartment and station level is the entering argument for tables which have been developed to give the corresponding gravitational attraction in milligals. The compartment cor- rections (199 in all) are then summed to yield the total terrain correction. The tables used in this research are modifications by the USGS of the work done by Swick in 1942. His tables are based, in turn, on Bullard's 1936 modification of the original Hay ford- Bowie paper that presented the combined effects of topography and isostatic compensation. These tables assume a removal/fill- in density of 2.670 g/cm^ , and give a 0.615 multiplication 54 factor for the corrections in oceanic compartments. (Since not air but sea water of density 1.027 g/cm^ fills oceanic compartments to sea level, only 1.64 3 g/cm^ additional mass density, or 61.5% of 2.670 g/cm^, is needed to complete the terrain correction density fill-in.) As one would expect, the tables' corrections decrease for a given elevation dif- ference with increasing distance from the station, and increase for a given zone with increasing elevation differences. Thus the elevation difference effectively adjusts the value of the numerator of equation (3) , while the zone specification does the same to the denominator. The effect of terrain upon a gravity value measured on the sea floor is more complex than its terrestrial counterpart; this is understandable in view of the inclusion of an additional medium, the sea water, surrounding the station. This effect must be computed in three stages; Figure 13 presents the analy- sis in cross-section. Had the sea floor (Fig. 13-A) been horizontal, a terrain correction would not be required. The existence of a uniform slab of 1.027 g/cirr material extending infinitely in all directions between station level and sea level can be neglected without changing the TC value to be calculated since the topo- graphic effect of such a slab of given density and of constant thickness equals zero. The first step shown in Figure 13 simplifies the problem and requires no actual calculations; it effectively replaces the sea water in Sector 1 with air and decreases the crustal density in Sector 4 to 1.643 g/cm^. The 55 k oy \ \ / / / / / / s / / / / / s o o II '/ / / / i +J ■H > to M U id ^ 0 m c 0 •H ^-^ -P • U<^"> H H fC QJ Eh to H ■H H -P < td ■>— « •P c 0 • CO u rm: CBA = gQ - gt + ET + D - FAC + BC + TC - CC (15) :, from equation (12) : CBA = SBA + TC - CC - (16) le CBA is the difference between observed gravity and the ilue expected had the station been located on a flat solid lrface at sea level, unaffected by astronomical attractions. >is final gravity anomaly refines the geological implications : the SBA and represents a useful tool that geologists can ill upon to infer substructure mass distribution. SHORELINE SURVEY Data reduction for the ten stations ashore proceeded in a ishion similar to that of the bottom survey, but with five cceptions . First, since calibration factors for the con- jrsion of counter units to milligals are unique to each irticular gravimeter, the equation for g for the Model G-17B ravimeter was not the same as equation (1) . Second, free-air 61 corrections were added (rather than subtracted) , since on land, mean sea level in general is closer to the earth's center than station elevation. Third, Bouguer corrections were subtracted (rather than added) , because part of the observed attraction was due to the material between station level and sea level. Thus, while underwater Bouguer correc- tions are composed of three parts, terrestrial Bouguer correc- tions need compensate for only a single attraction. Fourth, the three-step terrain correction analysis does not apply. Finally, since all elevations were within 10 ft of mean sea level, curvature corrections were not required. (A station elevation of 22 ft above or below sea level is required to produce a curvature correction of 0.01 mgal . ) The equation summarizing land station data reduction is then: CBA = gQ - gt + ET + D + FAC - BC + TC (17) Estimated error values for land station corrections are +0.02 mgal for gQ i +0.07 mgal for gt (corresponding to positioning accurate to within 1/20 n mile) , + 0.06 mgal/ft for station elevation, and, again, +0.30 mgal for the terrain correction elevation estimations. J. DATA PRESENTATION The values of the "mass-adjusted" free-air anomaly and complete Bouguer anomaly for both shoreline and bottom stations were recorded on charts next to appropriate station locations and scalar analyses were performed by hand with isolines spaced at a 5-mgal interval (Fig. 14 and 15) . Table III presents the values of these and two other gravity anomalies 62 of interest for the 92 stations; values of observed and theoretical gravity as well as values of the various correc- tions are presented in Appendix A. 63 ►~36°45'N Figure 14. "Mass-Adjusted" Free-Air Anomaly Map of Southern Monterey Bay. (Contour values are in milligals.) 64 I 121°55'W 50 FM ■36°45'N Figure 15. Complete Bouguer Anomaly Map of Southern Monterey Bay. (Contour values are in milligals.) 65 . TABLF III. DATA PRESENTATION. (VALU ES ARE IN MILLIGALS. STATION FAA FA A' SBA CBA 1 8. 378 9.139 9.997 12.959 2 1.783 3.761 5.349 3.135 3 -7.90 2 -5.444 -3.474 -C. 825 4 -16. 926 -14. 714 -12.941 -1 3.399 5 -14.256 -10. 267 -7.075 -4.492 6 -3.314 0.309 3.206 5.916 7 2.473 5. 996 8.8 08 11.64 8 8 6.C75 8. 157 9.815 12.780 9 6.966 8.947 10.521 13.6J7 10 5.273 0. 681 11.395 14.566 11 -6.959 -0.241 5.120 7.934 12 -4.313 1.253 5.690 8.454 13 -6.932 -1.971 1.981 4.656 14 3. 093 5. 920 8.163 11.015 15 -16.282 -11.975 -8.550 -6.003 16 -12. 734 -7.146 -2.69 7 -0.103 17 -10.975 -4. 576 0.52 0 3.200 18 -15.408 -8.755 -3.457 -0.792 19 -21.103 -14.961 -10. 370 -7.596 20 -22.216 -15.729 -10.561 -8.143 21 -22. 131 -15. 530 -10.269 -7.903 22 -20.362 -16.258 -12.977 -10.495 23 -21. 733 -19.727 -18.126 -15.723 24 -24.C94 -20. 366 -17.388 -15.071 25 -25.615 -23.745 -22.254 -19.985 26 -25. 753 -21.958 -18.924 -16.69C 27 -2 7.460 -25.419 -23.793 -21.697 23 -26.808 -23.369 -20.625 -18.544 29 -26. 698 -23. 023 -20.089 -18.001 30 -25.718 -20.716 -16.721 -14.5C6 31 -24.362 -18.562 -13.928 -11.678 32 -25.118 -20.234 -16.332 -14.076 33 -23. 154 -17. 785 -13.495 -11 .126 34 -19.543 -14.460 -10.397 -7.913 35 -27. 33.0 -25.268 -23.856 -21.936 36 -25. 863 -24. 333 -24.005 -22.122 37 -24.659 -23.225 -22.074 -20. 188 38 -23.634 -22.5 39 -21.606 -19.774 39 -19.796 -12. 362 -7.313 -4.862 40 -18.477 -11.256 -5.477 -2.851 41 -18.260 -11.495 -6.082 -3.488 4? -10.885 -3.632 2.169 4.984 43 -5.340 1.793 7.497 10.643 44 1.478 7.556 12.417 15.832 4 5 3.318 8.685 12.976 16.615 46 -1.939 5. 143 10.804 14.302 47 -5.387 2.036 8.368 11.821 48 -10. 3c3 -2. 669 3.478 6.625 49 -1.697 3.828 8.239 11. 104 50 -16. 389 -3.366 -2.194 0.633 6 6 TATICN FAA FA A SBA CBA 51 -18.504 -11. 198 -5.361 -2.797 52 -21.887 -15. 133 -9.683 -7.371 53 -23.793 -17.805 -13.021 -10.753 54 -23.333 -18.145 -13.630 -11.448 55 -25. 221 -21. 157 -17.911 -15.831 56 -27.646 -25.666 -24.088 -22.082 57 -25.623 -23.612 -22.311 -23.326 58 -25.037 -21. 165 -18.076 -16.042 59 -24.413 -21. 136 -18.524 -16.550 60 -24.643 -21. 073 -18.227 -16.348 61 -23.344 -21.460 -19.963 -18.136 62 -23.120 -21.828 -20.805 -13.997 63 -22.447 -21.598 -20.932 -19. 177 64 -22.353 -21.905 -21.157 -19.394 65 -38.000 -26. 040 -16.491 -13.723 66 -25.055 -21.527 -18.727 -16.597 67 -24. 388 -20.401 -17.234 -15.213 68 -24.047 -18.606 -14.278 -12.171 69 -23. 559 -17.831 -13.274 -11.173 70 -24.638 -19. 562 -15.527 -13.365 71 -23.324 -17. 013 -11.992 -9.751 7 2 -21.083 -15.065 -9.635 -7.374 73 -24.339 -18. 343 -13.335 -13.854 74 -24.639 -19. 860 -16.025 -13.838 75 -25.312 -13.422 -12.939 -10.558 76 -22. 713 -15.664 -10.353 -7.775 77 -21. 156 -13.459 -7.329 -4.872 73 -21.485 -14.178 -3.360 -5.968 79 -19. 065 -11.201 -4.936 -2.312 80 -21.757 -14. 503 -3.726 -6.312 81' -21.336 -14.255 -3.213 -5.554 82 -14.101 -12.753 -11.694 -9.088 A 18.111 18.332 21.842 B 15.358 15.338 19.148 C 12.838 12.335 16.615 D 10.890 13.613 13.823 E 9.344 9.087 12.40 7 F 4.975 4.937 7.777 G -23.471 -23.64 5 -21.145 H -27.662 -27.740 -25.620 I -25.079 -25.113 -23.023 J -23. 138 -23.373 -21.403 67 VI . DATA ANALYSIS AND GEOLOGICAL INTERPRETATION Figure 15 is in general overall agreement with the regional trend of CBA values on adjacent land areas as deter- mined by Fairborn (1963) , Sieck (1964) , Bishop and Chapman (1967) , and Ivey (1969) . The shape of the +10 and +15 mgal isolines is in excellent accord with the shape of the granitic contact north and east of Pt. Pinos (Fig. 3). Additionally, there is a discernible gradient increase from the +10 to -5 mgal isolines in the southernmost region of the Bay; this coincides with the Tularcitos Fault Zone. The previously assumed existence of -30 mgal isolines seaward of the Marina area (Chapman and Bishop, 196 7) was due to insufficient station coverage along the coast and no offshore coverage; the -25 mgal isoline in Figure 15 is confirmed by a CBA value of -25.620 at Station H. Monterey Peninsula CBA values were found to be slightly higher than those presented on the 1967 Santa Cruz Sheet, but, again, bottom gravimetry CBA values were verified by the independent land survey (Stations A, B, C, and D) . Of far greater geological significance than the actual CBA value associated with each isoline is the general shape and trend of these lines, an aspect of Figure 15 that all investigators concerned could easily accept. Possible error magnitudes are summarized in Table IV. These estimates are considered to be maximum values; it is felt that the final CBA values of the author's survey are accurate to within 1 mgal. 68 to rH CO ■H g -H d) 5h (d W o w CD -P CO g ■H -P CO W CD H3 P -P -H £ CO s 5h o u u w < CQ u -9 c> •H CD > 5H >H CD o o CO g >1 O CD U -p > O -P U U O P u CQ CO W u o u u w Q 13 Cd c o •H 4-> U P CN rH O o • • o o + 1+ o o rH rH O O + 1+1 >1 u >i fO U H rd p ^ u • P u C u < 0 u •H <: cr> +J c rd 5-1 -H tn CD TS ■H +J CO > CD CD ftj 2 « 2 o C7> a) rH t c o o CM s o rH + 1 + CD rH •H g o ■^ rH O + i: ct> CN rH P 4-1 O fS + 1 + O -P CN »H o ^r + 1+1 C 0 •H P CO > CD rH W \ .c -p o< CD a Em o m • o + 1 o n o + 1 I CO > £ CD W g c 0 ■H >1 +J +J P fO -H C > CD •H P -P CO U U w cd rd -mac £ g 0 P O -H CO U -P g p Eh CN LD O + 1 CO o + 1 to 5-1 o u u < CQ U TS CD -P CO g -H 4-> CO W g P g ■H X CO 69 It is readily apparent from Figure 15 that the north- eastern portion of the survey area is characterized by a deficiency of mass density relative to the southwestern region. This deficiency represents an increase in depth to the top of the dense granitic basement from southwest to north- east, and is in very good agreement with seismic reflection profiles presented by Greene in 1970 (Fig. 16 and 17) . Since a single geophysical method seldom assures a unique solution, this confirmation of previous seismic reflection interpretation is an important result of the author's study. The CBA pro- files in the figures lack the detail (particularly in the Tularcitos Fault Zone) of the corresponding seismic profiles; this is most probably due to the grid size of the station net- work. Additionally, there may exist an insufficient density contrast, although this possibility is remote in view of the apparent displacement and the density difference between the Monterey Formation and granodiorite below Monterey Bay. Figures 16 and 17 indicate an average complete Bouguer gravity anomaly decrease of 5 mgal for each 520 ft increase in depth to the granitic basement structure. Since the 11 n mile extent of profile A-B and corresponding 3700 ft drop in the level of the basement represent a slope at the interface be- tween the Monterey Formation and the granodiorite of only about 3°, we can approximate the difference in attractive forces at two points along that profile whose CBA values differ by 5 mgal by the attraction of a horizontally infinite slab 520 ft thick whose mass density is equal to the density 70 sw A NE B 2000-- 4000 Tularcilos Fault Zone FEET 0 h NAUTICAL MILES + 20 + 10-- -10-- -20 CBA Figure 16 Comparison of CBA and Depth of Granite Substructure as Determined by Seismic Reflection for Profile A-B (Upper profile after Greene, 1970). The location of Profile A-B is shown in Figure 15. 71 NAUTICAL MILES 1 Comparison of CBA and Depth of Granite Sub- structure as Determined by Seismic Reflection for Profile C-D (Upper profile after Greene, 1970 The location of Profile C-D is shown in Figure 15 72 difference between the two rock types. So doing permits equating a theoretical attraction of 2tT G pc h (where h is 520 ft and pc is the density contrast) with the associated 5 mgal inferred value, and yields a density contrast of 0.75 g/cm^. Since the slope of the igneous/sedimentary rock inter- face is fairly constant along the profile, the values of sub- layer terrain irregularity corrections at two such locations will be close and their difference can therefore be neglected. This analysis assumes, then, that the relation between CBA and depth to basement is due solely to the magnitude of the density deficiency existing in the upper formation; the attraction at the point farther toward the northeast will be less since a thicker layer of the (lighter) Monterey Formation exists below. A fairly accepted value for the density of the granodiorite is 2.75 g/cirr , but the density of the in situ Monterey Forma- tion is not accurately known. While this formation consists of multiple members , the Miocene marine shale predominates ; its water-saturated density was determined experimentally by the author to be 1.85 g/cm3 (true in situ density is higher since the lab samples were not under pressure) . Sieck (1964) found the dry density of Monterey shale to vary from 1.41 to 2.10 g/cm , with an average of 1.80 g/cm , and his measured density of Monterey sandstone averaged 2.10 g/cm^. Exami- nation of various references to shale densities in general indicates that it is a highly variable figure (depending on age, location, and many other factors); values for diatomaceous 73 shales in the San Jauquin Valley are between 0.9 and 1.1 g/cm3 , while some water saturated shale types can be as dense as 3.21 g/cm3 ( Jakosky , 1950). In view of these many factors, the 2.00 g/cm3 implied density of the Monterey Formation (2.75 g/cm3 granodiorite minus the gravity survey's calculated 0.75 g/cmJ density contrast) would appear quite acceptable as a true value . Having arrived at densities of 2.00 and 2.75 g/cm3 for the two major geological units in the area, it is now appropri- ate to consider the validity of using 2.67 g/cm3 for the assumed average crustal Bouguer density. Since the Monterey Formation is less than 1 km thick, while the granodiorite sub- structure is probably several times thicker, the average crustal density in the vicinity of southern Monterey Bay is much closer to 2.75 than to 2.00 g/cm . Thus, while the frequently used average crustal density of 2.67 g/cm3 can be unrealistic in many regions of the world, it is probably not the case here. Throughout the course of this research it was anticipated that isoline positions over the deep Monterey Submarine Canyon could be inferred from the results of the bottom surveys in ' the northern and southern halves of Monterey Bay. Both sets of data were obtained with the same equipment in an identical manner and reduced in a comparable fashion, so there is no reason to doubt the validity of making such an inference. Figure 18 is the result of combining the data from Cronyn ' s (19 73) survey with that of the author. 74 121°50'W •36°50'N ■36°45'N ■36°40'N -• ► NAUTICAL MILES Figure 18. Composite Gravity Map of Monterey Bay (Northern Monterey Bay contours after Cronyn, 1973). 75 The fact that the southernmost positions of Cronyn's known 0,-5, -10, and -15 mgal isolines must bend to meet the northern- most positions of those of the author supports the theory that a fault exists along the axis of the Monterey Submarine Canyon. The difference in lithologies on opposite sides of the Canyon led Martin and Emery (1967) to recognize the so-called Monterey Fault parallel with the Canyon and offset at numerous locations by transverse faults of different magnitudes. Although the fault's suggested orientation is at an angle to the trend of almost all of the many fault zones in central California, the proposed situation is seen elsewhere, as in the case of the Santa Ynez Fault region north of Los Angeles, where the strike of the San Andreas Fault changes. Further evidence of the existence of the Monterey Fault based on seismic reflection tracks across the Canyon has been presented by Greene (1970) . Although some of the advocates of the Monterey Fault suggest that the motion along the fault is dip-slip with the northern side having descended relative to the southern side, this is not uniquely indicated by the gravity data. On both sides of the Canyon (in the eastern portion of the Bay) the CBA isolines run north-south with the higher values seaward (Fig. 18 and 19-A). If the motion of the Monterey Fault is, in fact, primarily dip-slip, the shape of the CBA field over the Canyon would suggest a relative motion such that the southern side has descended rather than the northern side, as shown in Figure 19-B. This is supported by the fact that the motion of the northeastern side of the Tularcitos Fault is 76 A. Inferred CBA Trend (Plan View) N LOW Granite Blocks with Overlying Shale Removed B. Dip-Slip Fault Motion C. Right-Lateral Strike-Slip Fault Motion Figure 19. Two Possible Structural Explanations for the Inferred CBA Field in the Vicinity of the Monterey Submarine Canyon. 77 downward (Greene, 19 70) . If, on the other hand, the fault motion is primarily right-lateral strike-slip, the inferred CBA isolines over the Canyon may be explained as in Figure 19-C, wherein the northern side has moved east relative to the southern side. Specifying which of the two types of relative motion has occurred is not possible from the gravity infor- mation alone; quite possibly a combination of both types of motion has occurred, causing an oblique fault. It may well be that the intersection of the Monterey Fault and the Palo Colorado - San Gregorio system farther to the west corresponds to the area where a concentration of recent seismic epicenters is located. More geophysical and ocean- ographic study of this region is needed before an accurate structural model can be developed. 78 VII . FUTURE WORK i 0 It is recommended that CA-259 (the Monterey County Airport station) of the state gravity base station network be re- occupied in accordance with procedures outlined by Chapman (1966) to re-establish a correct value. Local construction there will be completed soon. It is suggested that additional gravity measurements be made in the vicinity of the Monterey Submarine Canyon. Where a large portion of the coverage needed might be precluded (with a bottom gravimeter) by excessive depths and/or slopes, surface ship gravimetry could be called upon to complete this task. One such survey consisting of several canyon crossings was conducted by USGS in November, 1972, aboard the USNS BARTLETT , but at this writing the results are not yet available for inclusion here. They will eventually help greatly in re- moving the question marks in Figure 18. At this writing the gravimeter and auxiliary equipment used for the Monterey Bay surveys is back aboard the R/V ACANIA. The author has thoroughly familiarized two new NPS students with the entire operation, and surveys are underway to extend shallow water coverage in two areas, one from Pt. Lobos to Pt. Sur and the other from Santa Cruz northwest to beyond Davenport. The goal of the project sponsors is to tie in these surveys with those conducted by the author and co- workers, so that a continuous picture of the off-shore Bouguer anomaly will become known. Hopefully this goal will be achieved 79 APPENDIX A: SUPPLt EMEf vJTA BCTTOM SURVEY STATION LATITUDE LONGITUDE 1 36 36 31 121 53 26 2 36 36 42 121 52 41 3 36 37 23 121 52 01 4 36 38 16 121 51 07 5 36 38 07 121 52 04 6 36 37 21 121 52 48 7 36 37 18 121 53 28 8 36 37 45 121 54 21 9 36 38 07 121 54 57 10 36 38 43 121 55 28 11 36 39 12 121 54 23 12 36 38 09 121 53 30 13 36 37 59 121 52 49 14 36 37 02 121 53 15 15 36 33 23 121 51 57 16 36 38 57 121 52 42 17 36 3 9 02 121 5 3 26 1 8 36 39 52 121 53 27 19 36 40 15 121 52 2 7 20 36 41 06 121 52 29 21 36 42 01 121 52 42 22 36 39 19 121 51 25 23 36 39 18 121 5D 12 24 36 40 06 121 50 40 25 36 40 36 121 49 30 26 36 41 11 12 1 50 19 27 36 41 38 121 49 17 23 36 41 56 121 49 58 29 36 42 50 121 50 23 30 36 42 02 121 5 0 58 31 36 41 30 121 51 33 32 36 41 04 121 50 58 33 36 40 25 121 51 33 34 36 39 32 121 51 52 35 36 43 24 121 49 17 36 36 44 09 121 48 43 37 36 47 121 49 17 38 36 45 29 121 43 46 39 36 41 22 121 53 13 40 36 41 04 121 53 43 41 36 40 34 121 53 24 42 36 4D D2 121 54 34 43 36 39 35 121 55 30 44 36 39 03 121 56 10 45 36 38 49 121 56 54 STATION INFORMATION G-OBSERVED 979895.028 979892.293 979385.315 979876.674 979835.502 979894.019 979899.353 979893.423 979899.636 979903.752 979904.074 979901.069 979896.036 979897.034 979884.839 979893.832 9 79398.615 979896.275 979889.308 9 79890.660 979892.480 979881.520 979872.538 979377.537 979870.061 979377.662 979870.289 979376.333 979873.629 9 79883.215 979386.657 979881.988 9 798 3 4.7 54 979336.054 979872.276 979871.901 979875.478 979376.404 979895. 163 979397.076 979894.934 979903.294 979907.755 979910.033 979903.932 ■THEORET. 9 79 3 83. L2 8 979883.391 979884.375 979885.646 979335.430 979884.327 979334.255 979384.902 979885.646 979886.293 979386.989 979885.473 979885.233 979833.871 979335.814 979836.629 979886.749 97 9 63 7.94 3 979388.500 979869.724 979 891 . 044 979337. 157 979337.133 979833.26'* 979889.004 970889.3*4 9 79 3 90.49 2 979cc0.924 979892.220 979391 .068 979390.3 00 979389.676 979383.740 979387.469 979393.036 979894.117 979895. C29 979396.038 979390.108 979839.676 979888.956 979688. 183 979887.541 579386.773 979386.432 80 STATION LATITUDE LONGITUDE 46 36 39 34 121 56 46 4 7 36 39 51 121 56 40 48 36 40 34 121 55 25 49 36 38 21 121 54 16 50 36 41 28 121 54 36 51 36 42 05 121 53 42 52 36 4? 50 121 53 CO 53 36 42 36 121 51 54 54 36 43 15 121 51 55 55 36 43 36 121 50 51 56 36 42 44 121 49 14 57 36 44 04 121 49 50 58 36 44 3D 121 53 47 59 36 45 11 121 50 29 60 36 46 46 121 49 21 61 36 45 44 121 49 32 62 36 46 19 121 48 47 63 36 46 57 121 48 10 64 36 47 50 121 47 47 65 36 48 10 121 48 27 66 36 4 7 23 121 49 02 67 36 46 12 121 50 34 68 36 45 16 121 51 36 69 36 44 32 121 51 52 70 36 43 50 121 51 32 71 36 43 38 121 52 30 72 36 44 26 121 52 56 73 36 45 55 12 1 51 48 74 36 46 51 121 50 36 75 36 46 46 121 51 42 76 36 45 28' 121 52 40 77 36 44 58 121 53 45 78 36 44 00 121 53 39 79 36 43 27 121 54 25 80 36 43 18 121 53 30 81 36 42 38 121 5 4 08 82 36 37 34 121 50 58 G-OBSERVED 979910.948 9 79910.41 7 979906. 144 979903.845 979901.714 979898.698 979894.520 979839.433 979889.267 979882.560 979871.423 979875.478 979883.371 979882. 851 979885.960 979879.721 979873.671 979878.671 979379.908 979904.709 979886.2^3 979886.906 97989 1. 118 979891.575 979887. 156 979892.605 979897.024 979894.333 979890. 567 979597.211 979893.511 979901.672 979898. 553 979902.182 979697.030 979397.232 979875.343 G-THEORET 979887.517 979837.925 979368.956 979835.766 579390.252 979891 .140 979892.220 979891.384 979392.820 979393.324 979892.076 979693.997 979894.621 979895.605 979397.837 979896.398 979397.238 979898.151 979899.424 979899.905 979398.776 979897.070 979395. 725 979894.669 979893.660 979893.372 979394.525 979896.662 979393. 0C7 979897.337 979896.014 979895.293 979893.900 979893. 108 979892.892 979891.932 97988^.638 81 STATION c T DEPTH FAC BC TC cr. 1 2 3 4 5 0. 02 0.00 ■0. 31 •0. 01 ■0.02 40.0 75.1 93.4 84.2 151.9 -3.841 -7.118 -8.832 -7.944 14.309 1.918 3.566 4.428 3.985 7.181 2 .9-8 2.82 2.69 2.58 2.65 •0.018 0. C34 0.041 ■0.038 0.067 6 7 8 9 13 ■0.02 ■0.02 ■0.02 ■0.02 ■0.02 138.1 134.5 79.7 76.0 13 0.4 ■12.986 ■12.636 -7.430 -7.054 12.166 6.520 6.336 3. 740 3.555 6.122 2.77 2.93 3.00 3. 12 3.23 0. 060 0.063 0.035 0.03^ 0.359 11 12 13 14 15 ■0.02 0.02 •0.02 3.32 0.01 256 212 189 1 38 165 24.025 19.835 17.710 13.35 3 15.347 12.079 10.003 8.913 5.369 7.732 2.93 2.86 2.76 2.93 2.62 3. 116 ■0.096 0. 085 0.048 •0.073 16 17 18 19 20 0.00 0.00 0.31 0.01 0.01 214.1 244.9 254.6 235.2 248 .2 19.937 22.841 ■23.744 21.921 •23.162 10.037 11.495 11.953 11.034 11.655 2 .69 2.79 2.78 2.58 2.53 -0.096 0.110 ■3.115 •0.106 0. 112 21 22 23 24 25 0.01 ■0.02 •0. 01 0.00 0. 31 252 156 76 142 71 •23.577 ■14.735 -7.178 13.347 -6.681 11.862 7.386 3.608 6.706 3.361 2.48 2.55 2.44 2.38 2.33 ■0. 114 0.068 •0.034 ■0.063 ■3.331 26 27 28 29 30 0.02 0.03 0.03 3.34 0.04 145.1 78. 1 131.4 14 3.4 191.0 ■13.596 -7.288 12.297 ■13.147 17.905 6.834 3.667 6.183 6.639 8.993 2.29 2.13 2.14 2.15 2.30 0.364 0.034 0. 059 0.062 •0.035 31 32 33 34 35 0. CD 0.06 3. 36 3.07 0.06 221.3 136.3 2 34 . 3 193.9 67.0 -20.763 -17.490 -19.228 -18.209 -6.329 10.434 8.7S6 9.659 9.146 3.174 2.35 2.34 2.46 2.57 1.95 0.100 C.C84 0.091 0.086 0.030 36 37 38 39 43 0.06 0.05 0.05 0.02 3. 32 39.1 54.4- 42.7 264.1 275.1 -3.707 -5.158 -4.053 24.870 25.897 1.857 2.535 2.023 12.483 12.999 1.90 1.91 1.85 2.57 2.75 0.017 ■3.024 •0.018 0.119 3.124 41 42 43 44 45 0.02 0. 01 0.01 3.3 3 0.00 257.8 276.4 271.9 231.8 204.7 •24.253 26.001 2 5.564 •21.781 19.232 12.178 13.054 12.836 10.933 9.t>58 2.71 2.94 3.27 3.52 3.73 3. 116 ■0.125 ■0. 124 3. 135 ■0.091 82 STATION E.T . DEPTH FAC BC TC CC 46 47 48 49 5 0 0.00 0.00 0. 00 -0.01 0.11 270.1 3 02.2 293.6 211.1 294.7 -25.369 -28.330 -27.550 -19.767 -27.661 12.743 14.255 13.841 9.936 13.896 3.62 3.59 3.28 2.96 2.96 -0.122 -0. 137 -0.133 -0.095 -0.133 51 52 53 54 55 0.10 0. 10 0.10 0. 39 0.09 278.7 258.8 228.5 217.1 155.2 -26.162 -24.295 -21.442 -2 0.3 7 0 -14.547 13.143 12.205 10.772 10.2 34 7.310 2.69 2.43 2.37 2.25 2.15 -0. 126 -0.113 -0. 102 -0. 098 -0.070 56 57 58 59 60 0.08 0.07 0. )6 0.05 0.04 75.7 76.8 147.9 125.3 136.5 -7.072 -7.172 -13.847 -11.703 -12.757 3.557 3.609 6.961 5.889 6.416 2.04 2.02 2.10 2.03 1.94 -0.034 -0.035 -0. 066 -0.056 -0.061 61 62 6 3 64 65 0.04 0. 03 0.02 0.01 -0. 01 72.3 49.8 33.0 37.0 456.8 -6.707 -4.5 83 -2.937 -3.352 -42.794 3.380 2.315 1.515 1.701 21.509 1. 89 1.83 1.77 1.78 2.98 -0.033 -0.022 -0.015 -0.017 -0.212 66 67 68 69 70 -0.02 -0.03 -0.04 -0. 04 -0.05 135.4 153.0 208.4 219.5 194.7 -12.55 2 -14. 194 -19.399 -20.425 -18.034 6.323 7.154 9.76 8 10.285 9.111 2.19 2.09 2.20 2.20 2.25 -0. 06 0 -0.069 -0.093 -0. 099 -o.oss 71 72 73 74 75 -0. 05 -0.0 5 -J. )6 -0. 06 -0.06 241.8 261.3 241.2 185.3 263.9 -22.507 -24.337 -22.447 -17.183 -24.576 11.333 12.253 11.304 8.664 12.373 2 .35 2.28 ? .29 2.27 2.50 -0.109 -C. 119 -0. 109 -0.083 -0. 119 76 77 78 79 8 0 -0.06 -0.06 -0. 06 -0.06 -0. D6 2 69.9 . 294.6 279. 7 300.9 277.7 -25.150 -27.475 -26.077 -28.078 -25.891 12.660 13.827 13.125 14.129 13.031 2.4 0 2.59 2.52 2.76 2.54 -C. 122 -0.133 -0.128 -0. 136 -0.126 81 82 -0. 06 -0. 06 290.1 52.4 -27.076 -4.746 13.623 2.408 2.79 2.63 -0.131 -0.024 83 APPENDIX A (Continued) SHORELINE SURVEY Theoretical Station Latitude o ' Longitude o ■ " Observed Gravity Gravi ty A 36 36 34 121 57 24 979900.880 979883.200 B 36 37 09 121 56 26 979899.280 979884.039 C 36 38 15 121 56 05 979898.380 979885.622 D 36 36 57 121 53 55 979893.790 979883.751 E 36 36 32 121 53 24 979 891.700 979883.152 F 36 36 16 121 52 12 979887.600 979882.768 G 36 39 45 121 49 18 979 863.740 979887.781 H 36 41 57 121 48 32 979863.050 979 890.948 I 36 44 53 121 47 57 979869 .880 979895.173 J 36 48 28 121 47 21 979876.420 979900.337 Station Elevation Earth Tide A 3.2' + 0.13 B 0 .6' +0.0 6 C 0.1' + 0.07 D 8.2' +0.0 8 E 7.5' +0.09 F 1.1' + 0.04 G 5.1' +0.09 H 2.3' +0.02 I 1.0' + 0.12 J 6.9' + 0.13 FAC BC TC 0.301 0.109 3.84 0.056 0.020 3.81 0.009 0.003 3.78 0.771 0.280 3.21 0.705 0.257 3.32 0.103 0.038 2.84 0.480 0.174 2.50 0.216 0.078 2.12 0.094 0.034 2.09 0.649 0.235 1.97 84 COMPUTER PROGRAMS CCMPUTFR PROGRAM FOR SHORELINE STATIONS 500 1000 2000 30 00 35C0 ( 5 (5 (5 ( 5 4000 5000 6000 ] TMPLICI DIMENSI { 10) ,CB 10KETC READ (5 READ READ READ READ FORMAT ( FORMAT ( FORMAT { FORMAT ( FORMAT! DO 4000 TLA( I ) = ARGM I ) ARG3U ) THEOt I ) -C.0000 FAC( I) = FAA( IJ = BC(T )= ( )/10000 SBA( I) = C8A( I ) = DO 5000 WRITE ( AA(I ), 3 CCNTINU FORMAT ( ,F6.3, 3 STOP END 3 (A-H,0-Z) (10) ,ELEV( 10),FAC( 10), FAA( 10),BC( 10), SBA LA(10) ,ARGA(10) , ARG3 ( 10 )» DEGREE! 10 ),GGB( 10) GCB( I ) .1 = 1. 10) (DEGREEU ) .1=1,10) (ELEV(I),I=1,10) (ET( I ) ,1=1,10) ( TC( I ) ,1=1,10) 7) 1) 2) t real: ON THEO A ( 1 0 ) , T 10) ,TC( .503) ( , 1000) .2000) .3000) , 3500) 8 F10.3) 8F10 10c3 10F4 10F4.2) 1=1. ID 3.14159 = CSIN(T = DS I N ( 2 =97 3 049 0 59D0'" ( 0.09406 GOBI I )- 2. 0 00- 3 0.000 FAA( I )- SBA( I ) + 1-1.13 6.6000) I .GOB (I ) .THEO (I ) , ET ( I ) , EL EV ( I ) , FAC ( I ) , F CI I ),SBA( I ),TC ( I ),C3A( I ) ///.2 0X.I 2.3 X.F 10.3, 3X,F1 3.3, 3X, F4. 2,3 X.F3.1 ,3X X,F7.3,3X.F6.3.3X,F7.3,3X,F4.2,3X,P7.3) DO-" ( DEGREE ( I )/ 180.0 DO) LA(I ) ) .000-TLA( I ) ) .000:' (1 ,ODO + (0.0052384DO-' ( ( ARGA(I ) )**2) ) (ARGB( I ) ) -2) ) D0 = ' (ELEVd )) THEO(I) +ET( I)+FAC(I ) .1415900- 6. 67D0 30.48D0^ 2.67D0* (ELEV( I ) ) BCbert Andrew Brooks, Sr R T DATE March 19 73 TRACT OR GRANT NO. JEC T NO. 7«. TOTAL NO. OF PACES 92 7b. NO. OF REFS 24 9a. ORIGINATOR'* REPORT NUMBER!*) 66. OTHER REPORT NO(S) (Any other number* tji&t may be a* signed this report) rRIBUTION STATEMENT Approved for public release; distribution unlimited, PLEMENTARY NOTE* 12. SPONSORING MILITARY ACTIVITY Naval Postgraduate School Monterey, California 9 3940 Eighty-two ocean bottom gravity stations in southern Monterey y were occupied in the summer and fall of 19 72 from the R/V ANIA. A land gravity survey of ten stations about the perimeter the Bay was conducted in the spring of 19 72. Gravimeters ployed were LaCoste and Romberg Models H6G and G-17B, respectively Conventional steps in data reduction are discussed, and a rrain correction theory unique to ocean bottom gravimetry is esented. The complete Bouguer anomaly (CBA) field for bottom and oreline surveys is included. The geological interpretation of the gravity data is discussed iefly. Sub-bottom structure of southern Monterey Bay as termined by seismic reflection is verified by the CBA field, d a calculated density contrast between the basement granodiorite d overlying sedimentary strata is found to be realistic. The ta supports the existence of a fault oriented beneath the nterey Submarine Canyon. FORM I NOV 65 01 -807-661 1 1473 (PAGE i) UNCLASSIFIED -? I Security Classification 1-31408 CLASSIFIED Security Classification KEY WO ROI 1 DLOGY •PHYSICS IVITY RINE GEOLOGY JTHERN MONTEREY BAY FORM t Nov e t 101 -807-6S2 1 1473 (BACK) UNCLASSIFIED 92 Security Classification A- 3 1*09 Thesis B80943 c.l ikd270 Brooks A bottom gravity survey of the shallow water regions of south- ern Monterey Bay and its geological inter- pretation. DUDLEY KNOX LIBRARY III I II oU * J