A BOTTOM GRAVITY SURVEY OF CARMEL BAY, CALIFORNIA Ant6nfo Pedro Dias Souto Library Naval Postgraduate School Monterey, California 93940 erey, baniornia A BOTTOM GRAVITY SURVEY OF CARMEL BAY, CALIFORNIA by Antonio Pedro Dias Souto Thesis Ad visors: Robert S. J. J. von Andrews Schwind March 1973 T kpp>w\) o £ CD (X o CD +-> CD £ -r-J > j-. a u CD CD TS C CNJ CD ■M f0 I — I 18 Slip Ring Assembly. Control Box Power Unit Winch 'A' fr« To ship's electrical supply Gasoline Engine Hydraulic P ump ^-Underwater Gravimeter Figure 4. Schematic Diagram of the Complete Bottom Gravimeter Installation 19 (0 -I— I c (d o o rd c CD B a -r-l 3 o* H CO CD +J CO < — i PL, 20 C . THE SURVEYS 1. Shoreline Survey The shoreline survey was conducted on 27 April 1972 with the LaCoste & Romberg G17B meter. A total of six stations were occupied: Cypress Point, Stillwater Cove, Jeffreys Bench Mark at Abalone Point, Monastery Beach at San Jose Creek, Whalers Cove and Point Lobos . Table I is a listing of the data obtained. 2. Bay Survey The bay survey was completed in three days, on 18 and 19 August and 3 October 1972. The navigation fixes to determine the location of the stations were made by visual bearings and radar distances. Accuracy is estimated at 50 ft. The rocky bottom and the kelp beds in part of the bay posed serious problems during the survey. Plate 4 shows the underwater unit after a station made over a kelp bed. Many planned stations could not be occupied and in some cases 20 lowerings and raisings of the underwater unit had to be made before finding a bottom slope with less than the allowed 15 inclination. These problems, along with electrical failures and the time available, made it possible to measure the gravity at only 55 stations. Figure 5 shows the geographical location of the stations, the latitude and longitude for which are tabulated in Table II. Table III is a summary of the observed data. The depth indicated is the one obtained by the pressure sensor in the underwater unit. 21 TABLE I SHORELINE STATIONS DATA Station Date Hour (PST) Elevation (ft) Observed gravity (mgal) Cypress Point 27 Apr 72 1239 14 979 906.01 Stillwater Cove 1206 14 979 902.84 Jeffreys B. M. 1338 : 30 979 902.76 Mission Beach 1400 5 979 902.31 Whaler's Cove 1419 3 979 904.12 Point Lobos 1435 5 979 904.41 Station Latitude N Longitude W Cypress Point 36° 34' 26 121° 58'.35 Stillwater Cove 36 33 98 121 56.52 Jeffreys B. M. 36 32 61 121 55.93 Mission Beach 36 31 52 121 55.42 Whaler's Cove 36 31 19 121 56.36 Point Lobos 36 31 .13 121 57.15 Station 1930 Theoretical gravity formula 1967 formula !mgal) (mgal) CBA 1930 1967 (mgal) (mgal) Cypress Point 979 879.91 979 866 42 26.10 39.60 Stillwater Cove 979 879.48 979 865 99 23.36 38.86 Jeffreys B. M. 979 877.52 979 864 03 25.24 38.74 Mission Beach 979 875.96 979 862 46 26.35 39.85 Whaler's Cove 979 875.48 979 861 98 28.64 42.14 Point Lobos 979 875.39 979 861 89 29.02 42.52 22 Platr- 4. [Jn i rwal i i nil aft' i a Station over a K"lp B' 23 N i 4 ^N- V • i 3 • 5 — • 6 > 11 "' \ • I • 7 • 2 X_J 1# 10* e fj\ 15 \ 35 „, 13: M- \ * 8. 9 41. \ • „ 42 \ 36. 37 39.4°- «. 16. \ J'» 38. 33. 34. 44- 48. 17- \ 45. \ 46- 4'-« . 50. * 18' * 21* 20. / 22. p^ 52. \ 23* V^>. 24. 25. \ 26. ^ 53. 54' 32. 30. 27 ]^ V^ 29. {X_r^^^ . n miles 1 I 0 1 1 1 i ^o l Figure 5. Station Location for Underwater Stations 24 TABLE II STATION LOCATION Station Latitude N 36° 33'. 58 36 33.66 36 34.02 36 34.11 36 33.90 36 33.78 36 33.62 36 33.42 36 33.44 36 33.75 36 33.82 36 33.71 36 33.52 36 33.53 36 33.54 36 33.33 36 33.29 36 33.05 36 33.05 Lor lgitu .de W 121° 57! ,29 121 57, ,45 121 58, ,01 121 58. ,28 121 58. ,19 121 57, ,93 121 57. ,70 121 57, .41 121 56, ,93 121 56, ,84 121 56, .84 121 56, .50 121 56, .53 121 56, ,13 121 56, ,53 121 56. ,26 121 55. ,92 121 55. ,85 121 56. ,12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 25 TABLE II (continued) 20 36° 3 2 '. 8 1 21 36 32.84 22 36 32.63 23 36 32.37 24 36 32.15 25 36 32.18 26 36 31.92 27 36 31.48 28 36 31.62 29 36 31.32 30 36 31.46 31 36 31.59 32 36 31.68 33 36 31.67 34 36 31.48 35 36 33.47 36 36 33.34 37 36 33.29 38 36 33.21 39 36 33.31 40 36 33.38 41 36 33.47 42 36 33.43 121° 55'.93 121 56.34 121 56.16 121 56.16 121 56.00 121 55.78 121 55.84 121 55.66 121 56.15 121 56.27 121 56.64 121 56.33 121 56.72 121 56.98 121 56.98 121 58.32 121 57.96 121 57.65 121 57.13 121 56.85 121 56.73 121 56.63 121 56.42 26 TABLE II (continued) 43 36° 33'.35 121° 56'.53 44 36 33.26 121 56.65 45 36 33.18 121 56.80 46 36 33.10 121 56.67 47 36 33.14 121 56.54 48 36 33.25 121 56.42 49 36 33.13 121 56.26 50 36 33.00 121 56.49 51 36 32.82 121 56.77 52 36 32.51 121 56.45 53 36 31.77 121 55.83 54 36 31.85 121 55.65 55 36 31.54 121 55.95 27 TABLE III OBSERVED DATA Station Date Hour Depth Observed gravity- Oc cupii 3d (PST) (ft) Cm gal) 1 18 Aug 72 1045 99 979 905.38 2 ii 1100 76 979 904.44 3 H 1135 65 979 904.56 4 it 1150 105 979 906.81 5 it 1215 83 979 905.38 6 it 1227 84 979 905.38 7 it 1240 106 979 906.13 8 it 1252 105 979 905.50 9 H 1300 112 979 905.31 10 ■I 1315 41 979 901.62 11 ■I 1325 34 979 901.38 12 ii 1359 49 979 901.56 13 ii 1410 62 979 902.38 14 ii 1430 32 979 900.31 15 19 Aug 72 1238 58 979 902.06 16 ii 1250 45 979 901.12 17 . H 1303 26 979 899.38 ' 18 ii 1312 26 979 899.25 19 ii 1320 44 979 900.75 28 TABLE III (continued) 20 19 Aug 72 1335 20 ■ 979 899.44 21 ii 1352 70 979 903.38 22 ii 1403 26 979 901.19 23 ir 1425 92 979 904.44 24 H 1436 109 979 904.88 25 •' 1442 44 979 901.38 26 ii 1510 252 979 911.50 27 ii 1550 35 979 900.44 28 ii 1646 145 979 906.31 29 n 1715 69 979 903.56 30 ii 1728 111 979 905.63 31 ■1 1740 145 979 907.00 32 H 1752 245 979 911.31 33 ■I 1833 202 979 909.87 34 ■I 1854 144 979 907.81 35 3 Oct 72 0810 155 979 907.81 36 ii 0845 144 979 906.88 37 M 0900 161 979 907.50 38 ■I 0913 170 979 906.56 39 H 0925 118 979 904.25 40 ■I 0935 87 979 903.00 41 ■I 0944 73 979 902.06 29 TABLE III (continued) 42 3 Oct 72 0952 60 979 901.44 43 1001 71 979 901.94 44 1008 91 979 902.88 < 45 1019 133 979 904.19 46 1028 120 979 904.50 47 1039 89 979 902.88 48 1051 70 979 901.69 49 1102 50 979 900.75 50 1116 95 979 903.56 51 1127 145 979 906.06 52 1144 143 979 905.94 53 1202 120 979 903.44 54 1213 59 979 900.81 55 1228 99 979 903.00 30 III. REDUCTION OF DATA The results of this survey are presented, as is usual, in the form of gravity anomalies. To obtain these anomalies corrections had to be applied to the measured gravity values and the corrected values compared with the computed theoretical value at each station. A. THEORETICAL GRAVITY For each station a value of theoretical gravity (THG) was computed for the reference spheroid. This is the value of gravity which would be expected if the earth were a perfectly uniform spheroid, fitted as closely as possible to mean sea level. The formula used is the internationally adopted standard (International Union of Geodesy and Geophysics, 1967): 2 2 THG = (978.03090+5.18552 sin L - 0.00570 sin 2L) gal where L is the latitude of the station. As most of the surveys are still based on the international gravity formula adopted in 1930 (International Association of Geodesy, Stockholm, 1930) , this formula was also used so that the results could be directly compared with earlier work. The 1930 formula is: THG = [978.0490(1 + 0.0052884 sin L - 0.0000059 sin22L] gal. B. EARTH TIDES The earth is not an infinitely rigid body and responds to the 31 gravitational attractions of the sun and the moon. The deformations are accompanied by a gravity change of measurable magnitude. The measured gravity values were corrected for this change using tables furnished by the USGS. C. INSTRUMENT DRIFT The instrument drift was periodically checked by reoccupation of a base station in Monterey Bay and the measured gravity value corrected for the observed drift. The measured gravity value at each station, after correction for earth tides and instrument drift, is considered to be the observed gravity (OG) . D. FREE AIR CORRECTION The free air correction accounts for the fact that the gravity measure- ment is not made at mean sea level. Near the surface of the earth the gravity gradient is negative upwards and has a value of 0.09406 mgal/ft (Heiskanen, 1967). For underwater stations the correction (FAC) is: FAC = (0.09406 D ) mg'al where D = (D -TD) ft a o in which D is the measured depth and TD is the tide height, both in feet. For land stations the correction is: FAC = (0.09406 x H) mgal where H is the elevation in feet. 32 This correction is always negative for underwater stations and always positive for land stations. E. BOUGUER CORRECTION The Bouguer correction (BC) assumes that the distance between the station elevation and the reference elevation is filled with an infinite horizontal plate of rock material. For underwater stations it is applied in two parts. First, the effect of the attraction of a plate of water above the meter (BC1) is removed using the formula: BC1 = 2irp GD w o where o is the density of the water, G the universal gravitational w constant and D the measured depth. Next, the volume is filled with a plate of rock using the correction (BC2): BC2 = 2770 GD r a where p is the density of the rock and D the depth below mean sea r a level as defined in the previous section, 3 3 Taking p = 1.027 gm/cm and p =2.67 gm/cm , the Bouguer correction is: BC = (0.0131 D + 0.0341 D ) mgal o a for D and D in feet. This correction is positive. o a 33 For land stations we obtain (Heiskanen, 1967): BC = (0.034 x H) mgal where H is the elevation in feet. This correction is negative. F. TERRAIN CORRECTION The assumption of an infinite horizontal plate as in the Bouguer correction is not realistic. Valleys and hills around the station decrease the observed gravity value and must be compensated for. Theoretically the mass of each deviation from the Bouguer plate has to be calculated and its effect on the gravimeter computed. In practice the Hayford-Bowie templates and tables are used. For underwater stations the tables have to be modified to compensate for the presence of water in the "valleys" (air is assumed on the tables) and for the excess mass introduced by the Bouguer correction in the already rock-filled zones between the station depth and mean sea level. Due to the irregularity of the bottom topography and the lack of de- tail of the hydrographic charts, the terrain correction (TC) is assumed to be accurate to only 0.5 mgal. This correction is always positive. G. CURVATURE CORRECTION The curvature correction (CC) is used to compensate for the assumption of a flat plate made in the Bouguer correction. This assumption is valid for short distances but must be corrected for greater distances. The formula used is: 34 CC = (0.0004462 D -3.282x10 D +1 . 27 x 10 15D 3) mgal a a a This correction is negative. H. COMPLETE PROCEDURE To summarize, the complete procedure of data reduction was carried out as follows: 1) Theoretical gravity was computed by the formulas: THG= (978.03090+5.18552 sin L-0.00570sin 2L) gal (1967) and THG - [978. 0490(1+0. 0052884sin2L-0.0000059sin22L)] gal (1930) 2) Earth tide values were obtained from the tables and applied to the measured values. 3) Instrument drift was calculated and applied linearly as a function of time to the measured values. 4) A free air correction was applied using the formulas: FAC = (0.09406 D ) mgal a for the underwater stations and: FAC = (0.09406 x H) mgal for the land stations, to reduce the observed value to the reference spheroid. 35 5) In the case of the sea floor stations, the Bouguer correction was applied -using the equation: BC = (0.0131 D + 0.0341 D ) mgal o a to remove the effect of the water above the gravimeter and introduce an 3 infinite horizontal plate of rock of density 2.67 g/cm . Figure 6 shows the plate of water removed, the plate of rock introduced and the result- ing densities after the application of the Bouguer correction. For the land stations the Bouguer correction was applied using the formula : BC = (0.034 x H) mgal. 6) The terrain correction was made by estimating the mean elevation of each compartment of the Hayford-Bowie templates and reading the correction from the tables. For the underwater stations the values of the tables were corrected as follows for the regions labeled in Figure 7: Region 1 - value read from Hayford-Bowie table, added; Region 2 - value read from Hayford-Bowie table multipled by [(2.67 - 1.03)/2.67], added, compensates for the presence of water instead of air; Region 3 - value read from Hayford-Bowie table multiplied by [1 - (4.31 - 2.67)/2.67], subtracted, compensates for the excess mass introduced by the Bouguer correction. 36 V ■o o 0 > e E « CD CD Q CD > o CD *+-! w tn 3 CO CD C£ -a c ro t3 CD "a a < c o o CD s- V-, o O CD Di a o CD 3 l/J 37 e «> 0£ O •r-t +-> (0 B Q) XI U CO c o U 2 38 7) Curvature correction applied using the formula: CC = (0.0004462 D - 3.282 x 10 8D 2+1.27 x 10 15D 3) mgal a a a to compensate for the curvature of the earth. Table IV lists the theoretical gravity values computed for each station using the 1930 and 1967 formulas and Table V lists the corrections applied to each sea-floor station. The theoretical gravity values for the land stations are listed in Table I. 39 TABLE IV THEORETICAL GRAVITY Station 1930 formula 1967 formula (mgal) (mgal) 1 979 878.91 979 865.42 2 979 879.02 979 865.54 3 979 879.54 979 866.05 4 979 879.67 979 866.18 5 979 879.37 979 865.88 6 979 879.20 979 865.71 7 979 878.97 979 865.48 8 979 878.68 979 865.19 9 979 878.71 979 865.22 10 979 879.15 979 865.66 11 979 879.25 979 865.76 12 979 879.10 979 865.61 13 979 878.82 979 865.33 14 979 878.84 979 865.35 15 979 878.85 979 865.36 16 979 878.55 979 865.06 17 979 878.49 979 865.00 18 979 878.15 979 864.66 19 979 878.15 979 864.66 40 TABLE IV (continued) 20 979 877.80 979 864.31 21 979 877.84 979 864.35 22 979 877.54 979 864.05 23 979 877.17 979 863.68 24 979 876.85 979 863.36 25 979 876.90 979 863.41 26 979 876.52 979 863.03 27 979 875.89 979 862.39 28 979 876.09 979 862.59 29 979 875.66 979 862.16 30 979 875.86 979 862.36 31 979 876.05 979 862.55 32 979 876.18 979 862.68 33 979 876.17 979 862.67 34 979 875.89 979 862.39 35 979 878.75 979 865.26 36 979 878.56 979 865.07 37 979 878.49 979 865.00 38 979 878.38 979 864.89 39 979 878.52 979 865.03 40 979 878.62 979 865.13 41 979 878.75 979 865.26 41 TABLE IV (continued) 42 979 878.69 979 865.20 43 979 878.58 979 865.09 44 979 878.45 , 979 864.96 45 979 878.33 979 864.84 46 979 878.22 979 864.73 47 979 878.28 979 864.79 48 979 878.43 979 864.94 49 979 878.26 979 864.77 50 979 878.07 979 864.58 51 979 877.82 979 864.32 52 979 877.37 979 863.88 53 979 876.31 979 862.81 54 979 876.42 979 862.93 55 979 875.98 979 862.48 42 TABLE V GRAVITY CORRECTIONS Station FAC BC TC CC (mgal) (mgal) (mgal) (mgal) 1 9.28 4.66 4.29 0.05 2 7.12 3.58 4.19 0.04 3 6.07 3.05 4.62 0.03 4 9.83 4.94 4.56 0.05 5 7.75 3.90 4.33 0.03 6 7.84 3.94 4.20 0.04 7 9.90 4.98 4.18 0.05 8 9.80 4.93 4.38 0.05 9 10.45 5.26 4.07 0.05 10 3.76 1.90 3.91 0.02 11 3.09 1.57 3.93 0.01 12 4.50' 2.27 3.94 0.02 13 5.71 2.88 3.92 0.03 14 2.88 1.46 4.01 0.01 15 5.44 2.73 3.91 0.03 16 4.20 2.11 3.97 0.02 17 2.42 1.22 3.97 0.01 18 2.41 1.21 3.95 0.01 19 4.10 2.06 3.87 0.02 43 TABLE V (continued) 20 1.48 0.93 3.90 0.01 21 6.54 3.29 4.21 0.04 22 2.39 1.21 4.06 0.01 23 8.59 4.32 4.22 0.04 24 10.18 5.12 4.52 0.04 25 4.06 2.05 4.14 0.02 26 23.61 11.86 5.01 0.09 27 3.16 1.60 4.72 0.02 28 13.47 6.78 5.13 0.06 29 6.30 3.19 4.68 0.03 30 10.24 5.17 5.14 0.04 31 13.43 6.77 4.68 0.06 32 22.84 11.49 5.53 0.09 33 18.78 9.46 5.74 0.08 34 13.33 6.72 5.64 0.06 35 14.39 7.25 4.46 0.06 36 . 13.37 6.73 4.83 0.06 37 14.97 7.54 4.58 0.07 38 15.83 7.97 4.48 0.07 39 10.95 5.52 4.05 0.04 40 8.04 4.06 3.96 0.03 41 6.73 3.40 4.02 0.03 44 TABLE V (continued: 42 5.52 2.79 4.02 0.03 43 6.57 3.31 3.98 0.03 44 8.46 4.26 4.00 0.03 45 12.42 6.24 4.33 0.06 46 11.20 5.63 3.99 0.06 47 8.30 4.17 3.94 0.04 48 6.53 3.28 3.97 0.03 49 4.66 2.34 3.89 0.02 50 8.91 4.47 3.93 0.04 51 13.62 6.84 4.45 0.06 52 13.47 6.76 4.58 0.06 53 11.32 5.68 5.28 0.05 54 5.61 2.81 4.61 0.02 55 9.39 4.70 5.02 0.05 45 IV. GRAVITY ANOMALIES A gravity anomaly is the difference between the corrected observed gravity and the computed theoretical value. In general, several different anomalies are considered depending on the corrections applied to the observed data. A. FREE AIR ANOMALY The free air anomaly (FAA) is the gravity value after the free air cor- rection is applied: FAA= (OG + FAC - THG) mgal. Figure 8 shows the free air anomaly chart for Carmel Bay using the 1930 formula for the theoretical gravity. B. SIMPLE BOUGUER ANOMALY The simple Bouguer anomaly (SBA) is the gravity value when the free air correction and the Bouguer correction have been applied: SBA = (OG + FAC + BC - THG) mgal or SBA= (FAA + BC) mgal. Figure 9 shows the resulting simple Bouguer anomaly chart obtained, again using the 1930 formula for the theoretical gravity. 46 Figure 8. Free Air Anomaly Map of Carmel Bay Based on the 1930 Formula for Theoretical Gravity 4 7 Figure 9. Simple Bouguer Anomaly Map of Carmel Bay Based on the 1930 Formula for Theoretical Gravity 48 C . COMPLETE BOUGUER ANOMALY The complete Bouguer anomaly (CBA) is the anomaly which results after the free air correction, Bouguer correction, terrain correction and curvature correction have been applied: CBA = (OG + FAC + BC + TC - CC - THG) mgal or CBA = (SBA + TC - CC) mgal. Figure 10 shows the complete Bouguer anomaly chart based on the 1930 formula. Table VI summarizes the above gravity anomalies for each station. In a similar fashion Table VII shows the gravity anomalies based on the 1967 formula for the theoretical gravity. The CBA for the land stations are shown in Table 1 . D. MASS COMPENSATED FREE AIR CHART Figure 11 shows the approximate differences that would be expected between the theoretical gravity values and the values observed by a sea- surface gravity meter and corrected for Eotvos and ship motion, based on the 1930 gravity formula, called the mass compensated free air values (MCV) . These values were obtained from the free air anomaly by correcting for the double Bouguer effect of the water. Thus: 49 MCV = [FAA + (0.262 D )] mgal a where D , as defined before, is in feet. a This figure is included so that a future sea- surface gravity survey of the bay can be directly compared with the results reported herein. 50 Figure 10. Complete Bouguer Anomaly Map of Carmel Bay Based on the 1930 Formula for Theoretical Gravity 51 TABLE VI GRAVITY ANOMALIES BASED ON THE 1930 FORMULA FOR THEORETICAL GRAVITY Station FAA SBA CBA (mgal) (mgal) (mgal) 1 17.15 21.82 26.06 2 18.29 21.87 .26.02 3 18.90 21.95 26.54 4 17.27 22.21 26.72 5 18.26 22.15 26.45 6 18.30 22.25 26.41 7 17.22 22.20 26.33 8 17.01 21.94 26.27 9 16.11 21.36 25.38 10 18.66 20.56 24.45 11 1-9.00 20.56 24.48 12 17.97 20.24 24.16 13 17.80 20.69 24.58 14 18.60 20.06 24.06 15 17.77 20.51 24.39 16 18.33 20.44 24.39 17 18.45 19.66 23.62 18 18.67 19.88 23.82 52 TABLE VI (continued) 19 18.48 20.54 24.39 20 19.76 20.69 24.58 21 18.97 22.26 26.43 22 21.21 22.41 26.46 23 18.64 22.96 27.14 24 17.84 22.95 27.43 25 20.42 22.47 26.59 26 11.35 23.21 28.13 27 21.36 22.97 27.67 28 16.72 23.50 28.57 29 21.59 24.78 29.43 30 19.51 24.68 29.78 31 17.52 24.28 28.90 32 12.26 23.75 29.19 33 14.90 24.36 30.02 34 .. 18.55 25.27 30.85 35 14.63 21.87 26.27 36 14.94 21.67 26.44 37 14.01 21.55 26.06 38 12.31 20.28 24.69 39 14.73 20.24 24.25 40 16.32 20.37 24.30 53 TABLE VI (continued 41 16.56 19.96 23.95 42 17.18 19.97 23.96 43 16.80 20.11 24.06 44 15.93 20.18 24.15 45 13.42 19.66 23.93 46 15.03 20.67 24.60 47 16.29 20.46 24.36 48 16.69 19.97 23.91 49 17.80 20.14 24.01 50 16.55 21.02 24.91 51 14.62 21.46 25.85 52 15.07 21.82 26.34 53 15.76 21.43 26.66 54 18.77 21.57 26.16 55 17.59 22.29 27.26 54 TABLE VII GRAVITY ANOMALIES BASED ON THE 1967 FORMULA FOR THEORETICAL GRAVITY Station FAA SBA CBA (mgal) (mgal) (mgal) 1 30.64 35.31 39.55 2 31.78 35.35 39.51 3 32.38 35.43 40.03 4 30.75 35.69 40.21 5 31.74 35.64 39.94 6 31.79 35.73 39.90 7 30.71 35.69 39.82 8 30.50 35.43 39.76 9 29.59 34.85 38.87 10 32.15 34.05 37.94 11 32.48 34.05 37.97 12 31.46 33.73 37.65 13 31.29 34.17 38.07 14 32.09 33.55 37.55 15 31.26 33.99 37.88 16 31.82 33.93 37.88 17 31.94 33.15 37.11 18 32.16 33.37 37.31 19 31.97 34.03 36.88 55 TABLE VII (continued) 20 33.25 34.18 38.07 21 32.46 35.75 39.93 22 34.70 35.91 39.96 23 32.14 36.46 40.64 24 31.33 36.45 40.92 25 33.91 35.96 40.08 26 24.85 36.71 41.62 27 34.86 36.46 41.17 28 30.22 37.00 42.07 29 35.09 38.28 42.93 30 33.01 38.18 43.27 31 31.01 37.78 42.40 32 25.76 37.25 42.68 33 28.40 37.85 43.51 34 32.04 38.76 44.34 35 .. 28.11 35.36 39.76 36 28.43 35.16 39.93 37 27.50 35.04 39.55 38 25.80 33.77 38.18 39 28.22 33.73 37.73 40 29.81 33.86 37.78 41 30.05 33.45 37.44 56 TABLE VII (continued) 42 30.67 33.46 37.45 .43 30.29 33.60 37.55 44 29.42 33.67 37.64 45 26.91 33.15 37.42 46 28.52 34.16 38.09 47 29.78 33.95 37.85 48 30.18 33.47 37.40 49 31.29 33.63 37.50 50 30.04 34.51 38.40 51 28.11 34.95 39.34 52 28.56 35.32 39.84 53 29.25 34.93 40.16 54 32.26 35.07 39.65 55 31.08 35.78 40.76 57 Figure 11. Mass Compensated Free Air Chart of Carmel Bay Based on the 1930 Formula for Theoretical Gravity V. INTERPRETATION The interpretation of gravity data is not an easy process. Gravity contours usually do not relate directly to structure contours and attempting to do so may result in highly erroneous interpretaion. Gravity data is not unique; by itself it is not a reliable source of information for the inter- pretation of subsurface geology. The more available data there is from other sources, the more specific can be the interpretation of the gravity data. It can be said that the interpretation of gravity data of itself is a speculative process. This assuredly is the case with the interpretation resulting from the present study. Information from other sources on the subsurface structure of Carmel Bay is very scarce. Simpson (1972) conducted seismic reflection profiling in the bay with a 3.5 kHz high resolution profiler and a 300 J spark er, but in the shallow regions the high reflectivity of the sand sediments and the lack of resolution in the first 6 fathoms of the records due to the sparker pulse and air bubbles masked any layering that might be expected. In consequence, the interpretation presented here can be considered to be done on the basis of gravity data alone. Figure 12 is a chart showing the interpolated and extrapolated com- plete Bouguer anomaly contours. The land gravity trends are based on work by Sieck (1961) . The two most evident features are the low anomaly between Pescadero Point and Abalone Point extending seaward in the direction of 59 n miles Figure 12. Complete Bouguer Anomaly Chart of Carmel Bay Including Extrapolations of Contours 60 the secondary canyon, and the bending of the anomaly lines in the southern part of the bay in the region of the Carmel Submarine Canyon. Figure 13 is a profile of the complete Bouguer gravity anomaly from Pescadero Point to Abalone Point (A-B on Figure 12). The difference of 1.5 mgal cannot be attributed to the presence of the Carmelo Formation inshore of the profile nor to the thin layer of sediments indicated by Simpson (1972). As mentioned before, the seismic work done in the area does not show any sedimentary rock layer, but the granodiorite basement shown in the geological map of Carmel Bay (Figure 2) is not compatible with the gravity profile. A more plausible explanation results if the pro- file is considered to be due to an erosional and depositional feature. It is possible that the Pleistocene glaciers might have cut the secondary canyon in the bay and sediments, probably derived from erosion of granodiorite and Carmelo Formation, filled the canyon. The thickness of the sediments is estimated to be over 500 m. Future work is needed to check this hypothesis. No evidence was found for the fault between Pescadero Point and Abalone Point as proposed by Bowen (1965) , but this fault, laying parallel to the regional trend of the area, could be masked by the effect of the layer of sediments proposed above. The bending of the anomaly lines in the southern part of the bay is believed to be due to faulting along the Carmel Submarine Canyon. This fault could be the seaward continuation of the one proposed by Simpson (1972) , running down the San Jose Creek Valley (Figure 2) . 61 It is evident that the submarine geology of Carmel Bay is not yet well known and much more work needs to be done before it can be well understood . 62 o CI E < CQ U ►25.5. ♦250. ♦24.5.. +24.0 ♦23.5.. B 2 Distance, n miles Figure 13. Complete Bouguer Gravity Profile from Pescadero Point (A) to Abalone Point (B), garmel Bay 63 VI. SUGGESTIONS FOR FURTHER STUDIES Further studies that would help to define the marine geology of Car- mel Bay should include: 1. magnetic measurements in the bay; 2. seismic refraction and reflection measurements in the bay; 3. carbon, carbonate and organic nitrogen analysis of sediments; 4. current and water column structure determinations within the bay, and 5 . periodic surveys of the Carmel Submarine Canyon with a narrow, beam profiler. 64 BIBLIOGRAPHY Bomford, B. G. 1962. Geodesy. 2d ed. Oxford University Press. Bowen, O. E. 1965. Point Lobos, a Geological Guide. California Divi- sion of Mines and Geology Mineral Information Service. 18(4). Dobrin, M. B. 1960. Introduction to Geophysical Prospecting. 2d ed. McGraw-Hill Book Company. Grant, F. S. , and G. F. West. 1965. Interpretation Theory in Applied Geophysics. McGraw-Hill Book Company. Griffiths, D. H. , and R. F. King. 1965. Applied Geophysics for Engineers and Geologists. Pergamon Press. Heiskanen, W. A. , and F. A. Vening Meinesz, 1958. The Earth and Its Gravity Field. McGraw-Hill Book Company. Heiskanen, W. A. and H. Moritz, 1967. Physical Geodesy . W. H. Freeman and Company. Lawson, A. C. 1893. The Geology of Carmelo Bay. Bull. Dept. Geol. , University of California , Berkeley 1:1-59. Martin, B. D. 1964. Monterey Submarine Canyon, California: Genesis and Relationship to Continental Geology. PhD Dissertation, University of Southern California, Los Angeles. (Unpublished Report). Martin, B. D. , and K. O. Emery. 1967. Geology of Monterey Canyon, California. Bull. Amer. Assoc. Petrol. Geologists. 51 (1 1) :22 81-2304 . Nili-Esfahani , A. 1965. Investigation of Paleocene Strata, Point Lobos, Monterey County, California. M. A. Thesis, University of California , Los Angeles. (Unpublished Report). Shepard, R. P. , and R. F. Dill. 1966. Submarine Canyons and Other Sea Valleys. Rand McNalley & Co., Chicago. Shepard, R. P., and K. O. Emery. 1941. Submarine Topography Off the California Coast: Canyons and Tectonic Interpretation. Geological Soc. Amer. Spec. Paper 31. Sieck, H. C. 1961. A Gravity Investigation of the Monterey-Salinas Area. University of California . (Unpublished Report). 65 Simpson, J. P. 1972. The Geology of Carmel Bay , California. M. S. Thesis. Naval Postgraduate School, Monterey. (Unpublished Report). Trask, J. B. 1854. Report of the Geology of the Coast Mountains and Particularly of the Sierra Nevada. Assembly Journal, Appendix Doc. . 9, 5th Session, State Legislature, Calif. 21, 22, 36. Trask, J. B. 1855. Report of the Geology of the Coast Mountains. Assembly Journal, Appendix Doc. 14, 6th Session, State Legislature, Calif. 28. Zardeskas, R. A. 1971. A Bathymetric Chart of Carmel Bay, California. M. S. Thesis, Naval Postgraduate School, Monterey, California. (Unpublished Report) . 66 INITIAL DISTRIBUTION LIST No. Copies 1 . Defense Documentation Center . 2 Cameron Station Alexandria, Virginia 22314 2. Library, Code 0212 2 Naval Postgraduate School Monterey, California 93940 3. Department of Oceanography, Code 58 3 Naval Postgraduate School Monterey, California 93940 4. Oceanographer of the Navy 1 The Madison Building 732 North Washington Street Alexandria, Virginia 22314 5. Office of Naval Research 1 Code 480-D Arlington, Virginia 22217 6. Professor Robert S. Andrews 10 Department of Oceanography, Code 58Ad Naval Postgraduate School Monterey, California 93940 7. Professor Joseph J. von Schwind 3 Department of Oceanography, Code 58Vs Naval Postgraduate School Monterey, California 93940 8. Dr. Rodger H. Chapman 1 Division of Mines and Geology Resources Building, Room 1341 1416 Ninth Street Sacramento, California 95814 9. Dr. Howard Oliver 1 United States Geological Survey 345 Middlefield Road Menlo Park, California 94025 67 10. Dr. S. L. Robbins United States Geological Survey 345 Middlefield Road Menlo Park, California 94025 11. Mr. H. Gary Greene United States Geological Survey 345 Middlefield Road Menlo Park, California 94025 12. Gravity Section Naval Oceanographic Office Washington, D. C. 20390 13. Mr. H. B. Parks LaCoste and Romberg, Inc. 6606 North Lamar Austin, Texas 14. Professor Warren Thompson Department of Oceanography, Code 58Th Naval Postgraduate School Monterey, California 93940 15. Master, R/V Acania Department of Oceanography Naval Postgraduate School Monterey, California 93940 16. LT. Robert A. Brooks, USN SMC 2820 Naval Postgraduate School Monterey, California 93940 17. LT. Brian S. Cronyn, USN USS Independence (CVA-62) Fleet Post Office New York, New York 09501 18. LCDR Antonio P. D. Souto Av. D. Luis I - 8 R/C Esq. Alfragide, Portugal 19. Director Geral Instituto Hidrografico Ministerio da Marinha Lisboa, Portugal 68 20. Biblioteca Central Ministerio da Marinha Lisboa, Portugal 21. LCDR Carlos F. D. Souto Caixa Postal 2089 Lourenco Marques , Mocambique Portuguese East Africa 22. LCDR Jose Saldanha Instituto Hidrografico Ministerio da Marinha Lisboa, Portugal 69 UNCLASSIFIED Security Classification _JJJJUjl !.■■ I IIIIII1IN I Mil I ■■! II III I I HIT DOCUMENT CONTROL DATA -R&D (Security classification of title, body of abstract and indexing annotation must be entered when the overall report Is classified) ORIGINATING ACTIVITY ( Corporate author) Naval Postgraduate School Monterey, California 93940 2B. REPORT SECURITY CLASSIFICATION Unclassified 26. GROUP REPORT TITLE A BOTTOM GRAVITY SURVEY OF CARMEL BAY, CALIFORNIA DESCRIPTIVE NOTES (Type ot report end, inclusive dotes) Master's Thesis; March 1973 AUTHOR(S) (First nam*, middla Initial, laat nema) Antonio Pedro Dias Souto REPORT DATE March 1973 CONTRACT OR GRANT NO. b. PROJECT NO. 7«. TOTAL NO. OF PAGES 71 7b. NO. OF REFS 18 9*. ORIGINATOR'S REPORT NUMBERISf Bb. OTHER REPORT NOISI (Any other numbers thai may be aeai&iod thla report) 0. DISTRIBUTION STATEMENT Approved for public release distribution unlimited. I. SUPPLEMENTARY NOTES 12. SPONSOBING MILITARY ACTIVITY 3. ABSTRACT Bottom gravity data was obtained on 55 stations down to the 50 fathoms depth contour to produce the first gravity anomaly charts of Carmel Bay. The techniques of data collection and reduction are discussed. No evidence was found for the fault between Pescadero Point and Abalone Point proposed by Bowen. A layer of sediments over 500 meters thick, probably of the Paleocene Carmelo series, is indicated extending seaward from Carmel Beach, partially filling the secondary canyon. A new fault is proposed along the axis of the Carmel submarine canyon. DD ,fn°orv\.1473 (PAGE " S/N 0101 -807-681 I 70 UNCiAssrni.'p Security Cl«»nflc«tlon 1- 3I40« UNCLASSIFIED CARMEL BAY GEOLOGY GEOPHYSICS GRAVITY MARINE GEOLOGY DD/r\.1473 (back, S/N 0101-807-6821 UNOIASSIFIED 71 Security Clattificalion » - J I 409 Thesi s S66614 c.l 143411 Souto A bottom gravity survey of Carmel Bay, Cal ifornita. - Thesis S66614 c.l 143411 Souto A bottom gravity survey of Carmel Bay » Cal ifornia. thesS66614 A bottom gravity survey of Carmel Bay, C 3 2768 002 01709 7 DUDLEY KNOX LIBRARY