1084 Subsurface Geologic Methods 



intra-basement feature is elongated in a southeast-northwest direction. 

 From the magnetic map (figure 556) it may be seen that the center of the 

 magnetic "high" is shifted about three miles to the south of the gravity 

 "high." It is apparent from the areal extent of this anomaly that it must 

 be due to a density contrast within the basement complex. 



Chromite Deposits, Cuba — The gravity meter was used in the ex- 

 ploration for chromite ore within a serpentine country rock by the Gulf 

 Research and Development Company in the Camaguey district, Cuba. 

 (See fig. 573.) It was found that a mass of ore of some 40,000 tons dis- 

 tributed from the surface down to 70 feet gave a gravity anomaly of 

 only 0.35 milligals. The density contrast between the serpentine and the 

 chromite was 1.5. Since this was considered to be a rather large deposit, 

 the anomalies to be expected over smaller bodies would be much smaller. 

 Twenty-meter spacing between gravity stations was adopted, and repeat 

 readings were made with the gravity meter until the probable error of a 

 single observation was 0.016 milligals. When it is realized that the con- 

 tour interval used was 0.02 milligals, the precision with which the survey 

 was carried out is even more evident. 



Stations numbering 5,320 were observed over an area of approxi- 

 mately 520 acres, and a total of 113 anomalies were designated.^^ 



Seismic Prospecting 



The seismic method of prospecting is credited with the discovery of 

 more oil fields than any of the other geophysical methods. The number 

 of crews in the field in this country has grown phenomenally since the 

 method was first used commercially by the Marland Oil Company in 1923. 

 In the later part of 1947, 450 crews were reported to be at work at a 

 cost to the oil industry of approximately $90,000,000. 



From 1924 to 1929 the Gulf Coast was intensively worked and re- 

 worked by the fan-shooting method, which resulted in the discovery of a 

 number of the shallower salt domes. In a period of several months a 

 dozen domes were discovered for the Louisiana Land and Exploration 

 Company. This feat is even more remarkable when we consider the 

 swampy, marshy terrain in which the work was conducted and the instru- 

 ments available at the time. 



After considerable research the technique of reflection shooting was 

 inaugurated with the discovery of three Oklahoma oil fields in rapid 

 succession during 1929. By 1937 there were 250 reflection crews in the 

 field compared with the four at work in 1929. The present high level of 

 seismic activity will undoubtedly continue until the day that a proved 

 direct method of oil finding is introduced. 



Principles 



The reflection seismograph is one of the few geophysical methods 

 that does not involve determining potential distribution or derivatives of 



** Hammer, S., Nettleton, L. L., and Hastings, W. K., Gravimeter Prospecting for Chromite in Cuba: 

 Geophysics, vol. 10, no. 1, pp. 34-49, Jan. 1945. 



