Subsurface Methods as Applied in Geophysics 1117 



variation of resistivity in the red-beds section, a contour map on the base 

 of the red beds was obtained. ^^ 



A modification of the resistivity method is the potential-drop-ratio 

 technique, in which the potential differences are not measured absolutely 

 as in the resistivity method but in the form of a ratio of successive differ- 

 ences. Sharper indications are obtained on vertical-formation boundaries, 

 and in favorable sedimentary sections the determination of horizontal 

 stratigraphic boundaries is more readily accomplished. A description of 

 the instrument, field procedure, and interpretation of results is presented 

 by Heiland.^^ This method is being used at present by Pemex to locate 

 anomalous areas associated with large faults that extend to the surface. 

 Rummerfield ^" reports that "Considerable success has been attained in 

 making well locations by this method." 



By using a direct current interrupted at ten-second intervals, Karcher 

 and McDermott ^^ proposed a method of deep electrical prospecting 

 known as the "eltran technique." By measuring the form of the resulting 

 potential when a sharp current pulse is introduced into the ground, de- 

 ductions may be made as to the electrical properties of the subsurface. 

 According to Nettleton ^^ there is considerable controversy as to the real 

 value of the electrical-transient methods. It appears that the method has 

 no greater depth range than the conventional resistivity methods, and 

 results obtained by the method should correspond with those observed 

 by conventional resistivity techniques. An "eltran" survey over the Sandy 

 Point field in Brazoria County, Texas, is shown in figure 592. About 

 fifty feet of subsurface closure is recognized in the producing area. 



Conclusion 



Since geologic structure must be inferred from the results of geo- 

 physical surveys, it behooves the interpreter to familiarize himself with 

 all geologic information concerning the area in which he is working. 

 It is sometimes suggested in the literature that geophysical interpretations 

 should be made on a mathematical and physical basis only without re- 

 course to geologic data. However, it must be remembered that mathe- 

 matical analyses require certain assumptions concerning the disposition 

 of physical properties of the subsurface materials. It remains for the 

 interpreter to make only assumptions that are geologically feasible in 

 order to lead to a particular solution which is probable in that area. 

 Future oil fields will be found through the combined efforts of the 

 geologist, paleontologist, and geophysicist working together in a spirit 

 of cooperation. 



^ England, C. M., A Resistivity Survey of the Monument Oil Field: Geophysics, vol. 8, no. 1, 

 p. 20, Jan. 1943. 



8^ Heiland, C. A., Geophysical Exploration, pp. 744-757, New York, Prentice Hall, 1940. 



"Rummerfield, B. F., Oil Exploration in Mexico: Mines Mag., vol. 38, no. 12, p. 35, Dec. 1948. 



*' Karcher, J. C, and McDermott, Eugene, Deep Electrical Prospecting: Am. Assoc. Petroleum 

 Geologists Bull., vol. 19, no. 1, pp. 64-77, Jan. 1935. 



*2 Nettleton, L. L., Geophysical Prospecting for Oil, p. 374, New York, McGraw-Hill Book Co., 

 Inc., 1940. 



