ADVANCES IN OIL PROSPECTING 147 
Figure 30 shows another good example of structure located by 
means of a resistivity profile. The traverse was made with a 4-terminal 
contacting arrangement using an electrode interval of 200 feet. As this 
interval is greater than the greatest thickness of the glacial till in the 
area, the observed apparent resistivities represent chiefly the varia- 
tions in resistivities below the glacial drift. Thus it was possible to 
get the effect of the anticline composed of Devonian limestone of high 
resistivity, flanked by Mississippian shales and sandstones of lesser 
resistivity. Hubbert states that the causes of the smaller variations 
in the resistivity peak are unknown; it is probable, however, that they 








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Fic. 31.—Results of ek vertical drilling with single-probe method (II) and 
corresponding geologic section in an area in New Mexico. Courtesy of Harry Aurand, 
Midwest Refining Company. 
are either due to variations in the composition of the glacial till, or 
due to the fact that this till occurs as filling in erosion channels on 
top of the structure. 
2 BY ELECTRICAL VERTICAL DRILLING 
There follows now a discussion of some examples of structural 
resistivity work, most of which were obtained recently and may not 
be found elsewhere in the geophysical literature. They illustrate well 
the great possibilities of the method of electrical drilling with the 
resistivity and potential-drop-ratio method under favorable con- 
ditions. 
Figure 31 shows the results of several resistivity traverses which 
were very carefully measured every 10 feet by means of the single- 
probe method (No. II of Hummel in Figure 7 or the Ehrenburg- 
Watson method). The geologic section consists at the surface of al- 
ternating shale, gypsum, and limestone beds with an average thick- 
477 
