162 C. A. HEILAND 
methods, namely, that the depth of geologic bodies must compare 
favorably with their size or thickness. The resistivity methods have 
been applied for some time in oil work, but interest in them has been 
aroused of late due to the perfection of technique which makes it 
possible to obtain a type of indication which may be interpreted 
more readily (the potential-drop-ratio method). 
Although the determination of actual resistivities of samples 
is not a factor as important in resistivity work as the determination 
of related physical properties in other geophysical methods, remark- 
able progress has been made in the design of apparatus which permits 
the determination of resistivities on samples in the field and in the 
laboratory, and on outcrops of geologic formations. 
The requirements on instruments and technique are not exces- 
sive in resistivity and potential-drop-ratio work; for moderate depth 
of penetration, moderate power sources are sufficient. The equip- 
ment for the determination of resistivities and potential-drop ratios 
is readily portable and easily operated. In both resistivity and po- 
tential-drop-ratio work, two distinct fields of application may be 
distinguished; first, the method of resistivity mapping, and second, 
the method of electrical vertical drilling. For the former, the 4- 
terminal Gish-Rooney equipment with one or two fixed electrode 
separations is usually employed, while for the electrical vertical 
drilling any one of the methods in resistivity work, and three poten- 
tial electrodes near one power electrode in potential-drop-ratio work, 
is employed. 
The results obtained thus far indicate that it is possible to locate 
readily geologic structure with the method of equi-resistivity mapping 
at not too great depth, and that detailed information may be obtained 
at any one point by the method of electrical vertical drilling with a 
fairly great degree of accuracy down to a depth of 1,500 feet with the 
present state of the technique, under favorable circumstances. 
While remarkable progress has been made in identifying and cor- 
relating indications due to oil beds in wells, the location of oil directly 
by measurements from the surface is still in its very early stages of 
development, and there is only one case where the direct location of 
oil iu very shallow depth from the surface by electrical measurement 
has been definitely established. 
The resistivity methods depend probably more than any other 
geophysical method on a very good correlation of the geophysical 
data with known geological results, and their chief application is 
492 
