110 GPA. ~HETEAND 
6b 2. VERTICAL ELECTRICAL DRILLING 
For this purpose, several methods of those already enumerated 
may be used. 
(1) The method that has been most commonly used is probably 
the 4-terminal Gish-Rooney-Wenner method (Fig. 7 J). The elec- 
trodes are arranged always symmetrically around the point to be 
drilled electrically; that is to say, this point is located where the 
electrode S in the Lee partitioning method would be placed. By ap- 
plying this modification of the Gish-Rooney method, one readily 
obtains the distribution of resistivity at the north and at the south 
(or any other two opposite directions) from the midpoint; that is to 
say, it is possible to determine by this method if there is a dip of the 
formations, or if formations come in north of this point that may be 
absent in the south. 
(2) The method that has been used perhaps as frequently as the 
4-terminal method for electrical vertical drilling is the single-current- 
probe method, that is, the investigation of the potential distribution 
in the vicinity of one current electrode only. For this purpose, either 
the electrode arrangement shown in J/ in Figure 7 may be used (ER 
equal to 2a) or the modification shown in JJa. As indicated in Figure 
7 IT, it is not necessary that the two potential electrodes be in line 
with the current electrodes; in fact, to eliminate completely the 
influence of the second current electrode, which is assumed to be at 
infinity, the line of measuring the potential should be located on a 
circle through F; with the distance £,E.2 as radius around £; or, if 
that is not feasible, a close enough approximation may be obtained 
by running the potential measurements on a line at right angles to 
EE. This is the method employed by Ehrenburg and Watson (with 
EP=a, ER= 22a) (reference list No. II). Gilchrist has applied (refer- 
ence list No. III;,) a modification of this last method by using two elec- 
trodes EF» at either side of #,, and passing currents of equal strength 
through both circuits (Fig. 7 77 6). Instead of using only one current 
base £2EF; at right angles to the potential line, any number of current 
bases, arranged in the form of a symmetrical star, may be employed. 
II. APPARATUS FOR RESISTIVITY WORK 
We now come to a description of the equipment used in resistivity 
work. In order not to make this paper unduly long, rather extensive 
use of figures will be made instead of giving lengthy descriptions of 
the apparatus. Further details may be obtained from the literature 
given in the reference list. 
440 
