BULLETIN OF THE AMERICAN ASSOCIATION OF PETROLEUM GEOLOGISTS 
VOL. 19, NO. 1 (JANUARY, 1935), PP. 64-77, 7 FIGS. 
DEEP ELECTRICAL PROSPECTING! 
J. C. KARCHER? anp EUGENE McDERMOTT? 
Dallas, Texas 
ABSTRACT 
By deep electrical prospecting is meant depths of a half mile or more. Most data so 
far published refer to depths appreciably shallower than this. A continuous current field 
is most satisfactory together with electrode spacing comparable with the depth of the 
insulating media. Electrical prospecting is here regarded in the light of its ability to de- 
termine the presence of insulating rather than conducting media. Use is made of re- 
sistivity determinations at various electrode spacings together with inductance and 
resistivity slope functions. Inductance is determined by measuring the time constant 
of the current decay. Three areas are considered. One, the Hugoton gas area, Stevens 
County, Kansas, is an ideal case for the electrical method. In the other two cases con- 
sidered, the Hebbronville area, Jim Hogg County, Texas, and the Anderson County, 
Kansas, area, the electrical method is quite ineffectual. 
Most of the published data on the electrical method of geophysical 
prospecting have been for rather short electrode spreads and conse- 
quently afforded information for relatively shallow depths. A further 
limitation as regards depth has been the use of an alternating current 
source to establish a field in the earth. In the following examples a 
continuous current was employed and interrupted at regular intervals 
of about 10 seconds. With the longer electrode spacings time con- 
stants of the order of one-tenth second were not unusual. It is there- 
fore obvious that for deep penetration it is undesirable to employ 
alternating current of frequencies higher than a few cycles per second. 
In view of the difficulty of obtaining large currents of such low fre- 
quency, the advantages of a continuous current are apparent. 
A current spread of one-half mile (distance between current elec- 
trodes) was used throughout. A pair of potential electrodes was 
placed at varying distances in line with the current electrodes and 
outside the current electrodes, as indicated in Figure 1. The distance 
from the inside current electrode to the inside potential electrode is 
the distance between current and potential electrodes referred to in 
what follows. The separation of potential electrodes was made as 
large as necessary to obtain a satisfactory value of voltage. This 
1 Read before the Geophysics Division of the Association at the Dallas meeting, 
March 23, 1934. Manuscript received, December 4, 1934. 
® Geophysical Service, Inc., Republic Bank Building. 
724 eh 
