DEEP ELECTRICAL PROSPECTING 73 
beyond the original resistivity determination adds but little to the 
value of the data. It would seem therefore that the limitations of the 
electrical method are apt to be more fundamental than the choice 
of instrument design or electrode arrangement. The following two 
cases support this view. 
HEBBRONVILLE AREA 
The center of the area surveyed lies about 10 miles southwest of 
the town of Hebbronville in Jim Hogg County, Texas. The electrical 
work in this area was completed previous to the drilling of any wells. 
The resistivity measurements were made in a manner similar to that 
in the foregoing illustrations. The location of resistivity points is in- 
dicated in Figure 4. The effective resistivity point is assumed to be 
half way between the inside potential electrode and the inside current 
electrode. The potential electrodes were placed at distances of .1, .2, 
-3, 4, 6,and .8 mile from the nearest current electrode. For the sake 
of clarity, not all of these positions are indicated in Figure 5, which 
represents a line of profiles east and west across the center of the 
area, east being at the right. The Rio Oil Corporation Armstrong 
No. 1 was drilled slightly east of the center of the profile. The broken 
connecting lines shown join corresponding points on the different pro- 
files, namely, the .1, .4, and .8 mile potential-electrode points. The .1 
mile point indicates a resistivity syncline where the .4 and .8 mile 
potential points evidence a resistance anticline, that is, high-resis- 
tance values. The .2 potential point not shown also demonstrates a 
resistivity syncline. It is therefore evident that the resistivity “high” 
indicated by the longer spreads is not the reflection of a surface con- 
dition but must be attributed to a subsurface effect at a depth of the 
order of the electrode separation, namely, .4 mile. This is assumed to 
be the case because the short electrode separations are affected op- 
positely and the longest separation, namely .8 mile, indicates a 
smaller resistivity ‘‘high” than the .4 mile position. This is the effect 
expected in the case of a poor insulator. The contour maps of Figure 
6 are the result of contouring all resistivity values in the one case for 
the .5—.4—500’ set-up and in the other for the .5—.8-1,000’ set-up. 
The Rio Oil Corporation well was drilled at the center of the 
“thigh” as shown in Figure 6. This well had no showing of gas or oil 
and upon correlation with the two Sun Oil Company Martinez wells 
later drilled at the west, was found to be structurally much lower 
than either. The following datum values on the Vicksburg were 
kindly furnished by the Sun Oil Company. 
Sun Oil Company Martinez No. 1 — 710 
Sun Oil Company Martinez No. 2 — 930 
Rio Oil Corporation Armstrong No. 1 —1390 733 
