70 J.C. KARCHER AND EUGENE McDERMOTT 
accurately known, lies at a depth somewhat greater than one-half 
mile below the surface. A number of lines of electrical profiles were 
run in east and west directions, traversing the east edge of the field. 
This particular portion of the field is shown in Figure 2, together with 
the resistivity values and the inductance function values for the long- 
est electrode spread on each profile. A profile consists of four potential 
electrode positions, namely, .4, .5, .6, and .7 mile from the inside po- 
tential electrode to the inside current electrode. The separation of the 
two potential electrodes ranged from 500 feet to 1,000 feet, the mini- 
mum value necessary to secure a good record being chosen in each 
case. This choice is not critical. The distance between current elec- 
trodes was .5 mile. These, according to a previous statement of no- 
menclature, are designated as: 
The north-south trending line through R. 36 W. indicates the elec- 
trically determined east edge of the gas field. 
In the upper portion of Figure 3 are shown the resistivity deter- 
minations for the line of profiles indicated in Figure 2 along the town- 
ship line. Although only the two end values are indicated on the pro- 
files, the profiles were determined from the four values specified 
above. The profiles run from right to left, the shorter electrode spac- 
ing being on the right. The relative disposition of the electrodes is 
indicated for one profile. 
Attention is called to the fact that resistivity values for the .5—.4- 
500’ set-up are on the average equally high, while some are even 
higher, east of the gas field than over the gas sand. On the other hand, 
the .5—.7—-1,000’ resistivity values are on the average decidedly higher 
over the gas sand than outside the bounds of the field. As a result of 
this, the shape of the profile curve is markedly different in the two 
cases. One of the curves in the lower portion of the figure represents 
the inductance function. This follows the same generai pattern as that 
of the .5-.7—-1,000’ resistivity line above, but the change across the 
field border is of greater magnitude. In this particular case all indices, 
the longer spread resistivities, inductance function and resistivity- 
slope function clearly indicate the presence of the insulating gas sand 
with a remarkable degree of certainty. This field amply satisfied the 
condition postulated, namely that the minimum dimensions of the 
insulating layer should be at least several times the depth. It is evi- 
dent also that, in sucha case as this, further manipulation of the data 
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