66 J.C. KARCHER AND EUGENE McDERMOTT 
space is filled with fresh water, gas, or oil, the apparent resistivity of 
the rock as observed by the ordinary methods of electrical prospecting 
will be large. Non-porous limestone, therefore, and porous sands where 
the minera] content of the water is small will indicate high resistivity. 
This, together with the fact that the areal extent of oil- or gas-filled 
media is in general small compared with the depth of such media 
below the surface, suggests at the outset that there are real limita- 
tions to the use of the electrical method as far as its use for the direct 
detection of reasonably deep oil- and gas-saturated media is con- 
cerned. Likewise the use of the electrical method for exploring the 
subsurface structurally at any depth likely to be of value will find its 
limitations in the lateral variation of resistivity due to changing po- 
rosity of the various media and mineral content of subsurface waters, 
the first mentioned probably offering the greater obstacle. Granite, 
due to its small porosity, exhibits a very high resistivity. It should, 
therefore, be possible to determine subsurface structure in the granite 
provided the lateral extent of the uplift and the amount of relief are 
sufficiently large in comparison with the depth of the surface of the 
granite. 
If it is assumed that the electrical method is to be used to discover 
the presence of electrically resistant media, such as oil and gas sands, 
it is safe to generalize to the extent of saying that to determine the 
presence of such media with a reasonable degree of certainty the 
smallest lateral dimension of the high-resistance medium must be at least 
several times the depth of the medium below the surface. The Hugoton gas 
field in Stevens County, Kansas, as here demonstrated, definitely 
comes under this category. Without much doubt the East Texas field 
also should be included, although this is now impossible to determine 
due to the large number of cased wells in the field. The electrical 
method is of no value if cased wells are in the vicinity of any of the 
electrodes. 
The resistivity was determined from the following relation: 

IpA IpA 
AV = _ 
a(x? — A?) (X22 — A?) 
wAV I 
p= == 
IA I I 
726 
