68 J.C. KARCHER AND EUGENE McDERMOTT 
where AV is the potential difference between potential electrodes in 
volts, J is the current flowing from the current electrodes in amperes, 
p is the resistivity in ohms per centimeter cube, A is the half current 
spread in centimeters, X; and X2 are the distances from the center 
of the current spread to the potential electrodes. It is obvious that if 
the medium is homogeneous the value of p will be independent of the 
position of the potential electrodes (X; and X-2). As the ground is non- 
homogeneous the value of p will depend on the position of the poten- 
tial electrodes. The variations from normal as the potential electrodes 
are moved permits an approximation of subsurface resistivities. 
As the current does not reach its maximum immediately upon 
application of a voltage on the current electrodes, or does not im- 
mediately return to zero on removal of the voltage, the path of the 
current through the ground has an effective inductance. The current 
rises and decays logarithmically in point of time, exactly the charac- 
teristic of an electrical circuit comprising an inductance and resistance 
in series. Now L/R is the time constant of the circuit and represents 
the time required for the current to decay to 1/e of its initial value. 
This may easily be measured from the photographic record of the 
voltage and R may be approximated from the resistivity measure- 
ments. Thus we arrive at a quantity which is a functoin of the induc- 
tance and is here designated the inductance function. 
Although the shorter spreads are affected principally by the near- 
surface conditions, an abnormal surface resistance will have some 
residual effect on the longer spreads. A quantity, therefore, which is 
a function of the resistivity as determined by the longer spread and 
the slope of the resistivity profile will further eliminate the near-sur- 
face effects. This we will call the resistivity-slope function. The slope 
is measured by the angle the resistivity profile makes with the ver- 
tical and increases in a clockwise direction. 
Electrical surveys of three areas are here described. In one area 
the Hugoton gas field, resistivities, inductance and resistivity-slope 
functions are all determined and compared. In the other two areas, 
the Hebbronville area in Jim Hogg County, Texas, and the Anderson 
County area in Kansas, only the resistivities are used. J. V. Polk, of 
Geophysical Service, Inc., deserves credit for the field work, most of 
which was conducted under his direction during 1932. 
HUGOTON GAS AREA 
The Hugoton gas field is one of the largest gas fields known, cover- 
ing the western part of Stevens County, Kansas, and extending into 
several adjacent counties. The gas sand, the thickness of which is not 
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