ELECTRICAL PROSPECTING METHODS 373

sisted of four electrodes in line, with the two potential electrodes
outside the two current electrodes (Fig. 175).

The interpretation of the results depends upon determining in
some way the time constants, such as the rate of build-up or of
die-down of the potential. Variations in this rate are expected
to correlate with variations in the electrical properties of the
subsurface. In some of the earlier proposals it was expected that
sharp nicks or irregularities in the curves would be found which
would result from penetration of the current to conductors at
different depths.1 However, in the later developments this
principle seems to have been abandoned, and the interpretation
of results is based on measurements of the time constants.

CURRENT INTERRUPTER

OR APPARATUS WAVE FORM

TO CONTROL WAVE FORM MEASURING CIRCUIT

FIG. 175.—Electrode configuration and schematic principle of the eltran system.

The time constants have been measured in two different ways.
In the first work, measurements were made of the actual form of
the recorded transient voltage. Another system measured the
resultant transient from two symmetrical and oppositely dis-
posed current sources.2 In these measurements the variations
in ground characteristics were correlated with variations in the
time required for the current to build up or to die down to a
certain fraction of its final value. A later method, which has
been called "sawtran,"3 applies a square wave to the ground.
The resulting voltage between the potential electrodes is then
run through a distorting electrical network by which the voltage
is restored to the same square wave form as the impressed current.
The distorting network -then compensates for the amount of
distortion produced in the part of the circuit that flows under-
ground. A numerical value for the amount of distortion can be
obtained from the values of the electrical variables used in the
1 Blau, 1933.
2 Statham, 1936.
3 Klipsch, 1939,