Bulletin 54. Geophysical Methods of Exploration and their ...

GEOPHYSICAL METHODS OF EXPLORATION 43
token as equal to the depth of the disturbance. This system has
been used by both the Bureau of Public Roads and the War Department
in making determinations of the depth to bed rock in
connection witk various civil engineering problems. (For a typical
curve, see Fig. 14).
There are several different methods of making resistivity measurements.
One called the Porous Pot method involves the use
of ordinary direct current and this necessitates the use of nonpolarizing
potential electrodes known as porous pots. These pot
electrodes are o2 unglazed pottery with an electrolyte of copper
sulphate solutiou surrounding copper electrodes. They are necessary
on account of potential effects that are obtained due _o
chemical reactions between metallic potential stakes and ground
moisture when the former are driven into the ground.
Other methods, notable the Gish-Rooaey aud Megger, n_ake
use of a commutator which reverses the current often enough
to get rid of the stake effects by reversing the polarity. Still
other methods, as the Lee Partitioning method, derive their
names front slightly different arrangements of electrodes. In
this case three potential electrodes arc used with one at the center
of the spread that remains fixed while the others are moved,
and observations are made of the potential drop over each potential
pair. One advantage of this configuration is that it indicates
the direction of change.
Electrical resistivity measurements have also been used to
detect the presence of possible oil-bearing sands in drillholes.
The electrodes are lowered into a well and the resistivity is measured
as the systent is lowered, much as in surface measurements.
By measuring only the potential drop and plotting the changes,
the location of porous sands can be determined due to the infiltration
of the drilling _nud into the sands and the resultant
potential drop caused by the current infiltration at these points.
The two sets of data give what ]s known as an electrical log.
Sands or other porous t{orizons are thus indicated by areas of
potential drop, and these sands carrying oil can be distinguished
front the water sands by the high resistivity of the former. Geologic
information of economic value can thus be obtained directly
without the expense of cores by this method. (For an illustration
of apparatus and results see Fig. 15). Some of the common
resistivity values for rocks and minerals are given, in the following
table.
NEW JERSEY GEOLOGICAL SURVEY