Geology Reference
In-Depth Information
The fact that the current electrodes, even when insulated from the ground,
are charged, implies the existence of an electric field that can cause charged
particles in the ground to move. This current flow would also be brief,
persisting only until equal and opposite reverse potentials were established.
If, however, the power-source polarity were then to be reversed, there would
be a reverse flow of charge until a new equilibrium was established. An
alternating voltage of sufficiently high frequency will thus cause alternating
current to flow in the ground, despite the presence of the insulators. This is
capacitive coupling. The current flow in the ground is also coupled to the
receiver aerial, providing a signal.
6.6.2 Instrumentation
The Geometrics OhmMapper
TM
(Figure 6.14), which is probably the most
widely used CCR instrument, can be used to illustrate the basic principles.
A 16.5-kHz alternating current is supplied to a dipole aerial that, in standard
configurations, is made up of a 2, 5, 10 or 20 m length of cable. The signal
is received at a second, similar aerial towed ahead of the first and separated
from it by a non-conductive linkage. The linkage is usually an integer
Figure 6.14
Towing the OhmMapper
TM
. Photo reproduced courtesy of
Geometrics Inc.
