RESISTIVITY METHODS - Field Geophysics - page 132

6.6 Capacitive Coupling
There have been many attempts to make galvanic resistivity profiling con-
tinuous or semi-continuous. Towed electrode arrays rely on devices such as
spiked wheels to make contact with the ground, but achieving and main-
taining sufficiently low contact resistances is a major challenge for these
systems. They function poorly on surfaces that are frozen or very dry. An
alternative approach is to use capacitive coupling, in which electrical fields
due to alternating currents flowing in insulated conductors cause currents to
flow in the ground without direct contact. The aerials of such systems can
be freely dragged along the ground, either manually or mechanically, and
resistivity can be measured continuously.
6.6.1 Capacitive principles
Capacitive coupling relies on the ability of alternating current to pass through
a capacitor (Figure 6.13). In capacitively coupled resistivity (CCR), a cable
or metal sheet forms one plate of the capacitor while the ground behaves as
the other.
In a CCR survey, what would have been the electrodes in a conventional
electrical survey are removed from the ground and insulated from it. If the
power source is then connected, current will flow only until the electrical
potentials produced by the charges on the current electrodes are equal and
opposite to those produced by the source. The ability of the system to store
charge in this way is termed its electrical capacity and is measured in farads.
Figure 6.13 Principle of capacitively coupled resistivity (CCR) operation.
(a) Simple parallel plate capacitor capable of storing charge, and passing
alternating current. (b) OhmMapper TM aerial (consisting of insulated co-
axial cable) and electronics nacelle. (c) Complete OhmMapper TM assembly.