Field Geophysics by John Milsom & Asger Eriksen

Author:John Milsom & Asger Eriksen
Language: eng
Format: epub
Publisher: John Wiley & Sons
Published: 2011-06-18T00:00:00+00:00

6.6 Capacitive Coupling

There have been many attempts to make galvanic resistivity profiling continuous or semi-continuous. Towed electrode arrays rely on devices such as spiked wheels to make contact with the ground, but achieving and maintaining 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) OhmMapperTM aerial (consisting of insulated coaxial cable) and electronics nacelle. (c) Complete OhmMapperTM assembly.

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