Conductivity Measurements in Dissipative Media with Electrically Short Probes

In a dissipative medium, the radio-frequency input conductance of a linear dipole, whether a bare wire or an insulated wire with its terminals short-circuited to the medium, is simply related to the conductivity of that medium provided the electrical length of the dipole is short. Model measurements were made on dipoles in saline water of known conductivity to test the theory. The feasibility of deducing conductivity of the solution by using both types of antennas as probes was demonstrated. The technique has been utilized to evaluate the electrical conductivity of rock media from measurements on probes inserted into vertical drill holes. Examples are given of measurements variously to depths of 4000 ft. In one of the cases the rock type was (fractured) granite and, in another, anorthosite. In a geophysical crustal sense, data are typical of some near-surface or sedimentary rock conditions. Since such rock media are rarely uniform in their electrical characteristics, the deduced conductivity will be a gross average conductivity of that volume of the rock approximately within the antenna region of the probe.