Rock electrical characteristics deduced from depth attenuation rates (in drill holes)

A radio technique is described for obtaining an estimate of the electrical properties of rock media below the earth's surface when there exist access drill holes which penetrate well into such media. The technique is that of measuring the depth attenuation rate of the field between transmitting and receiving antennas as their separation is varied. Experiments conducted with both transmitting and receiving antennas situated in a drill hole within the rock are described. Measurements were made with small vertically polarized antennas located in the lower rock portions of 1000-foot drill holes on Cape Cod, Mass. An example of results obtained at frequencies near 150 kc is given. Antenna separation distances of up to about 500 feet were used. The method yields reasonably well-defined values of conductivity for a single frequency when the loss tangent is large.