Electromagnetic narrowband signals were transmitted through the Earth at 27 coal mines located throughout the United States. From these tests, apparent Earth conductivity values were derived based upon a homogeneous half-space model of the Earth. The derived conductivity values were found to be inversely proportional to the transmitted frequency and mine depth. This result is not consistent with the assumption of a homogeneous half-space Earth. A linear regression model relating the logarithm of the conductivity to the mine depth was formulated. The author found that the apparent Earth conductivity could be estimated from a linear combination of the transmitted frequency and mine depth.
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