Previous publications have outlined the general theory of electromagnetic wave propagation along an interface such as the air-Earth boundary and described model experiments in which this theory was verified. Equipment similar to that described for the laboratory experiments has been adapted for field use. Two large loops, one for transmitting and the second for receiving, are laid on the surface of the ground and a current square wave is passed through the transmitting loop. A similar arrangement of loops is placed on the surface of a metallic model. The signals from the two receiving loops are mixed in opposition. The layering in the model is then manipulated until the difference signal is a minimum, which condition signifies that the actual earth has been approximated in the model, after account is taken of the appropriate scaling factors. At five stations in Dallas County, Texas, this technique was used to determine the depth of the Austin chalk-Eagle Ford shale contact, which varies from 50 to 400 feet, within an average accuracy of about five per cent.

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