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Abstract

A method utilized with an underground conductor detection system for calibrating a downhole transmitter to compensate for detuning of the transmitter antenna by geologic formations. The method comprises sending a synchronization signal to the transmitter antenna, measuring the current flow through the transmitter antenna and adjusting the current to a constant level, and measuring the phase difference between the transmitter antenna current and the synchronization signal. A receiver transmitter is calibrated by sending a synchronization signal to a radiating antenna in the receiver that in turn sends a calibration signal to the receiver antenna that is directed over the entire signal pathway back to surface located signal processing equipment. Another method of underground conductor detection sends a surface wave to the downhole receiver to cancel the effect of the surface wave modulation on a target wave being radiated by the underground detector. Another method of underground conductor detection utilizes a receiver tuned loop antenna oriented orthogonal to the magnetic dipole of the transmitter antenna for discriminating against reception of a primary wave. Another method of detecting anomalous geological zones such as tunnels, utilizes a low to medium frequency tomographic scan to cancel the effect of geological conductivity noise in a high to very high frequency tomographic scan of a region suspected of containing the anomalous geological zone.

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