| 摘要 |
An apparatus and method for detecting vertically or horizontally oriented underground electrical conductors such as electrical wiring or rails in tunnels thin conducting ore veins surrounded by less conducting rock, or boreholes filled with conductive water or lined with a conductive casing. The apparatus includes a surface or downhole transmitter and a downhole receiver connected to a coherent frequency source unit by fiber optic cables. The source unit generates two phase synchronized frequency signals in the range of one hundred to three hundred kHz. The receiver includes a vertical or horizontal magnetic dipole antenna, e.g. a ferrite rod antenna, and the transmitter includes a vertical or horizontal magnetic dipole antenna in the downhole configuration or at least one long cable or loop antenna in the surface configuration. The method includes generating a first electromagnetic field which induces current flow in the electrical conductor surround by the less conducting rock and then detecting a component of a second electromagnetic field generated by the induced current flowing in the conductor, as a received signal at the receiver. The received signal is processed by synchronous detection in the coherent frequency source unit so that phase shift and amplitude data can be extracted from the received signal. The method can also be used to determine if the distance separating two drillholes is constant. In this case, the preferred operating frequency is above 10 MHz.
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