Gradiometer antennas for tunnel detection
David A. Hill · 1992
The use of gradiometer antennas for detection of long conductors and detection of empty tunnels is analyzed.For reception in vertical boreholes, the gradiometer consists of two vertical electric or magnetic dipoles with a vertical separation.Both sum and difference responses are useful, but the difference response has the potential advantage of suppressing the primary field and making the scattered field easier to detect.The difference response is most effective in suppressing the primary field for a parallel scan where the transmitting antenna and receiving gradiometer are always at the same height.Gradiometers are most advantageous at low frequencies where the scattered field is small compared to the primary field.