Gradiometer antennas for detection of long subsurface conductors

Abstract

The use of gradiometer antennas for detection of long conductors in tunnels is analyzed. For reception in vertical boreholes, the gradiometer consists of two vertical magnetic dipoles with a vertical separation. The source is a vertical magnetic dipole located in an adjacent vertical borehole. 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.