Abstract
The authors present an electromagnetic gradiometer that uses a SQUID (superconductor quantum interference device) sensor as the receiver in an active, electromagnetic detector of nonferrous as well as ferrous objects. The gradiometer pickup loops sit in the center of magnetic coils that generate a time-varying magnetic field inducing eddy currents in conductive bodies. The gradiometer measures the secondary magnetic field of the eddy currents. The SQUID's sensitivity at frequencies below 1 kHz makes electromagnetic metal detectors practical in the marine environment, where the electrical conductivity of seawater precludes the use of conventional systems. The authors describe a prototype system that attains a detection range of 10 m in seawater for targets 50 cm in diameter. It operates at frequencies from 1 Hz to a few hundred Hz. Uses of the electromagnetic gradiometer include locating naval mines buried in sea bottom sediments and hunting undersea treasure. The system's response to the seawater itself enables application to airborne electromagnetic bathymetry. Over land, its sensitivity to crustal conductivity contrasts makes it suitable for airborne mineral exploration. >
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