Electromagnetic detection of buried metallic objects using quad–quad conductivity

Abstract

Apparent conductivity computed from in‐phase and quadrature components has been used successfully to detect buried metallic objects such as unexploded ordnance (UXO). The conductivity computation uses magnetic susceptibility calculated from the lowest‐frequency in‐phase data obtained at a specific sensor height. Over magnetic soils, however, the in‐phase component may fluctuate with varying sensor heights. Uncertainties in sensor height, common with handheld or cart‐mounted sensors in rough terrain, can produce errors in the computed magnetic susceptibility, which, in turn, causes errors in apparent conductivity.To overcome these limitations, we have developed an algorithm to compute the quad–quad apparent conductivity from the quadrature components at two frequencies. Our results show that the quad–quad technique has several advantages for detecting metal targets in magnetic terrains: it is (1) insensitive to the magnetic polarization currents; (2) it is immune to sensor motion over magnetic soil; and (3) it is biased to metal objects and can detect small and/or deep metal targets. The first two properties suppress the noise caused by magnetic terrain and sensor motion and thus yield a quiet background. The last property emphasizes metal objects as sought anomalies over geologic variations.