Apparent conductivity computed from in-phase and quadrature components has been<br>successfully used in detecting buried metallic objects such as unexploded ordnance (UXO). The<br>conductivity computation uses the magnetic susceptibility calculated from the lowest-frequency in-phase<br>data obtained at a specific sensor height. Over magnetic soils, however, the in-phase component may<br>fluctuate with varying sensor heights. Uncertainties in sensor height, which are common with handheld<br>sensors or cart-mounted sensors in rough terrain, can produce errors on the computed magnetic<br>susceptibility, which, in turn, causes errors in apparent conductivity, resulting in false anomalies.<br>We introduce the quad-quad conductivity that is computed from the quadrature components at<br>two frequencies. Compared with the traditional apparent conductivity calculated from in-phase and<br>quadrature components at a given frequency, the quad-quad technique has several advantages for<br>detecting metal targets in magnetic terrains: it is (1) insensitive to the magnetic polarization currents; (2)<br>immune to sensor motion over magnetic soil; and (3) biased to metal objects and, thus, able to detect<br>small and/or deep metal targets. The first two properties suppress the noise caused by magnetic geology<br>and sensor motion and, thus, yield a quiet background. The last property emphasizes metal objects as<br>sought anomalies over geologic variations.


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