Many specialized applications in near-surface geophysics require greater spatial and amplitude sensitivity than conventional geophysical systems can provide. Recently developed boom-mounted airborne magnetometer and electromagnetic systems are designed to map unexploded ordnance or other small metallic objects over large tracts of government land. These systems operate at altitudes of 1.5–2.0 m to detect ferrous objects with a mass as small as 2 kg. In this low-altitude environment, factors controlling the applicability of a vertical gradient configuration must be reassessed. Our results demonstrate the superiority of measured vertical gradient over calculated gradient (from total field configurations) for data acquired at these low altitudes. We believe the effectiveness is related to reduction of secondary gridding effects associated with several types of positioning errors, and reduction of correlated rotor and compensation noise.

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