High-resolution surveying with small-loop requency-domain electromagnetic systems: Efficient survey design and adaptive processing

The range of near-surface applications whereby the electrical and magnetic properties of the subsurface are investigated expands continually [1], and, particularly for terrestrial applications, increasingly higher spatial sampling densities are deployed [2]. Within the broad range of geophysical methods, small-loop frequency-domain electromagnetic (FDEM) instrumentation is often used to record variations of multiple EM properties simultaneously [3]-[7]. Such instruments typically incorporate at least one transmitter in combination with one or more receivers. The resulting transmitter-receiver pairs are commonly configured in either a horizontal coplanar (HCP), vertical coplanar (VCP), vertical coaxial (VCA), or perpendicular (PRP) orientation [8], [9]. A distinction can be made between symmetrical (e.g., HCP, VCP, and VCA) and asymmetrical (e.g., PRP) coil configurations [10], [11] in relation to survey heading. Although the former render an isotropic response unaffected by the acquisition direction, an asymmetrical configuration will produce different responses based on survey heading.