Induction-response functions for frequency-domain electromagnetic mapping system for airborne and ground configurations

A helicopter-towed electromagnetic (EM) sensor of the type typically flown in frequency-domain surveys may be towed over the ground on a trailer. The in-phase and quadrature components measured by a trailer-towed sensor will be dramatically different from those determined when the sensor is flown. For the airborne case, the in-phase and quadrature curves of the induction-response function are governed by the superposed-dipole behavior, in which flying height substantially exceeds the transmitting-receiving coil separation. In contrast, for a ground-based sensor, the coil separation substantially exceeds the sensor height, yielding the infinitely separated dipole case in which the sensor height is negligible compared to the coil separation. These two end cases -the superposed dipole and the infinitely separated dipole-yield EM amplitudes and phase angles that are very different from each other. For the superposed-dipole case of the airborne sensor, the in-phase component reaches a high positive value as the EM response approaches the inductive limit, whereas for the infinitely separated dipole case of the ground sensor, the in-phase component reaches a large negative value. Consequently, there must be a sensor height where the in-phase becomes zero at the inductive limit. This critical height is approximately 35% of the coil separation for the maximum coupled horizontal coplanar coils of the helicopter-towed EM system. The resolution of the ground sensor is superior to that of the airborne sensor, whereas the depth of exploration for the airborne sensor is superior to that of the ground sensor. This finding is to be expected from a consideration of the size of the footprint of the sensor.