Footprints of airborne electromagnetic systems over onedimensional Earths

Abstract

We have used an inductive-limit model to compare footprint sizes for a variety of common airborne electromagnetic survey geometries. The model incorporates the flight height and orientation of the transmitter, and accounts for electromagnetic coupling between the induced current system and the receiver. Horizontal magnetic dipole transmitters are shown to have a smaller footprint than vertical magnetic dipole sources. Given typical survey heights for helicopter and fixed wing airborne electromagnetic systems, the helicopter VCX geometry has the smallest inductive-limit footprint (40 m), and the fixed-wing, towed bird system the largest (550 m for a system measuring the vertical component of B-field). We also present preliminary calculations of frequency-domain airborne electromagnetic footprint sizes for the case of finite frequency or half-space conductivity. The original definition of the footprint is extended to be the side length of the cubic volume, centred below the transmitter, which contains the induced currents responsible for 90% of the secondary field measured at the receiver. Inphase footprint size for a horizontal coplanar helicopter frequency-domain system is shown to increase from around 3.7 times the flight height at the inductive limit to >9 times the flight height for induction numbers <0.7. The analysis also shows that the quadrature footprint is approximately two-thirds that of the inphase footprint, suggesting a higher spatial resolution for this component.