Footprint for frequency-domain airborne electromagnetic systems

Abstract

An airborne electromagnetic (AEM) survey often covers hundreds of square kilometers. Huge amounts of survey data make 2D/3D data inversion very difficult. However, due to the compact configurations of AEM systems, the sensitive area for each single survey station is much smaller than the whole survey area, which makes it possible to only invert partial survey data. The sensitive area is called the footprint. Based on “moving-footprint” technology, the entire survey can be divided into subareas and the data are first inverted individually and then combined to form the inversions of the entire survey area, so that the cost for forward and inverse modeling can be vastly reduced. Contrary to previous electromagnetic (EM) footprints defined only for an EM transmitter or for a perfectly conductive earth, we defined the frequency-domain AEM footprint by considering a complete AEM transmitter-receiver system over an earth with limited conductivity. We used the tensor Green’s function to calculate the secondary magnetic field from the induced underground current and evaluate the EM footprint as the volume in which the induced current contributes 90% to the total secondary magnetic field at the EM receiver. Numerical experiments for horizontal coplanar and vertical coaxial coil configurations revealed that among all influence factors on the AEM footprint, the flight altitude was dominant, with a high flight altitude corresponding to a large EM footprint, whereas the transmitting frequency and earth resistivity played a secondary role and in a combined way of induction number, with the low frequency or high earth resistivity (small induction number) corresponding to large EM footprints.