The sensitivity functions of airborne frequency domain methods

Ground based electromagnetic (EM) methods, mainly TEM, have for many years successfully been employed for hydrogeological investigations in Denmark. Lately a great deal of attention has also been paid to airborne electromagnetic methods, and different preliminary test flights have already been accomplished (Christensen et al., 2000). In the summer 2001 a few selected areas in Denmark were surveyed using a frequencydomain helicopter (HEM) system. At the same time a research group were set up to make an exhaustive analysis of frequency-domain systems and to examine their applicability under the given conditions in Denmark. The purpose of the analysis is, among other things, to get a deeper understanding of the resolution capabilities of the systems. In this study the current density distribution and the sensitivity functions for a frequencydomain system are illustrated and commented on. All examples are calculated for a 50 Wm homogenous half-space and for five frequencies (385, 1500, 6200, 25700 and 102000 Hz). The depicted plots represent, however, a vast number of combinations of half-space resistivities and frequencies, as they are identical as long as the product of frequency and conductivity kept the same.