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The resolution capabilities of airborne electromagnetic (AEM) frequency-domain systems are traditionally analyzed in terms of the footprint, which provides a simple measure of the lateral extent of the earth volume involved in a given measurement. However, considerably more detailed insight into the system resolution capabilities can be obtained by studying the 3D sensitivity distribution as defined by the Fréchet derivatives. A qualitative analysis of the 3D sensitivity distributions for six typical magnetic dipole-dipole configurations demonstrates that the spatial resolution characteristics differ widely and that the optimal coil configuration for practical investigations depends on the expected target characteristics. For all six coil configurations, the 3D sensitivity distributions reveal significant sign changesdownwards and outwards from the center, stressing the necessity of reliable starting models for successful inversion of frequency-domain AEM data. Likewise, the central zone of sensitivity for the in-phase component is always larger than for the quadrature, indicating an inferior lateral resolution of the former. A new sensitivity footprint is defined, based on the at-surface behavior of the sensitivity distribution, simply as the area where the sensitivity is at least 10% of its maximum value. For the vertical coaxial (VCA) coil configuration, the size of the sensitivity footprint in the [Formula: see text]-direction (perpendicular to the flight path) is approximately a factor of two larger than in the [Formula: see text]-direction (along the flight path), while there is virtually no difference for the horizontal coplanar (HCP) coil configuration. The ratio of the HCP to VCA sensitivity footprint exceeds one in both [Formula: see text]- and [Formula: see text]-directions, suggesting that the VCA coil configuration has the best lateral resolution.

Ground-based electrical and electromagnetic methods are used systematically for quantitative hydrogeologic investigations in Denmark. In recent years, a desire for faster and more cost-efficient methods has led to growing interest in the possibility of using airborne systems, and in 2001 a number of test flights were performed using a frequency-domain, helicopter-borne electromagnetic (HEM) system. We perform a theoretical examination of the resolution capabilities of the applied system. Quantitative model parameter analyses show that the system only weakly resolves conductive, near-surface layers but can resolve layer boundary to a depth of more than 100 m. Modeling experiments also show that the effect of altimeter errors on the inversion results is serious. We suggest a new interpretation scheme for HEM data founded solely on full nonlinear 1D inversion and providing layered-earth models supported by data misfit parameters and a quantitative model-parameter analysis. The backbone of the scheme is the removal of cultural coupling effects followed by a multilayer inversion that in turn provides reliable starting models for a subsequent few-layer inversion. A new procedure for correlation in the model space ensures model sections with slow lateral variations in resistivity, normally assumed in sedimentary environments. A field example from a Danish survey demonstrates that the interpretation scheme can produce satisfactory results within the limitations of the system.