Comparison of towed electromagnetic with airborne electromagnetic and electrical resistivity tomography in a hydrogeophysical context

Abstract

SUMMARY Electrical and electromagnetic methods are well suited for mapping the top 100 m of the subsurface, particularly electrical resistivity tomography (ERT) and Traseint electromagnetic (TEM). Both methods can provide comparable resolution and depth of investigation for generating continuous 2-D resistivity profiles. TEM measurements taken continuously from moving platforms, whether towed on the ground or airborne, can generate 2-D-like resistivity sections similar to those produced by ERT profiling. However, despite the fact that both ERT and TEM can map the electrical resistivity of the subsurface, their results differ due to fundamental differences in physical principles, sensitivity, system geometry and instrumentation. The main objective of this paper is to provide a one-on-one comparison of the newly developed towed TEM system, tTEM, against ERT and airborne TEM, in our case a SkyTEM system. First, we performed the comparison in terms of model resolution using synthetic data and models. For all methods, synthetic data were generated using a 1-D forward response, and inversions were carried out using smooth layered models in a laterally constrained inversion framework. Overall, the inversion results are comparable across the three methods, and they all capture the key features of the synthetic models. The ERT and tTEM cross-sections from two field cases show very comparable results even in the top 5 m where thin resistive layers (∼60 Ω·m) are clearly mapped by both methods. However, the resistivity of the resistive layer is better resolved using ERT than tTEM because of the high sensitivity close to the surface in the ERT case. In the deeper part of the section, tTEM tends to resolve the boundaries of conductive layers (resistivity < 10 Ω·m) better than the ERT method. Compared to SkyTEM, tTEM has a better vertical and horizontal resolution especially in the top 20 m. The better tTEM resolution compared to SkyTEM is primarily due to a smaller footprint and denser data sampling. Depth of investigation-wise the SkyTEM system is superior compared to tTEM due to its larger magnetic moment.