Inversion of long-offset TEM soundings near the borehole Münsterland 1, Germany, and comparison with MT measurements

Abstract

SUMMARY Data from a long-offset transient electromagnetic (LOTEM) survey in northwest Germany around the borehole Munsterland 1 are interpreted using 1-D inversion. Special emphasis is given to the fact that the best-fitting model is not necessarily the most realistic one. We therefore investigate different inversion algorithms in order to obtain a more geologically meaningful result. The 1-D inversion results using a second-order Marquardt algorithm show a three-layer structure, with a conductor embedded between two more resistive layers. This is in good agreement with the known lithology and the well logs. ‘Profile inversion’ uses the result from one site as the starting model for the next. The application of the technique yields a smoother section than the individual inversions, and corresponds more closely to the (presumably) layered structure in the area. If desired, using a parameter weighting scheme makes it possible to force consistency at the cost of a larger misfit, yielding a smooth section even in a part of the area where the data are influenced by a 3-D anomaly. Occam inversion is a smoothness-constrained technique which is independent of the starting model. The result of the Occam inversion shows the same features as the Marquardt inversion, including effects which are attributed to 3-D distortions in the data. Joint inversions of the LOTEM data were carried out with magnetotelluric data from the same area. In that case, the LOTEM electric field is recognized as crucial for the resolution of the resistive layer in the section.