none 10.1190/1.1443327 Society of Exploration Geophysicists Society of Exploration Geophysicists 186 4729904 5981 2025121112015362900 10.1190 2025-12-11T17:01:58Z 2002-10-11T15:19:45Z 61 Geophysics 1942-2156 0016-8033 09 01 1992 09 01 1992 57 9 Andreas Hoerdt Univ. Koeln, Inst. Geophys. und Meteorol., Cologne, Federal Republic of Germany Vladimir L. Druskin Schlumberger Doll Res., United States Leonid A. Knizhnerman Cent. Geophys. Exped., USSR Kurt-Martin Strack West. Atlas Int., United States Abstract The interpretation of long-offset transient electromagnetic (LOTEM) data is usually based on layered earth models. Effects of lateral conductivity variations are commonly explained qualitatively, because three-dimensional (3-D) numerical modeling is not readily available for complex geology. One of the first quantitative 3-D interpretations of LOTEM data is carried out using measurements from the Munsterland basin in northern Germany. In this survey area, four data sets show effects of lateral variations including a sign reversal in the measured voltage curve at one site. This sign reversal is a clear indicator of two-dimensional (2-D) or 3-D conductivity structure, and can be caused by current channeling in a near-surface conductive body.Our interpretation strategy involves three different 3-D forward modeling programs. A thin-sheet integral equation modeling routine used with inversion gives a first guess about the location and strike of the anomaly. A volume integral equation program allows models that may be considered possible geological explanations for the conductivity anomaly. A new finite-difference algorithm permits modeling of much more complex conductivity structures for simulating a realistic geological situation. The final model has the zone of anomalous conductivity aligned below a creek system at the surface. Since the creeks flow along weak zones in this area, the interpretation seems geologically reasonable. The interpreted model also yields a good fit to the data. 09 01 1992 09 01 1992 09 01 1992 1127 1137 10.1190/1.1443327 https://pubs.geoscienceworld.org/geophysics/article/57/9/1127/106388/Interpretation-of-3-D-effects-in-long-offset https://pubs.geoscienceworld.org/seg/geophysics/article-pdf/57/9/1127/3162462/1127.pdf https://pubs.geoscienceworld.org/seg/geophysics/article-pdf/57/9/1127/3162462/1127.pdf http://geophysics.geoscienceworld.org/cgi/doi/10.1190/1.1443327