Structural mapping in basin-and-range-like geology by electromagnetic methods: a powerful aid to seismic

A case history is presented where electromagnetic (EM) methods were applied as a complement to seismic, for structural mapping in basin-and-range-like geology: 366 five-component magnetotelluric (MT) soundings were carried out together with 331 transient soundings (TDEM) along seismic lines. Due to high structural complexity, seismic shows a number of limitations. For the same reasons, MT is highly perturbed and three specific interpretation techniques were comprehensively applied: 1. a classical correction of static effect using TDEM sounding, to determine the high-frequency nondistorted apparent resistivities and thus the corrected tensor; 2. a so-called regional correction based upon the same concept as the static effect, to transform distorted resistivity curves due to the horst/graben situation into plausible 1D curves, through the use of nomograms built for 2D H-polarization situations; 3. a stripping technique which made it possible to map areas where a deep conductive Mesozoic shale was present below carbonates, at a depth of 3 km. After the best MT interpretation was obtained along each line, it was integrated with seismic and with the results from two boreholes. A crude empirical law relating resistivity and acoustic velocity was established and the MT horizons were plotted on the two-way traveltime seismic sections. The final integrated cross-sections obtained are undoubtedly of greater use to the explorationist than the initial seismic sections alone and two wells were accurately predicted.