Constraining seismic parameters with a controlled-source audio-magnetotelluric method (CSAMT)

Abstract

SUMMARY Precise seismic two-way traveltime (TWT) determinations are often limited by the knowledge of the overburden thickness and its seismic velocity. Values between 500 and 1800 m s−1 are conceivable, according to the water content. The CSAMT method has been successfully applied to determine the thickness of the Quaternary fluvio-glacial deposits and Tertiary Molasse (interbedded marls and siltstones) overlying Mesozoic carbonates along a 3 km profile through a syncline in the Swiss Jura fold and thrust belt. The homoclinal structure within the syncline is well constrained by a petroleum exploration Vibroseis profile which indicates a SSE-dipping Mesozoic series and a similarly oriented increasing thickness of Molasse and Quaternary. The CSAMT places tight constraints on the apparent resistivities of the three formations: Quaternary—300–1000 ωm; Molasse—5–15 ωm; Mesozoic—100–1000 ωm. Furthermore, modelling of the CSAMT sounding data provides absolute thicknesses for both the Quaternary (80–20 m) and the Molasse (10–210 m) layers, revealing a true dip of the top Mesozoic substratum of 7° to the SSE. The seismic velocities of the Quaternary (weathering layer) and the Molasse can thus be determined at 800 and 2100 m s−1, respectively.