CONSTRAINING SEISMIC PARAMETERS WITH A CONTROLLED-SOURCE AUDIO-MAGNETOTELLURIC METHOD (CSAMT)

Precise seismic two-way traveltime (TWT) determinations are often limi ted by the knowledge of the overburden thickness and its seismic veloc ity. 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 in dicates a SSE-dipping Mesozoic series and a similarly oriented increas ing thickness of Molasse and Quaternary. The CSAMT places tight constr aints on the apparent resistivities of the three formations: Quaternar y-300-1000 Omega m; Molasse-5-15 Omega m; Mesozoic-100-1000 Omega m. F urthermore, modelling of the CSAMT sounding data provides absolute thi cknesses 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 degre es to the SSE. The seismic velocities of the Quaternary (weathering la yer) and the Molasse can thus be determined at 800 and 2100 m s(-1), r espectively.