OUTCROP TOPOGRAPHY AS A PROXY OF ACOUSTIC-IMPEDANCE IN SYNTHETIC SEISMOGRAMS

Two-dimensional seismic models of geologic data are usually based on s implified impedance functions: large ''seismic-scale'' lithologic bloc ks exhibit uniform impedance values, and abrupt changes in impedance o ccur at the boundaries of these lithologic blocks. For outcrop-based s eismic models, erosional slope topography may be one possible proxy fo r impedance that is relatively easy to measure in outcrop. In this pap er, we use terrestrial photogrammetric techniques to establish the rel ationship between outcrop topography, expressed as the rock slope angl e, and impedance for a marl-limestone terrain in the Vercors, southeas t France. The photogrammetric surveys were combined with sedimentologi c descriptions and petrophysical measurements (including P-wave veloci ty, bulk-density, clay content, and porosity). The slope angle along a particular vertical profile was then converted into a pseudoimpedance log, which was subsequently used to construct 1-D synthetic seismogra ms. A comparison of these new seismograms with published seismic model s of the same area revealed the benefits of the new approach, in parti cular for seismic modeling using high-frequency source wavelets.