Outcrop topography as a proxy of acoustic impedance in synthetic seismograms

Two-dimensional seismic models of geologic data are usually based on simplified impedance functions: large ''seismic-scale'' lithologic blocks exhibit uniform impedance values, and abrupt changes in impedance occur at the boundaries of these lithologic blocks. For outcrop-based seismic models, erosional slope topography may be one possible proxy for impedance that is relatively easy to measure in outcrop. In this paper, we use terrestrial photogrammetric techniques to establish the relationship between outcrop topography, expressed as the rock slope angle, and impedance for a marl-limestone terrain in the Vercors, southeast France. The photogrammetric surveys were combined with sedimentologic descriptions and petrophysical measurements (including P-wave velocity, 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 seismograms. A comparison of these new seismograms with published seismic models of the same area revealed the benefits of the new approach, in particular for seismic modeling using high-frequency source wavelets.