PSEUDO-UNCONFORMITIES IN SEISMIC MODELS OF LARGE OUTCROPS

Traditionally, seismic modeling has concentrated on one-dimensional bo rehole modeling and two-dimensional forward modeling of basic structur al-stratigraphic schemes, which are directly compared with real seismi c data. Two-dimensional seismic models based on outcrop observations m ay aid in bridging the gap between the detail of the outcrop and the l ow resolution of seismic lines. Examples include the Dolomites (North Italy), the Vercors (SE France), and the High Atlas (Morocco). The sei smic models are generally constructed using the following procedure: ( a) construction of a detailed lithologic model based on direct outcrop observations; (b) division of the lithologic model into lithostratigr aphic units; (c) assignment of petrophysical properties to these litho stratigraphic units; (d) ray tracing to compute time- or depth section s of reflectivity; (e) convolution of the reflectivity sections with s ource wavelets of different frequencies. The lithologic detail modeled in the case studies led to some striking results, particularly the di scovery of pseudo-unconformities. Pseudo-unconformities are unconformi ties in seismics, but correspond to rapid changes of dip and facies in outcrop. None of the outcrop geometries studied were correctly portra yed seismically at 25-Hz peak frequency. However, in some instances th e true relationship would gradually emerge at peak frequencies of 50-1 00 Hz. The examples given in this study demonstrate that detailed, out crop-derived, seismic models can reveal what stratigraphic relationshi ps and features are likely to be resolved under ideal or less-ideal co nditions, and what pitfalls may befall the interpreter of real seismic data.