Pseudo-unconformities in seismic models of large outcrops

Abstract

Traditionally, seismic modeling has concentrated on one-dimensional borehole modeling and two-dimensional forward modeling of basic structural-stratigraphic schemes, which are directly compared with real seismic data. Two-dimensional seismic models based on outcrop observations may aid in bridging the gap between the detail of the outcrop and the low resolution of seismic lines. Examples include the Dolomites (North Italy), the Vercors (SE France), and the High Atlas (Morocco). The seismic 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 lithostratigraphic units; (c) assignment of petrophysical properties to these lithostratigraphic units; (d) ray tracing to compute time- or depth sections of reflectivity; (e) convolution of the reflectivity sections with source wavelets of different frequencies. The lithologic detail modeled in the case studies led to some striking results, particularly the discovery of pseudo-unconformities. Pseudo-unconformities are unconformities in seismics, but correspond to rapid changes of dip and facies in outcrop. None of the outcrop geometries studied were correctly portrayed seismically at 25-Hz peak frequency. However, in some instances the true relationship would gradually emerge at peak frequencies of 50–100 Hz. The examples given in this study demonstrate that detailed, outcrop-derived, seismic models can reveal what stratigraphic relationships and features are likely to be resolved under ideal or less-ideal conditions, and what pitfalls may befall the interpreter of real seismic data.