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Along the southern margin of the High Atlas of Morocco, Early Jurassic carbonate platforms developed in a transtensional, rifted basin. One of these platforms was the Djebel Bou Dahar platform, which grew to a vertical relief of 500 m, drowned, and was onlapped by deep-marine sediments.
Exceptional outcrops allowed detailed lithologic models of the platform to be made with a resolution of a few meters. We constructed a geometric model from building blocks comprising particular rock types. We estimated P-wave velocities and bulk densities of these lithologies from hand specimens, and we assumed these properties to be homogeneous and invariant throughout the building blocks. We constructed time sections of reflectivity from vertical incidence and image-ray migration techniques. These time sections were convolved with 25 to 200-Hz Ricker wavelets to produce synthetic seismic sections.
Large-scale features, such as the platform and the drowning unconformity, were resolved regardless of the frequencies and modeling techniques used. Broad, low-frequency wavelets resulted in profiles with apparent truncation of bedding and basinward shifts in the point of onlap. Smaller scale features, such as reef knolls and sand bodies at the platform margin, were poorly resolved at frequencies up to 50 Hz. However, we inferred the existence and location of these smaller features from the disturbance and distortion of other reflections. This distortion is most obvious in image-ray migration modeling because of the refraction of rays through the small-scale features.
Geometric effects caused lateral amplitude variations due to wavelet interference along the unconformity reflection where no petrophysical changes were modeled.
The seismic models of the Djebel Bou Dahar in its present stage may be good analogs for seismic lines of deeply buried carbonate platforms. To make a comparison with real seismic lines of less deeply buried platforms possible, we reconstructed both the geometry and petrophysical properties, and the subsequent seismic modeling of the Djebel Bou Dahar carbonate platform as it was prior to the onset of deep burial. At this stage, impedance contrasts were at their maximum.
This study clearly demonstrates the significance of modeling the seismic response of a detailed, outcrop-based, stratigraphic model, and shows that seismic modeling of specific sequence stratigraphy can reveal what features are likely to be resolved under ideal conditions and what pitfalls may await the interpreter.
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