Chapter 15: Seismic Modeling of a Carbonate Platform Margin (Montagna della Maiella, Italy): Variations in Seismic Facies and Implications for Sequence Stratigraphy
Flavio S. Anselmetti, Gregor P. Eberli, Daniel Bernoull, 1997. "Seismic Modeling of a Carbonate Platform Margin (Montagna della Maiella, Italy): Variations in Seismic Facies and Implications for Sequence Stratigraphy", Carbonate Seismology, Ibrahim Palaz, Kurt J. Marfurt
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Synthetic seismic sections across the exposed Cretaceous-Miocene carbonate platform margin of the Montagna della Maiella (central Italy) explain the seismic facies of a carbonate platform margin system and show the limitations of relating seismic sequences to depositional sequences. To define a layered impedance model, velocities and densities of 186 minicores from all major out-cropping lithologies were determined. The impedance model was converted to synthetic seismic data by applying a computer-simulated model that uses the normal incidence ray-tracing method at variable frequencies, amplitude gains, and noise levels. The resulting synthetic seismic sections show a mostly transparent platform that is onlapped along the escarpment by a succession of high-amplitude slope reflections. The different reflectivities of platform and slope can be explained by their differences in impedance contrasts. The small impedance contrasts within platform carbonates results in weak reflections nearly indistinguishable from noise, whereas the large impedance contrasts within the slope and basin carbonates yield coherent high-amplitude reflections. The seismic image with incoherent to transparent platform, high-amplitude slope reflections, and recognizable prograding units is similar to observed seismic data across other steep carbonate platform margins (e.g., Great Bahama Bank and Adriatic Sea).
In outcrop, seven unconformity-bounded supersequences were mapped. Comparison with the synthetic seismic section shows that, at a frequency of 20 Hz, only five of these depositional super-sequences can be recognized using seismic unconformities. With an increase in frequency, an increasing number of unconformities become visible, and at a frequency of 60 Hz, all seven are imaged. The synthetic seismic sections also reveal that some of the seismic unconformities are pseudo-unconformities—they do not exist in outcrop, but the seismic image shows erroneous or nonexistent geometric patterns. These are a result of the thinning of layers below seismic resolution. These observations document the problem of seismically imaging depositional sequences. Depending on the dominant frequency, an erroneous number of sequences might be interpreted. This limitation must be taken into account when making sequence stratigraphic interpretations based solely on seismic information.