ANSELMETTI, FLAVIO S., GREGOR P. EBERLI, and DANIEL BERNOULLI
Seismic sequence analyses performed on synthetic seismic sections across a steep-sided carbonate platform margin (Montagna della Maiella, Italy) indicate that the record of seismic sequences is strongly dependent on the seismic acquisition parameters and the depositional morphology. In particular, variations in seismic frequency affect the seismic stratigraphic image in a way that the total number of seismic sequences increases with increasing frequency.
The lithologic content and the geometries of seven unconformity-bounded supersequences could be mapped on well-exposed, valley flank-sized outcrops of the Cretaceous to Tertiary platform-to-basin transition. Velocity and density measurements on approximately 200 outcrop samples revealed the petrophysical signature of all lithologies. These data, together with the geometries, provided the database to construct a layered impedance model which was convolved with a seismic wavelet using the normal incidence raytracing method and frequencies between 20 and 60 Hz. Seismic unconformities were traced on the synthetic seismic section allowing for a seismic sequence analyses at all chosen frequencies. Accurate sequence stratigraphic interpretation was complicated by the steep non-depositional escarpment that separates the Cretaceous platform deposits from the adjacent basinal sediments. The escarpment is seismically seen as one onlap surface, i.e. one sequence boundary, while in reality, time lines merge and eventually cross this surface. At low frequencies (20 Hz), only five of the seven depositional sequences can be resolved on the seismic section, because two of the six unconformities are not seismically recorded. A frequency increase to 40 and 60 Hz results in six and seven detected seismic sequences, respectively. The fact, that in this case a frequency of 60 Hz is needed to image all depositional sequences documents the pitfalls of using sequence analyses to make age correlations based solely on seismic data. Low seismic frequency prevents thin depositional sequences and/or those with subtle geometric unconformities to be seismically detected.