Abstract

Seismic models of outcrops have shown that near the limits of resolution the seismic reflection tool records rapid facies changes as false lap-outs. We examined one such outcrop, the Triassic platform flank of Picco di Vallandro (Durrenstein) in northern Italy, to develop criteria for recognizing false lap-out patterns. Using measured sections, photo mosaics of the continuous outcrop, and petrophysical measurements on samples, two contrasting impedance models were constructed. One model, supported by field observation, shows interfingering between basin marls and slope carbonates. The second model shows onlap of basin marls against slope carbonates, but with similar overall geometries to the first model. Vertical incidence seismic models were produced at frequencies between 50 and 150 Hz and with displays of seismic amplitude, as well as two seismic attributes-instantaneous phase and reflection strength. At 50 Hz, the two lithologic models are seismically indistinguishable, both showing onlap of basin units onto the slope. At 90 Hz, displays of instantaneous phase begin to reveal the stratigraphic interfingering, whereas seismic amplitude displays require frequencies of 150 Hz to do so. Reflection strength accurately portrays the distribution of the two major facies in the entire range of examined frequencies, but does not show sufficient detail to reveal interfingering vs. onlap. The same trends were visible with finite difference modeling of the full waveform. Here, the differences between interfingering and onlap become visible at 150 Hz in the phase model, where seismic amplitude displays at this frequency are still ambiguous. The phase model, however, fails to differentiate between slope dip and the basin sediment wedge, whereas this wedge is clearly mapped by the reflection strength. A combination of instantaneous phase and reflection strength provides a decisive advantage over conventional amplitude displays in recognizing rapid facies changes in seismic reflection data.

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