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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
1Manuscript received February 2, 1998;
revised manuscript received December 15, 1998; final acceptance January
10, 1999.
2Vrije Universiteit, Institute of
Earth Sciences, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands; e-mail:
[email protected]
ABSTRACT
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.
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 (Dürrenstein) in northern Italy, to develop
criteria for recognizing false lap-out patterns.
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