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Predicting Geometry and Stacking Pattern of Thinly Interbedded Depositional Systems

Abstract

I introduce a seismic lithology and seismic geomorphology (seismic sedimentology)-based approach of interpreting the spatial geometry and stacking pattern of superimposed, seismically thin beds. The method is applicable to conventional, low-frequency seismic data, and requires knowledge of the basic shape of the seismic wavelet and lithology-impedance relationship (impedance model). The method is primarily aimed at those beds that are broad enough to be horizontally resolved, but so thin as to only be vertically detected with given bandwidth. The method assumes the data have been converted from time slices in the time (or depth) domain to stratal slices in the relative geologic time (or Wheeler) domain. A simple one-bed model illustrated that a thin-bed depositional system can be characterized by a seismic-geomorphologic pattern of the same spatial shape on sequential relative geologic-time (stratal) slices but the amplitude, phase and polarity will vary depending upon the known, estimated seismic wavelet. This phenomenon can be captured and evaluated in the thin bed's response window in Wheeler domain. If multiple thin-bed units are present in the response window, seismic responses from vertically adjacent units will interfere with the “true” seismic-geomorphologic pattern of any single thin-bed unit. The composite waveform for each of the units can be restored in variable quality, depending on its geomorphologic character, thickness, and stratigraphic position. Relative traveltime differences of thin-bed waveforms reveal the depositional history (stacking pattern) of the thin-bed sequence. A field-data test confirmed that within a stratigraphic interval of a composite seismic event (λ/4 or 15 m) at least 2, possibly 3, thin fluvial channel sandstones (2–6 m each) could be identified and their spatial localities and distribution unraveled.