Characterization of seismic patterns of cave responses: A forward modeling study based on the digital outcrop model of the Cristal cave, NE Brazil

Abstract Seismic modeling of carbonate reservoir analogs bridges the gap between outcrop and seismic scales. A forward modeling study based on the Cristal cave karst system (São Francisco Craton, Brazil) is presented. A digital outcrop model (DOM) of the Cristal cave, lithostratigraphic columns, and petrophysical measurements were used to build 2D models of cave systems. The shapes, dimensions, and horizontal spacing of the caves were varied so that 12 different scenarios were composed, which may also include silicified halos. The resulting raw seismic sections were processed following a conventional seismic workflow. Taking into consideration only the resolvability along horizontal and vertical directions, four patterns of cave responses are distinguished: (1) tabular string of bead responses (SBRs) caused by caves with relatively small sizes and horizontal spacings, as in the actual DOM of the Cristal cave, which cannot be resolved either horizontally or vertically; (2) short SBRs caused by caves with relatively small sizes and large horizontal spacing so that they can be resolved horizontally but not vertically; (3) superposed diffraction patterns caused by relatively large caves and small spacing so that they can be resolved vertically but not horizontally; and (4) isolated diffraction patterns caused by relatively large caves and large spacing so that they can be resolved horizontally and vertically. Pull-down effects are observed in most of these patterns. Silicified halos enhance the signals of the small caves but attenuate the signals of large caves. A seismic cube was simulated in the scenario of caves with relatively small sizes, but with large horizontal spacing. A strong correlation between the cave system and the SBRs is evidenced by the seismic cube. The insights gained from this study offer valuable guidance for interpreting cave systems in seismic images.