Improving the interpretability of air-gun seismic reflection data using deterministic filters: A case history from offshore Cape Leeuwin, southwest Australia

Abstract

To identify drilling targets for an Integrated Ocean Drilling Program project to investigate high-latitude black shales required reinterpretation of legacy seismic data. The original processing had identified the major structures but was of insufficient resolution to map the more-subtle markers at the top of the shale sequence. By reprocessing these 2004 vintage 2D air-gun marine seismic reflection data we show that the application of filters determined from deepwater data yields subbottom geological imaging superior to statistical methods and arguably better than modeled source deconvolution methods, particularly for recovery of low frequencies. The data were acquired to the southwest of Australia in an area with swells that are typically 2–4 m and cause distortions to the predicted source and receiver response functions. These distortions cannot be incorporated in an idealized modeled source function; hence, we have opted to design the deterministic filters from the seismic data. We applied the deconvolution in two steps: a prestack filter to suppress the air-gun bubble pulse signal and a poststack filter to suppress the notches in the amplitude spectrum caused by the free-surface reflections at the source and the receiver. Through this strategy, we expanded the seismic data bandwidth at the low and high frequencies and improved resolution. The tie with the single borehole in the area was significantly improved and has enabled a more-confident interpretation of the shale horizons.