Case History Improving the interpretability of air-gun seismic reflection data using deterministic filters: A case history from offshore Cape Leeuwin, southwest Australia Colin Sargent 1 , Richard W. Hobbs 1 , and Darren R. Gro ¨ cke 1 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 insuf- ficient 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 appli- cation of filters determined from deepwater data yields subbot- tom 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 can- not 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 spec- trum 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 sig- nificantly improved and has enabled a more-confident interpre- tation of the shale horizons. INTRODUCTION Deconvolution is a fundamental step in the processing of seis- mic reflection data, and despite advances in acquisition systems and processing algorithms, it can still be problematic. The func- tion of a deconvolution filter is to improve resolution, and its uses include suppressing short-period multiples, compressing the source function, and compensating for acquisition filters. Decon- volution is also important in the suppression of longer-period multiples, e.g., surface-related multiple elimination (SRME) (Verschuur et al., 1992), which are more robust if the source function is known or the data are preconditioned to compress the source function to a band-limited spike. Modern acquisition systems provide the means to directly estimate the source during the survey (Ziolkowski et al., 1982), which is then used to design the deconvolution filter, but this technology is not avail- able for the majority of the legacy data that are the focus of this paper. For these data, trace-by-trace statistically derived filters based on Weiner theory are still widely used even though the underlying assumptions are violated. Although developments of other types of filter design and optimization can yield improved resolution, e.g., maximum entropy and time-varying deconvolu- tion (Van der Baan, 2008), here we develop a simple yet robust prestack deconvolution filter strategy specifically targeted to compress the source function. Our prestack filter does not require time-varying compensation or transformation of the data into the s-p domain to ensure periodicity as it is nondirectional. Then a poststack filter is used to maximize resolution. The objective of the research is to delineate a thick sequence of Cretaceous black shale in preparation for a proposal to the Manuscript received by the Editor 22 January 2010; revised manuscript received 28 August 2010; published online 20 May 2011. 1 Durham University, Department of Earth Sciences, Durham, U. K. E-mail: colin.sargent.uk@googlemail.com; r.w.hobbs@durham.ac.uk; d.r.grocke@ durham.ac.uk. V C 2011 Society of Exploration Geophysicists. All rights reserved. B113 GEOPHYSICS, VOL. 76, NO. 3 (MAY-JUNE 2011); P. B113–B125, 10 FIGS., 2 TABLES. 10.1190/1.3554396 Downloaded 23 May 2011 to 129.234.252.65. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/