Hybrid seismic inversion: A reconnaissance tool for deepwater exploration

Abstract

When exploring new areas, it is desirable to have a robust inversion methodology that can obtain reliable results in a reasonable time. In addition, it is also desirable that such inversion be run with or without any a priori well information. This is particularly important in deep water where well information is sparse and seismic data is the only information on which the initial exploration strategy can be based. This paper describes a combination of prestack and poststack inversion that allows efficient inversion of large data volumes in the absence of well information. This hybrid methodology first runs prestack genetic algorithm (GA) inversion at discrete locations over the entire data volume. Detailed elastic models obtained from prestack inversion then constrain background trends for poststack inversion. The prestack GA inversion is applied to prestack CMP gathers (Mallick, 1999). In essence, this generates a random population of elastic earth models within a specified search interval and computes synthetic data using each random model. Synthetic data are then matched with observations. Based on the degree of match between synthetic and observed data, random models are then modified using a GA search algorithm. This process—modifying models, computing synthetics, and matching with observations—continues until convergence. Experience has proven that prestack inversion is sensitive to parameterization, as demonstrated in the inversion of the real data set in Figure 1a. The zone of interest is a carbonate layer at 2.9–3.1 s. We ran prestack inversion over the time interval 2.4–3.4 s. Knowledge of the regional geology suggested mainly sand/shale lithology at 2.4–2.9 s and carbonate/shale lithology at 2.9–3.4 s. Our prestack inversion used search windows consistent with this lithology. Figures 1b and 1c show two prestack inversion results. The main difference between them is that twice as many layers were used to describe the elastic earth …