Wide-Angle reflectivity: another amplitude dimension

Abstract

Deep-water seismic data contains high signal-to-noise ratio reflections from angles of 60 degrees and beyond. Wide-angle reflections contain information, not available in conventional 0 to 30 degree seismic data, which can be used to improve structural images and subsurface property estimates. These reflections add a third dimension to the two-dimensional reflectivity model that underpins conventional AVO analysis. The viability of acquiring such reflections in the North West (NW) Shelf of Australia was confirmed in 1997 by a deepwater walkaway VSP that also revealed significant accompanying earth transmission effects. NW Shelf marine, wide-angle (pre-critical) reflections from 3 km target depths have been acquired with maximum offsets of about 5 km. Processing wide-angle data into subsurface reflectivity required extensions to several processing applications including amplitude recovery (divergence, elastic/inelastic earth transmission), reflection angle estimation, velocity analysis, and deconvolution. Wide-angle information was explicitly retained for interpretation by producing wide-angle stacks to complement zero offset projection stacks and 0?30 degree stacks. Quantification of wide-angle reflectivity is proceeding with improvements to AVO gradient estimation and with weighted stack inversions for independent estimates of density, compressional and shear velocities across reflector interfaces. Translation of these event responses into layer-property constraints can be achieved through modelling or by various attribute analysis techniques. The reward for acquiring deep-water, wide-angle information is reduced resource uncertainty due to increased confidence in the detection and the translation of amplitude anomalies into subsurface causes. A significant remaining challenge is to detect and compensate for earth transmission effects, induced by small-scale lateral earth heterogeneity that can perturb the earth's reflectivity response.