Wide-Angle reflectivity: another amplitude dimension

Deep-water seismic data contains high signal-to-noise ratio reflections from angles of 60 degrees and beyond. Wide-anglereflections contain information, not available in conventional 0 to30 degree seismic data, which can be used to improve structuralimages and subsurface property estimates. These reflections add athird dimension to the two-dimensional reflectivity model thatunderpins conventional AVO analysis. The viability of acquiringsuch reflections in the North West (NW) Shelf of Australia wasconfirmed in 1997 by a deepwater walkaway VSP that alsorevealed significant accompanying earth transmission effects.

NW Shelf marine, wide-angle (pre-critical) reflections from 3km target depths have been acquired with maximum offsets ofabout 5 km. Processing wide-angle data into subsurfacereflectivity required extensions to several processing applicationsincluding amplitude recovery (divergence, elastic/inelastic earthtransmission), reflection angle estimation, velocity analysis, anddeconvolution. Wide-angle information was explicitly retained forinterpretation by producing wide-angle stacks to complement zerooffset projection stacks and 0–30 degree stacks. Quantification ofwide-angle reflectivity is proceeding with improvements to AVOgradient estimation and with weighted stack inversions forindependent estimates of density, compressional and shearvelocities across reflector interfaces. Translation of these eventresponses into layer-property constraints can be achieved throughmodelling or by various attribute analysis techniques.

The reward for acquiring deep-water, wide-angle information isreduced resource uncertainty due to increased confidence in thedetection and the translation of amplitude anomalies intosubsurface causes. A significant remaining challenge is to detectand compensate for earth transmission effects, induced by smallscalelateral earth heterogeneity that can perturb the earth'sreflectivity response.