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Effect of finely-layered stiff carbonates on a seismic response. Northern Carnarvon basin synthetic study

Fine layering is known to cause both seismic attenuation and VTI-anisotropy. In typical geological environments, the contrast in elastic properties of adjacent layers rarely exceeds 30% so that the layer-induced effects are negligible. However, it’s not true for the overburden of Northern Carnarvon basin (Northwest shelf of Western Australia) that is characterized by very stiff carbonates alternated with more porous and softer rocks. In this paper, we present a workflow for preliminary analysis of seismic wavefield and, in particular, effects of layer-induced scattering attenuation and anisotropy in the target area. The workflow is based on the walk-away VSP full-wave modelling (5-100 Hz) for the flat-layered elastic models that are constructed using logs of four wells, namely, Dampier 1, Parker 1, Wilcox 2 and Withnell 1. We show that particular sequences with carbonates produce significant amplitude loss and degradation of spectrum of a transient seismic pulse. Maximum attenuation is observed for Withnell 1 borehole and is characterized by the drop of 30% in the centroid frequency in the 200 m interval. Anellipticity parameter n is estimated by fitting of the moveout curves and varies from 0.1 to 1.5. In addition, the modelling reveals a very complex wavetrain with energetic reflected and converted waves at large offsets. All these effects should be taken into account in seismic processing and imaging.