Friday, December 29, 2017

Processing for AVO Interpretation

Any properly acquired seismic survey, new or old, can be processed for AVO analysis. The goal of processing is to preserve reflected pulse shape and amplitude. Changes in pulse with offset can then be interpreted in terms of lithology or fluid contrasts at the reflector. Data destined for stratigraphic interpretation or lithostratigraphic inversion also benefit from true amplitude processing. Every seismic data set creates its own processing problems, requiring a tailor-made processing sequence. Here is a typical AVO processing sequence, one that works for the data sets described in this article.

Basic Steps

True Amplitude Recovery (TAR)compensates for amplitude loss caused by wavefront spreading and low transmission quality (Q) of the rock through which the seismic wavefront travels.

Frequency wave number (F-K) filtering is required to attenuate coherent noise generated by near-surface or seabed features such as rigs, buildings and seabed channels. Ground roll, or surface waves, common in land data, cannot usually be removed with this method. Correctly designed receiver arrays can solve this problem.

Generalized  Radon Transform (GRT) demultiple reduces amplitude of multiples (interbed or water column reverberations) relative to primary energy. Conventional demultiple techniques do not preserve true amplitudes, nor do they eliminate all multiples. The GRT demultiple separates seismic arrivals by differences in their apparent velocities, then suppresses multiples by an inverse transform of only part of the data.

Deconvolution creates a new trace with wiggles that indicate the location (in time) and the strength of each reflector. Surface-consistent deconvolution reduces pulse shape distortion because the filter is the same for each shot and receiver location.

Surface-consistent scaling and residual statics correct amplitudes and arrival times of raypaths distorted by near-surface anomalies, such as those caused by the unconsolidated ("weathered") zone on land or a rough ocean bottom.

Velocity analysis and normal moveout (NMO) create and apply the velocity model that aligns wiggles from all offset. In conventional seismic processing, velocity analyses are made every 2 to 3 km. Because most AVO anomalies are caused by velocity variation, closely spaced velocity analysis are required every 0.25 km.

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