Analysis of converted refractions for shear statics and near-surface characterisation

Abstract

Converted-wave refraction statics is an algorithm that incorporates both compressional (P-) and shear (S-) wave refraction events to correct S-wave static errors in multi-component seismic data. Converted refraction (PPS) events are picked on an inline component shot record in the same way as first-breaks are picked on a normal vertical component record. These picks are then analysed using the reciprocal method to create a near-surface model from which S-wave receiver statics are derived. The derived PPS refraction statics have a similar short-wavelength character to the S-statics obtained via statistical analysis of converted-wave reflections. Based on standard P-wave practice, we believe that an optimal production approach will include converted-refraction analysis, followed by converted-wave residual statics. Although the thrust of this work has been towards derivation of S-wave statics, an interesting auxiliary output is also available. Based on theoretical modelling, the S-to-P time-depth ratios can be tuned to provide P-to-S velocity ratios (and hence dynamic Poisson's ratios) for the near-surface. This has interesting implications for lithological and rock-strength analysis in the mining, environmental and engineering contexts.