Shear-wave elastic impedance

It is well known that if we are able to estimate acoustic impedance (AI) and a parameter related to shear-wave velocity from seismic data, our ability to discriminate between different lithologies and fluid phases will increase. Prestack inversion on individual CDP gathers and inverting directly for VP , VS , and density have been tested in several ways, but the estimated parameters are often poorly determined. A more robust approach is to apply poststack inversion on partial stacks. For inversion of the near-offset stack, AI can be calculated directly from well logs. However, for the far-offset stack, we need to derive an equivalent of the acoustic impedance that can be used to calibrate the non-zero-offset seismic reflectivity. Connolly (1998, 1999) derived such an equivalent—elastic impedance (EI)—and demonstrated how, by using elastic impedance logs (which requires shear-wave logs), he was able to perform well calibration and inversion of far-offset data. This article describes a new function—shear-wave elastic impedance (SEI)—for linking converted-wave stacks to wells using a linearization of the Zoeppritz equations. SEI is similar to EI but adapted to P-S converted seismic data. It can be computed from acoustic log data ( P - and S -wave velocities and density) and used for well calibration, wavelet estimation, and inversion of P-S reflectivity data leading to improved interpretability of converted wave data. (Equation 4 in Appendix 1 is the key mathematical formula in this approach to SEI). To test the SEI technique, we used a 3-D four-component (4-C) ocean-bottom cable (OBC) survey acquired in 1997 that covers approximately 10 km2 of the Statfjord Field (Rogno et al., 1999). Both P -wave and converted-wave data were 3-D prestack time-migrated. The hydrophone ( P ) and the vertical geophone ( Z ) component were summed to attenuate receiver side water-layer reverberations and both data sets ( PZ …