WEIGHTED STACKING FOR ROCK PROPERTY ESTIMATION AND DETECTION OF GAS

Abstract

Amplitude versus offset concepts can be used to generate weighted stacking schemes (here called geo‐stack) which can be used in an otherwise standard seismic data processing sequence to display information about rock properties. The Zoeppritz equations can be simplified and several different approximations appear in the literature. They describe the variation of P‐wave reflection coefficients with the angle of incidence of a P‐wave as a function of the P‐wave velocities, the S‐wave velocities and the densities above and below an interface. Using a smooth, representative interval velocity model (from boreholes or velocity analyses) and assuming no dip, the angle of incidence can be found as a function of time and offset by iterative ray tracing. In particular, the angle of incidence can be computed for each sample in a normal moveout corrected CMP gather. The approximated Zoeppritz equation can then be fitted to the amplitudes of all the traces at each time sample of the gather, and certain rock properties can be estimated. The estimation of the rock properties is achieved by the application of time‐ and offset‐variant weights to the data samples before stacking. The properties which can be displayed by geo‐stack are: P‐wave reflectivity (or true zero‐offset reflectivity), S‐wave reflectivity, and the reflectivity of P‐wave velocity divided by S‐wave velocity (or ‘pseudo‐Poisson's ratio reflectivity’). If assumptions are made about the relation between P‐wave velocity and S‐wave velocity for water‐bearing clastic silicate rocks, then it is possible to create a display which highlights the presence of gas. Copyright © 1987, Wiley Blackwell. All rights reserved