Weighted Stacking for Rock Property Estimation and Detection of Gas*

Abstract

SMITH, G.C. and GIDLOW, P.M. 1987, Weighted Stacking for Rock Property Estimation and Detection of Gas, Geophysical Prospecting 35,993-1014.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.

Fractal and Chaos in Exploration Geophysics

“…The proposed analysis can help the hydrocarbon trapping research and analysis, fracture detection and fractured reservoirs analysis. We suggest application of the proposed philosophy at real seismic AVO data and its attribute, for example the Intercept, The gradient, the Fluid Factor proposed by Smith and Gildow [34] and the attribute product Intercept*Gradient .…”

Frequency-dependent AVO attribute: theory and example

“…The angles of incidence around 2.0 s are 0-16°, substantially less than 30°, which is the requirement for the two-term AVO approximation of Smith and Gidlow (1987). Nevertheless, the first 45 traces are able to delineate the changing seismic amplitude with offset.…”

“…In this paper, we introduce a scheme to calculate a frequency-dependent AVO attribute in order to estimate seismic dispersion from pre-stack data, and apply it to North Sea data. The scheme essentially combines the two-term approximation of Smith and Gidlow (1987) with the method of spectral decomposition based on the Wigner-Ville distribution, which is used to achieve high resolution. The result suggests the potential of this method for detection of seismic dispersion due to fluid saturation.…”

Amplitude versus offset analyses of the deep sedimentary structures at the northern flank of the Porcupine Basin, SW of Ireland

“…The Zoeppritz equation can be simplified by making some approximations that describe the variation of P-wave reflection coefficients with the angle of incidence as a Int J Earth Sci (Geol Rundsch) function of the P-wave velocity, the S-wave velocity and the densities above and below the interface (Smith and Gidlow 1987).…”

“…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 CDP-gather (Smith and Gidlow 1987).…”