Chapter 2: Petrophysics
John P. Castagna, Michael L. Batzle, Kenneth M. Tubman, James E. Gaiser, Michael D. Burnett, M. L. Batzle, T. K. Kan, 1993. "Petrophysics", Offset-Dependent Reflectivity–Theory and Practice of AVO Analysis, John P. Castagna, Milo M. Backus
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Quantitative calibration of amplitude variation with offset data by seismic modeling requires S-wave velocity information. The purpose of this paper is to review the means of acquiring S-wave velocities in boreholes by: (1) laboratory measurements on core samples, (2) pseudo-shear velocity logs derived from empirical trends and conventional well log data, (3) conventional full-waveform sonic logs, (4) direct S-wave sonic logs, and (5) multicomponent vertical seismic profiles with “point” or horizontal sources.
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Offset-Dependent Reflectivity–Theory and Practice of AVO Analysis
“The P-wave reflection coefficient at an interface separating two media is known to vary with angle of incidence. The manner in which it varies is strongly affected by the relative values of Poisson’s ratio of the two media. For moderate angles of incidence, the relative change in reflection coefficient is particularly significant when Poisson’s ratio differs greatly between the two media.
Theory and laboratory measurements indicate that gas sands tend to exhibit abnormally low Poisson’s ratios. Embedding a low velocity gas sand into sediments having ‘normal’ Poisson’s ratios should result in an increase in reflected P-wave energy with angle of incidence. This phenomenon has been observed on conventional seismic data recorded over known gas sands.”
With these words, W. J. Ostrander ushered in a new era in seismic interpretation. Although many workers were aggressively moving forward with amplitude variation with offset (AVO) analysis before 1982, Ostrander’s presentation at the 52nd Annual International Meeting of the Society of Exploration Geophysicists popularized the concept as an exploration tool. The presentation proved to be a precursor to the avalanche of literature on the subject which has appeared during the past decade.