Chapter 5: Inversion
Published:January 01, 1993
Emin Demirbag, Cahit Çoruh, John K. Costain, James J. Carazzone, Leonard J. Srnka, 1993. "Inversion", Offset-Dependent Reflectivity–Theory and Practice of AVO Analysis, John P. Castagna, Milo M. Backus
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The estimation of P- and S-wave seismic velocities and densities of earth layers from the data sets recorded at the surface has long been one of the main research interests in reflection seismology. Although structural interpretations of the subsurface with sophisticated data processing and interpretation techniques have been facilitated, there remains a lot more to be done in obtaining quantitative information about the seismic parameters to lead to lithological interpretation that will open the ways to direct hydrocarbon exploration, groundwater exploration, and understanding the deep earth crust.
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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.