Chapter 4: Interpretation
Published:January 01, 1993
Edward F. Chiburis, Ming-Ren Hong, John P. Castagna, Charles J. Sicking, Jose Regueiro, K. H. Wrolstad, Robert Estill, 1993. "Interpretation", Offset-Dependent Reflectivity–Theory and Practice of AVO Analysis, John P. Castagna, Milo M. Backus
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The past several years have seen steps taken in the application of amplitude variation with offset (AVO) techniques to prestack reflection data. These steps were obviously prompted by an attempt to exploit information contained in the additional dimension and by knowing the theoretic amplitude changes which can occur due to different lithologies and contained fluids. Offsets of 2000 m for both land and marine programs provide sufficient sampling to detect AVO changes, which may be related to change in formation properties. Regardless of source-and receiver-array attenuation effects and of the use of vertical-component receivers on land, the offset dimension can provide more significant information relevant to reservoir and stratigraphic properties than is usually appreciated.
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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.