Chapter 3: Processing
Jeffrey R. Resnick, Peter C. Luh, Z. Shang, J. A. McDonald, G. H. F. Gardner, 1993. "Processing", Offset-Dependent Reflectivity–Theory and Practice of AVO Analysis, John P. Castagna, Milo M. Backus
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Clues to the lithology and fluid content of an exploration target may be revealed by analyzing variations of seismic reflectivity with source-receiver offset or reflection angle. Field recordings of seismic data, however, do not directly indicate target reflectivity, because numerous other factors (for example, geometric spreading, variations in shot strength or receiver coupling, multiple reflections, ground roll, transmission loss, dissipation, and noise) affect seismic amplitude (Ostrander, 1984; Dey-Sarkar, et al., 1986).
The contaminating influence of many of these factors can be ameliorated through data processing. Regrettably, the benefits of data processing cannot be realized without some risks: removing noise or other unwanted influences from seismic data may also distort or damage the underlying signal. Therefore, when data are processed for amplitude variation with offset (AVO) or amplitude variation with angle (AVA) analysis, the interpreter must know how the processing has affected the final results. To minimize processing artifacts and aid the interpreter's understanding of the processing strategy, displays of the data at the completion of processing and at several intermediate stages of the data processing flow are especially helpful.
Figures & Tables
“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.