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The AAPG/Datapages Combined Publications Database

GCAGS Transactions


Gulf Coast Association of Geological Societies Transactions
Volume 54 (2004)

EXTENDED ABSTRACT: Conforming and Non-Conforming Sands - An Organizing Framework for Seismic Rock Properties


Young, Roger A.1 and LoPiccolo, Robert D.1


A new classification for AVO (amplitude versus offset) has been proposed to include all possible combinations of near and far trace polarities and amplitudes. This classification admits all sands regardless of the sign of the impedance contrast of the sand with the overlying shale or the direction (positive or negative) of change in impedance contrast reflectivity with increasing offset.

Previous AVO classifications have assigned gas sands to various classes (one through three, four, or five - depending on the author). These classes cover only a subset of the possible combinations of reflectivity that can occur with offset. Generally, the tops of sands in these classes are characterized by negative reflection coefficients or by positive reflection coefficients, which become negative with increasing offset. In the new classification these sand classes are defined as conforming types, whereas sands with positive reflection coefficients on both near and far offsets or those, which have negative reflection coefficients becoming more positive with increasing offset, are described as non-conforming types.

By providing a complete classification of AVO types a solid framework is established for systematically investigating rock and fluid properties, of each of the resulting ten AVO types, with seismic data. An understanding of the characteristics of the two categories of sand types and the variations of AVO response for the different sands within the categories is critical in accurately assessing any geologically-interesting property.

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