A comprehensive AVO classification

Abstract

In this paper a new classification for AVO responses is proposed. This classification covers all possible AVO responses and is independent of the fluid content of the beds. In 1989 Rutherford and Williams introduced a threefold classification of AVO (amplitude versus offset) characteristics for seismic reflections from the interface between shales and underlying gas sands. The classification scheme they proposed has become the industry standard and has proven its validity and usefulness in countless exploration efforts. The Rutherford and Williams classification is primarily based on the sign and magnitude of the reflection coefficient (R 0) at the top of the sand. These authors went on to observe that, owing to the high Poisson's ratio values for the typical encasing shales, relative to the gas sands, the AVO response of all of these gas-sand classes would be characterized by an increasingly negative reflection coefficient with increasing offset. Their classification is explicitly defined for gas sands. In 1997 Castagna and Swan proposed AVO crossplotting wherein an estimate of the normal-incidence reflectivity is plotted against a measure of the offset-dependent reflectivity. Using this approach Castagna and Swan graphically illustrated the continuum between the classes and defined the characteristics of the classes using what they termed AVO intercept and AVO gradient. They also added a class 4, which they believed was originally included in Rutherford and Williams' class 3 definition. The authors of the present paper propose to expand and partly redefine the currently used classifications. In order to distinguish among the schemes the term types, as opposed to classes, is used to name the different categories. And, in order to preserve congruity, where the schemes overlap, the sands, which Rutherford and Williams would call class 1, 2, or 3, are referred to as types 1, 2, or 3. Type 4 of the proposed classification includes parts of the domains, which Castagna and Swan assigned to their class 4 and their class 3. The proposed classification is intended to provide an unambiguous way of sorting the entire range of combinations of normal-incidence reflectivity and offset-dependent reflectivity, irrespective of the cause of the offset-dependent amplitude variations. A type 5 replaces part of Castagna and Swan's class 4, and types-1 through-5 are added which complete the spectrum. It is further suggested that AVO effects are best described as being not just between shales and underlying sands, but between nonshale lithologies and the overlying rock (whether it be shale or not). Because nonshale lithology is rather an awkward term, the name "sand" will be used in the following discussion, but the reader is asked to bear in mind that a larger and not especially accurate definition of sand is implied. Clearly, the classification goes beyond gas sands in its intended application. Variation in AVO should be considered a physical phenomenon, the root cause of which requires interpretation.

Reflective body observed by multi-channel reflection survey off Niijima and Kozushima, Japan

“…6c shows amplitude variations of both cases of Figs 6a and 6b on a graph with the relative amplitude versus the angle. As many authors reported, the former is a well-known pattern of waterbearing rocks while the latter is one of the typical patterns of gas-bearing rocks e.g., 20,21) .…”

Integrated Avo, Elastic Seismic Inversion and Petrophysical Analysis for Reservoir Characterization: A Case Study of Gas Field, South Sumatera Basin

“…The classifications expand and redefine the commonly used AVO type from Rutherford-Williams and Castagna-Swan. The difference between Young-Piccollo and Rutherford-Williams scheme is in the sand layer, which is designated to be conforming or nonconforming [10]. The conforming sands are re-divided to consider AVO response type 5.…”

“…The conforming sands are re-divided to consider AVO response type 5. The AVO response type is determined based qudrant classification that consider the AVO attributes (intercept and gradient) [10].…”

Variation of physical properties beneath a fault observed by a two‐ship seismic survey off southwest Japan

“…A classification of AVO effects. Ten types of AVO effects are conceptually shown for gas‐bearing and nongas‐bearing rocks [ Young and LoPiccolo , 2003]. …”

“…On the other hand, in Figures 8b and 8d, the amplitudes are negative at small angles and change to positive values at large angles. The former is categorized in type −1 and the latter is type −5 according to the classification of nongas‐bearing sands by Young and LoPiccolo [2003].…”