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.

Dish structure is defined by the presence of thin, subhorizontal, flat to concaveupward, argillaceous laminations in siltstone and sandstone units. It is commonly associated with vertical or nearly vertical cross-cutting columns and sheets of massive sand termed pillars. Both form commonly in sediment ranging in grain size from coarse-grained siltstone to coarse-grained, conglomeratic sandstone. In sedimentation units greater than about 0.5 m thick, dish structure is faint and neither cuts across nor is cross-cut by other sedimentary structures. In thinner units dish structures commonly cut across primary flat laminations, climbing-ripple cross-laminations, and convolute laminations. l)ish and pillar structures form during the consolidation of rapidly deposited, underconsolidated or quick beds. During gradual compaction and dewatering, semi-permeable laminations act as partial barriers to upward-moving fluidized sediment-water slurries, forcing horizontaI flow beneath the laminations to points where continued vertical escape is possible. As water seeps upward through the confining laminations, fine sediment, planar, and low-density grains are filtered out and concentrated in the sediment pore spaces. The resulting clay-and organic-enriclaed laminations are flat dishes that may be later deformed by the upward pressure of flow around their margins and central subsidence as underlying sediment and water escape. Pillars form during forceful, explosive water escape. It is suggested that tbe shapes of dishes and pillars within an individual bed can be related to its original water content, thickness, and grain size; to the rate and magnitude of dewatering including consideration of water entering the bed from underlying consolidating sediments; and to the types and distribution of earlier-formed sedimentary structures. l)isb structures cannot be used directly to infer transport or depositional processes. \Vhere dishes are associated with or cut across primary sedimentary structures, the latter indicate deposition from currents.The study indicates that coarse-grained terrigenous sediments often have prononneed and complex consolidation histories. Many rapidly deposited beds undergo partial liquefaction mad fhfidization during consolidatiou but retain sufficient strength to resist wholesale downslope flowage in response to gravity. DISH AA'D PILLAR 5~TRUCTURES48S dishes in coarse-grained lithie sandstone. Pigeon Point Formation (Cretaceous), California. (c) Finegrained dish-structured bed showing complex and extended consolidation history; oldest dishes (1) are strongly concave, discontinuous, and have commonly been blurred by the upward flow of fluidized sediment through associated pillars; later-formed dishes (2) are flatter, more continuous, and separated vertically by sand showing well defined substructure--the dark dish lamination, an underlying zone of white claypoor sand, and an overlying zone of gray argillaceous sand. Beds showing this degree of complexity are rare and suggest more than one period of water expul...

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