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A three-term (3T) amplitude-variation-with-offset projection is a weighted sum of three elastic reflectivities. Parameterization of the weighting coefficients requires two angle parameters, which we denote by the pair [Formula: see text]. Visualization of this pair is accomplished using a globe-like cartographic representation, in which longitude is [Formula: see text], and latitude is [Formula: see text]. Although the formal extension of existing two-term (2T) projection methods to 3T methods is trivial, practical implementation requires a more comprehensive inversion framework than is required in 2T projections. We distinguish between projections of true elastic reflectivities computed from well logs and reflectivities estimated from seismic data. When elastic reflectivities are computed from well logs, their projection relationships are straightforward, and they are given in a form that depends only on elastic properties. In contrast, projection relationships between reflectivities estimated from seismic may also depend on the maximum angle of incidence and the specific reflectivity inversion method used. Such complications related to projections of seismic-estimated elastic reflectivities are systematized in a 3T projection framework by choosing an unbiased reflectivity triplet as the projection basis. Other biased inversion estimates are then given exactly as 3T projections of the unbiased basis. The 3T projections of elastic reflectivities are connected to Bayesian inversion of other subsurface properties through the statistical notion of Bayesian sufficiency. The triplet of basis reflectivities is computed so that it is Bayes sufficient for all rock properties in the hierarchical seismic rock-physics model; that is, the projection basis contains all information about rock properties that is contained in the original seismic.

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The method of Tikhonov regularization is commonly used to obtain regularized solutions of ill-posed linear inverse problems. We use its natural connection to optimal Bayes estimators to determine optimal experimental designs that can be used with Tikhonov regularization; they are designed to control a measure of total relative efficiency. We present an iterative/semidefinite programming hybrid method to explore the configuration space efficiently. Two examples from geophysics are used to illustrate the type of applications to which the methodology can be applied.

A quantitative framework for relative rock physics includes a definition of relative elastic property that is rooted in inverse theory and that can be used practically to compute the properties. The framework also includes a set of rules that quantify how operations on absolute properties affect their relative counterparts. From these rules, a comprehensive table of relative elastic properties can be generated, all of which are expressed in terms of relative P-velocity, S-velocity, and density. Finally, the framework includes empirical and model-based rock-physics rules that can be applied directly to the relative properties without reverting to their absolute counterparts. That provides a practical route to such interpretation techniques as direct-to-seismic fluid substitution. The framework offers a quantitative alternative to workflows centered on absolute seismic inversion. In some cases, relative rock-physics workflows are preferable to absolute flows because they avoid statistical complications associated with the nonstationarity of the mean of absolute elastic properties.