Estimation of angle-dependent reflectivity is a critical step in the larger inverse problem of mapping subsurface material properties from surface seismic reflection data (e.g., Lumley and Beydoun, 1991). The latter is, in turn, important for hydrocarbon exploration and detection, and crucial for reservoir characterization and monitoring in the development and production phases.

Stolt and Weglein (1985) gave a comprehensive review of linearized seismic
migration and inversion techniques, which we recommend as useful background
information to the angle-dependent reflectivity analysis we present here.
Bleistein (1987) addressed the angle-dependent reflectivity
problem based on the work of Beylkin (1985).
By a suitable choice of a Kirchhoff integrand, Bleistein
was able to show that his trial solution reduced to an estimate of
angle-dependent reflectivity under the stationary phase approximation.
However, it is not clear that the solution is robust in the sense
of an *l*_{p}-norm estimate, nor that it can be derived in a straightforward
inverse theory approach, starting with an explicit forward model for
reflection and diffraction.

More recently, de Bruin (1992) and
Cunha (1992) approached this problem from a finite-difference perspective.
de Bruin estimates angle-dependent reflectivity using an -*x* migration
scheme. However, de Bruin processes the data in constant ray parameter
sections, and so cannot estimate the reflection angles directly from the data,
without a posteriori information on the geologic structure and dip.
His suggestion of a second-pass
procedure of evaluating the reflection angles after picking migrated
reflector horizons may represent an inefficient and unstable process.
Cunha (1992) applied a reverse-time migration procedure to estimate
angle-dependent reflectivity. As with de Bruin, Cunha needs to invoke
and evaluate a set of new imaging conditions to estimate reflectivity,
but, unlike de Bruin, Cunha is able to directly estimate reflection
angles from the data. Possible disadvantages to Cunha's method are its
prohibitive computational expense, and its potential for instability
due to its formulation in the common-shot domain.

In this paper we derive a robust least-squares estimate of angle-dependent reflectivity. We start from an elastic representation theorem-based forward modeling theory of generalized diffraction and reflection, and invoke a straightforward application of least-squares inverse theory. The method does not require the arbitrary definition of any particular imaging conditions, and both reflectivity and reflection angle are directly estimated from the data without a priori knowledge of structural dip. The results can be interpreted as a least-squares generalization of Bleistein's approach, and cost little more than a standard Kirchhoff prestack migration to implement.

This paper proceeds as follows. We first derive a forward theory relating angle-dependent seismic reflection data to a generalized reflection model which combines elements of Zoeppritz plane-wave reflection and Rayleigh-Sommerfeld elastic diffraction. On the basis of this forward model, we formulate a least-squares inverse problem to estimate the angle-dependent reflection coefficient as a function of subsurface coordinate and offset. This analysis leads to two coupled normal equations for reflectivity and reflection angle, as a function of the recorded seismic data. We decouple the two equations using a stationary phase approximation. We finally solve the reflectivity and angle equations separately using a standard Gauss-Newton gradient optimization technique and an approximate diagonal Hessian operator.

11/16/1997