We have developed a Kirchhoff prestack depth migration method which is
consistent with elastic wave propagation through a background model
in which one can specify values of P-wave velocity (*V*_{p}), S-wave
velocity (*V*_{s}), density (), and the anelastic attenuation factor
(*Q*). The method correctly estimates the Zoeppritz elastic
reflectivity coefficient *R*_{pp}, and the associated specular reflection
angles ,
in synthetic studies based on an actual field data application
with drilling information. This method is a true amplitude *depth*
migration in that the *R*_{pp} and estimates are valid
for laterally heterogeneous velocity models. The algorithmic implementation
is computationally efficient in that it takes 6.0 cpu hours on a
Sun Sparcstation 1^{+} to migrate
140 marine shot gathers (16,800 traces) into a 2x2x3 km^{3} target depth
image volume (1 million pixels) in a 1-D background model, each
of *R*_{pp} and as a function of depth,
horizontal distance and source-receiver offset.
However, this time can increase
significantly for complex 2-D migration velocity models which require
intense raytracing calculations.

A method has also been developed to invert gathers in order
to produce depth images of subsurface elastic parameter variations.
Parameterization choice issues have been investigated, and it has been found
that the Elastic Impedance parameter set (*I*_{p}, *I*_{s}, ) is the most
robust set for our purposes. The other less favorable parameterizations
investigated were Velocities, Elastic Moduli, Lamé Parameters,
*V*_{p}/*V*_{s} Ratio, and the industry-standard ``AVO'' Parameterization
(*A*, *B*, *C*). In all parameterizations, local
*relative changes* in elastic properties are recovered,
and not the elastic property values themselves.
A quantitative measure of
``confidence'' has been developed to appraise the quality of the
inversion images. Both the elastic
parameter inversion and confidence criterion methods have been successfully
tested on synthetic data and validated through the field data application.
The elastic parameter inversion takes about 10 cpu minutes on a
Sun Sparcstation 1^{+} for
the field data application discussed in this report.

Further details concerning the migration/inversion method, the confidence estimates, and the field data results are in preparation for journal publication, and are likely to appear in subsequent SEP reports (pending proprietary data release).

12/18/1997