Coda wave interferometry and the equilibration of energy in elastic media.

Multiple-scattered waves usually are not useful for creating deterministic images of the interior of elastic media. However, in many applications, one is not so much interested in making a deterministic image as in detecting changes in the medium. Cases in point are volcano monitoring and measuring the change in hydrocarbon reservoirs during enhanced recovery operations. Coda wave interferometry is a technique wherein changes in multiple-scattered waves are used as a diagnostic for minute changes in the medium. This technique was developed previously for scalar waves; however, the application of this technique in geophysics, nondestructive testing, and other applications where elastic waves are used, requires the extension of the existing formulation of coda wave interferometry to include conversions between P and S waves. Here, a simple model for the equilibration between P and S waves incorporates into the theory of coda wave interferometry the mode conversions that are inherent to multiply scattered elastic waves.