Ward identity for elastic wave transport in disordered media

For elastic wave transport in non-uniform static media, we derive a Ward-Takahashi identity that is a consequence of energy conservation. Making use of this basic identity, and with the help of an integral equation, essentially equivalent to the Bethe-Salpeter equation, we derive another version of the Ward identity that is important in describing the multiply scattered, diffusive transport of elastic waves in disordered media. (C) 1997 Published by Elsevier Science B.V