  We investigate the problem of recovering the initial displacement f for a
solution u of a linear, isotropic, non-homogeneous elastic wave equation, given
measurements of u on [0,T] x \partial \Omega, where \Omega\subset\R^3 is some
bounded domain containing the support of f. For the acoustic wave equation,
this problem is known as thermoacoustic tomography (TAT), and has been
well-studied; for the elastic wave equation, the situation is somewhat more
subtle, and we give sufficient conditions on the Lam\'e parameters to ensure
that recovery is possible.
