  We consider an inverse problem in elastodynamics arising in seismic imaging.
We prove locally uniqueness of the density of a non-homogeneous, isotropic
elastic body from measurements taken on a part of the boundary. We measure the
Dirichlet to Neumann map, only on a part of the boundary, corresponding to the
isotropic elasticity equation of a 3-dimensional object. In earlier works it
has been shown that one can determine the sheer and compressional speeds on a
neighborhood of the part of the boundary (accessible part) where the
measurements have been taken. In this article we show that one can determine
the density of the medium as well, on a neighborhood of the accessible part of
the boundary.
