  In this work, we are concerned with the inverse scattering by interfaces for
the linearized and isotropic elastic model at a fixed frequency. First, we
derive complex geometrical optic solutions with linear or spherical phases
having a computable dominant part and an $H^\alpha$-decaying remainder term
with $\alpha <3$, where $H^{\alpha}$ is the classical Sobolev space. Second,
based on these properties, we estimate the convex hull as well as non convex
parts of the interface using the farfields of only one of the two reflected
body waves (pressure waves or shear waves) as measurements. The results are
given for both the impenetrable obstacles, with traction boundary conditions,
and the penetrable obstacles. In the analysis, we require the surfaces of the
obstacles to be Lipschitz regular and, for the penetrable obstacles, the Lam\'e
coefficients to be measurable and bounded with the usual jump conditions across
the interface.
