  We propose a direct imaging method based on the reverse time migration to
reconstruct extended obstacles in the half space with finite aperture elastic
scattering data at a fixed frequency. We prove the resolution of the
reconstruction method in terms of the aperture and the depth of the obstacle
embedded in the half space. The resolution analysis is studied by virtue of the
point spread function and implies that the imaginary part of the
cross-correlation imaging function always peaks on the upper boundary of the
obstacle. Numerical examples are included to illustrate the effectiveness of
the method.
