  We propose a new reverse time migration method for reconstructing extended
obstacles in the planar waveguide using acoustic waves at a fixed frequency. We
prove the resolution of the reconstruction method in terms of the aperture and
the thickness of the waveguide. The resolution analysis implies that the
imaginary part of the cross-correlation imaging function is always positive and
thus may have better stability properties. Numerical experiments are included
to illustrate the powerful imaging quality and to confirm our resolution
results.
