  We consider the resolution of the single frequency reverse time migration
(RTM) method for extended targets without the assumption of the validation of
geometric optics approximation. The resolution analysis, which applies in both
penetrable and non-penetrable obstacles with sound soft or impedance boundary
condition on the boundary of the obstacle, implies that the imaginary part of
the cross-correlation imaging functional 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.
