In planar bulk acoustic wave devices, an interdigital transducer (IDT) is generally used as the source of excitation. Grooves, which convert an incident SAW (surface acoustic wave) to bulk waves and vice versa, also have the potential to be used as an alternative (if secondary) source of excitation. A typical shallow rectangular groove etched into a crystalline substrate perpendicular to the propagation direction is considered. The groove is modeled as an interior perturbation to the substrate. Coupled-mode equations are developed and solved for the reflected SAW and the scattered bulk waves. The results are compared to those of the half-space geometry. The interior-perturbation approach produces a good match with earlier published results that used the boundary-perturbation method.