ASA 125th Meeting Ottawa 1993 May

4pUW4. A simple ray model of plane-wave reflection from a thin sediment layer.

David M. F. Chapman

Defence Res. Establishment Atlantic, P.O. Box 1012, Dartmouth, NS B2Y 3Z7, Canada

N. Ross Chapman

Defence Research Establishment Pacific, FMO Victoria, BC V0S 1B0, Canada

Plane-wave reflection from a thin, elastic solid sediment layer over a hard basement is modeled by a simple ray model that includes only two ray families: a family of compressional waves that transit the sediment layer any number of times before returning to the water; and a similar family that includes doubly converted compressional waves. (The latter are compressional waves that are converted to shear waves at the basement, make one or more round-trip transits of the sediment layer, and are converted back into compressional waves at the basement.) The theory is applied to homogenous sediment layers with low shear speeds, and it provides valuable insight into the physical processes governing reflection from several seabed environments of interest. The coherent sum of all the rays belonging to these families leads to a closed-form expression for the overall reflection coefficient that includes several characteristic features also simulated by full wave-theoretical numerical models, such as bottom-loss resonances related to sediment shear waves and large losses associated with the excitation of interface waves at the sediment/basement boundary. Calculated examples are provided.