Surface wave focusing and acoustic communications in the surf zone

The forward scattering of acoustic signals off of shoaling surface gravity waves in the surf zone results in a time-varying channel impulse response that is characterized by intense, rapidly fluctuating arrivals. In some cases, the acoustic focusing by the curvature of the wave crest results in the formation of caustics at or near a receiver location. This focusing and the resulting caustics present challenges to the reliable operation of phase coherent underwater acoustic communications systems that must implicitly or explicitly track the fluctuations in the impulse response. The propagation physics leading to focusing are studied with both experimental data and a propagation model using surface wave profiles measured during the collection of the experimental data. The deterministic experimental and modeled data show good agreement and demonstrate the stages of the focusing event and the impact of the high intensity arrivals and rapid fluctuations on the ability of an algorithm to accurately estimate the impulse response. The statistical characterization of experimental data shows that the focusing by surface gravity waves results in focused surface reflected arrivals whose intensity often exceeds that of the direct arrival and the focusing and caustic formation adversely impacts the performance of an impulse response estimation algorithm.