Analysis of mid-frequency acoustic signals interacting with a rough sea surface at very low grazing angles

Estimates of channel impulse response from a shallow water acoustic propagation experiment in the southeastern Arabian Sea off the west coast of India (off Kerala) are presented. The experiments involved strong thermocline gradients and a tri-layer structure that led to interactions of broadband mid-frequency (5-10 kHz) acoustic signals with a rough sea surface at very low grazing angles (1.9°-2.4°). Measurements consist of sea surface-reflected arrivals recorded at receiver depths along two propagation tracks of range ∼900 m. The sea surface arrival is modeled as a combination of coherent surface reflection coefficient derived from second-order small slope approximation (SSA) theory and higher-order SSA-based bistatic scattering cross section (incoherent component) considering a 3D geometry. A model utilizing the fourth-order SSA-based cross section for 2D surfaces is presented, applicable at very low grazing angles. Results show that inclusion of the third- and fourth-order SSA terms improves accuracy across a broader range of scattering angles and accounts for multiple scattering effects at very low grazing angles. A high correlation coefficient between the ensemble averaged measured and the modeled surface-reflected arrival pulse indicates very good agreement in terms of the peak value and the time spread of the signal.