Midfrequency sound propagation and reverberation in a deep ice-covered ocean

Two experiments on midfrequency acoustic transmission under ice in the Beaufort Sea are discussed. The APL-ICEX14 measurements were made using 3500 Hz 5s-long CW pulses for fixed geometry of source and receiver at ∼30 m depths and ∼720 m range. The NPS-ICEX16 experiment was performed using CW pulses and LFM sweeps from two mobile 950–3000 Hz transmitters and five spatially separated hydrophones at different combinations of source-receiver ranges (0.3–10km) and depths (45–183m). For analysis of recorded timeseries, a modeling approach is suggested that considers several types of arrivals contributing to the received signal at different time intervals. The direct arrivals corresponding to nearly horizontal propagation are described using a hybrid PE-&ray-based approach, which employs a fast PE code for propagation in a stratified ocean with flat ice-free surface to account for effects of structure and dynamics of the arctic surface duct and then adds a field of an image source whose strength is defined by a complex reflection loss factor of the ice cover at small grazing angles. The direct signal is followed by reverberation coda that is modeled by bottom- and ice-bounced arrivals with corresponding reflectivity and scattering strengths at steep angles. Potential applications to remote sensing in ice-covered environments are discussed. [Work supported by ONR.]