Mid-frequency acoustic transmission and communication in the Arctic: the Nansen Basin under-ice experiments

The Arctic Ocean is undergoing rapid environmental changes, and measurement and monitoring of its physical ice-ocean variables is becoming increasingly important. To operate under conditions of ice cover, oceanographic measurement platforms must typically rely on acoustics for communication and navigation, and it is of interest to explore limitations for such. In this context, this paper presents data from mid-frequency (0.5-1.7 kHz and 4–8 kHz) under-ice acoustic transmission and communication experiments at two sites in the Nansen Basin of the Arctic Ocean, in August 2019 and March 2023, respectively. The areas of experiment were characterized by stable ice-cover (78% and 99% ice concentration, respectively), under-ice oceanographic ducts (depths ~25–180 m) and water depths 2700–4000 m. Measured impulse responses at ranges of 8, 21, and 42 nmi showed considerable delay spread between distinct groups of arrivals due to direct, surface-ducted, and bottom-reflected paths. The relative strengths of these groups depend on duct and under-ice properties, and source-receiver range. In general, in-duct paths at shallow grazing angles are attenuated with range due to repeated under-ice reflection loss, while bottom-reflected paths can remain strong to longer range. The interpretations of impulse responses are supported by numerical modeling using the Bellhop ray model with an under-ice reflection model that includes rough-surface scattering. Of the tested communication schemes, a coherent multiband modulation performed best in these Arctic environments. Bottom-reflected arrivals play an important role in this analysis. They can either degrade modem performance by interfering with the surface-ducted reception or enable error free decoding of the bottom reflection alone.