Experimental investigation of surface acoustic duct propagation variability near the Atlantis II Seamounts using autonomous surface vehicles

Detection of underwater targets in the upper ocean is often influenced by surface acoustic ducts, which enable long-range guided sound propagation. The physics of surface acoustic duct propagation, notably its minimum cutoff frequency, has been extensively studied in past studies. However, quantifying the influence of environmental parameters on the temporal and spatial variability of the sonic layer depth (SLD) and the associated energy leakage below the SLD remains under investigation. A collaborative experiment was conducted in August 2024 during the NESMA-IOP 2 cruise to address this topic. A dual-frequency band (∼130 Hz and ∼1.3 kHz) acoustic source was deployed ∼15 m deep from the drifting R/V Revelle while continuously profiling the water column down to 350 m with a fast CTD system to capture the local sub-mesoscale and internal-wave ocean variability. Simultaneously, two autonomous surface vehicles, called Wave Gliders (WG), each instrumented with a towed acoustic module, located above and below the effective sonic layer depth (∼50 m) at ∼12 and ∼100 m, recorded the source transmissions at ranges between 1 and 10 km. A third WG profiled the water column down to 150 m with a CTD assessing the spatial variability of the SLD. Experimental detection ranges are compared to numerical predictions. [Work sponsored by ONR.]