Retrieval of empirical Green’s functions from ambient sound recorded by deep-water receivers during the 2023–2024 NESMA experiment

During the 2023 and 2024 New England Sea Mounts Acoustics (NESMA) experiments, the Naval Postgraduate School deployed a network of near-bottom Moored Autonomous Noise Recorders (MANRs) near the Atlantis II Seamounts. MANRs recorded ambient sound continuously for 2 months in 2023 and 6 months in 2024. Sub-millisecond timing accuracy was maintained with on-board chip-scale atomic clocks. This paper investigates the retrieval of acoustic empirical Green’s functions (EGFs) from cross-correlations of ambient sound simultaneously recorded by MANR#2 and MANR#3 located on steep seamount flanks and MANR#4 located on an abyssal plain. MANRs were located 4 m above the seafloor at depths of 2994, 4067, and 4443 m, respectively. Stacking cross-correlation functions from approximately 100 h of noise recordings is found to produce EGFs with stable causal and acausal direct arrivals in the 15–50 Hz frequency band. We then track the temporal evolution of the reciprocal and non-reciprocal components of the passively measured acoustic travel time, which contain information about the evolution of sound speed and flow velocity along the propagation path. Acoustic observations of the environmental variability in deep water in the vicinity of Atlantis II Seamounts are compared to tide-resolving MITgcm hydrodynamic simulations. Work supported by ONR.