Low-frequency geoacoustic inversion of abyssal plain deep sediment properties near the Atlantis II Seamount

Impulsive airgun transmissions collected during the New England Seamount Acoustics pilot experiments are used to investigate deep sediment structure in the Sohm abyssal plain south of the Atlantis II Seamount. Airgun signals from a moving source were recorded on a fixed vertical line array, deployed at a depth of 5050 m, which was equipped with six receivers spanning water depths from 304 to 1434 m and environmental sensors. Distinct arrivals consistent with sub-bottom eigenray paths are identified at ranges up to 15 km. Geoacoustic inversion is performed using the relative arrival time (ΔT) between single-bottom-interacting and corresponding single-sub-bottom-interacting eigenray arrivals. Modeled ΔT is computed using BELLHOP eigenrays in the water column combined with analytic formulas for layered sediment models in the sub-bottom. Constant sound speed and n2 linear parameterizations are evaluated for geoacoustic inversion. Layer parameters, including thicknesses, sound speeds, and gradients, are estimated using adaptive simplex simulated annealing, and model selection is performed using the Bayesian information criterion. Results favor a model with a constant sound speed upper layer over a deeper n2 gradient layer, indicating the presence of two resolvable sub-bottom layers with a combined thickness of approximately 500 m and sound speeds consistent with consolidated sediments.