Seafloor characterization in the 2023 NESMA pilot experiment using sound sources of opportunity and a single near-bottom hydrophone

Summary Detailed geoacoustic characterization of the seabed typically relies on controlled sound sources and multi-element hydrophone arrays with well-known source and receiver positions. This study explores the opposite scenario, where underwater noise is recorded by a near-bottom, single-hydrophone receiver, with only limited knowledge of the sound sources. Three Moored Autonomous Noise Recorders (MANRs) were deployed on the flanks and in the trough of the Atlantis II Seamounts during the 2023 New England Seamounts Acoustics (NESMA) Pilot experiment, at depths of 2573-4443 m and 4 m above the seafloor. Wideband noise from ships passing above MANRs generated Lloyd’s mirror-like interference patterns, enabling estimation of local seafloor roughness near the MANRs. Root-mean-square (rms) roughness did not exceed 0.16 m. On a broader scale, seafloor roughness and sound speed in the bottom along the propagation track were estimated by analyzing multiple acoustic arrivals from a high-energy impulsive event approximately 60-74 km northeast of the MANRs, caused by the failure of another mooring. The results indicate a generally smooth seafloor, with roughness less than 0.2–0.35 m rms away from seamount flanks. Reflection coefficient analysis suggests a lower bound of the sediment sound speed of 1717–1776 m/s, consistent with a fine-sand seabed.