Characterization of Acoustic Scattering at the New England Seamounts
Abstract
The Gulf Stream plays a critical role in mediating the temporal and spatial variability of sub-mesoscale and finer scale physical oceanographic processes at the New England Seamounts. The New England Seamounts are a chain of over twenty submarine mountains that rise to a depth of around 1000-2000 m. The Seamounts are separated by geography and by physical oceanographic processes driven by theGulf Stream. Processes such as meanders in the Gulf Stream, cold and warm core eddies, intense upwelling and circum-seamount eddiesdrive a diverse physical and biological environment at the Seamounts. Upwelling from flow over the rapidly changing bathymetry makes the Seamounts biological hotspots of increased productivity compared to the surrounding waters. At deep seamounts such as AtlantisII, this productivity manifests as a dense deep scattering layer (DSL) and a robust fish population on the flanks and summit of theseamount. The DSL at Atlantis II is coincident with the SOFAR channel and the abundance of DSLs changes on temporal scales from daily # in the form of diel vertical migration, to seasonally and inter-annually. The fish populations, both in the DSL and at the flanks and summit of the seamount, have gas filled swimbladders and echoes from these fish can be the dominant source of reverberation over a range of important active Navy sonar frequencies (1- 10 kHz). Effects of absorptivity due to the gas-filled swimbladders on transmission loss at mid-frequencies can also be significant, as can the impact of complex scattering from large aggregations of gas bladder bearing fishes on echo-statistics. Changes to the phase of transmitted sound as it passes through fish populations can lead to errors in beam forming and target localization if not properly accounted for.Accounting for the biological populations in long range acoustic transmission at the seamounts suffers from a paucity of information about fish populations and the physical oceanographic processes that drive the distribution and abundance of acoustically important species. Due to their isolation, the deep-water seamounts in the New England Seamount chain are a relatively understudied part of the ocean. The environmental complexity associated with the seamounts poses increased challenges to the Navy ASW operations.The work proposed here focuses on understanding the impact of extreme spatial and temporal environmental (physical and biological) variability on the scales of a few hundred meters to 10s of kilometers as it pertains to long range propagation of Navy mid-frequency active sonar (MFAS). A coordinated seamount acoustic experiment is proposed involving high resolution, synoptic measurements of physical and biological processes using a combination of shipboard broadband backscatter, CTD and ADCP measurements, fish trawls, AUV-based sonar and oceanographic measurements, a novel towed vehicle with broadband split-beam arrays spanning 1-450 kHz and a moored Biosonar for long time scaleobservations of DSLs. A thorough mapping of the DSL and flank and summit fish populations is proposed, as well as determination of the dominant physical oceanographic processes that drive distribution and abundance of acoustically important fish species.The proposed experiment will be coordinated with the seamount acoustics projects being proposed by YT Lin, Julien Bonnel and Lauren Freeman. These combined projects will provide critical information about the acoustic environment at Atlantis II seamount, an understudied and strategically important part of theocean. Approved for public release.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Dec 15, 2023
- Source ID
- N000142412077
Entities
People
- Andone Lavery
Organizations
- Office of Naval Research
- United States Navy
- Woods Hole Oceanographic Institution