NESMA

The ONR-funded Task Force Ocean: New England Seamounts Acoustics Effort is a multi-PI, multi-institution effort to understand the impact of geological, physical, and biological variability on acoustic scattering, propagation, attenuation, coherence, and inversion methods. This specific ship-time request involves a pilot test and engineering effort focused on assessing the capabilities of a towed instrument platform, Deep-See, and a REMUS 600, for performing measurements of acoustic scattering in the Gulf Stream meanders expected to influence the seamounts. 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 the Gulf Stream. Processes such as meanders in the Gulf Stream, cold and warm core eddies, intense upwelling and circum-seamount eddies drive 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 Atlantis II, this productivity manifests as a dense deep scattering layer (DSL) and a robust fish population on the flanks and summit of the seamount. The DSL at Atlantis II is coincident with the SOFAR channel and the abundance of organisms within 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. 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 is 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. This project focuses on understanding the impact of extreme spatial and temporal environmental variability on the scales of a few hundred meters to several 10s of kilometers as it pertains to long range propagation of Navy mid-frequency active sonar (MFAS) and low-frequency passive sonar.