TFO Geoacoustic Inversion and Acoustic Characterization of Seamounts

The ONR-funded Task Force Ocean: New England Seamounts 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 ship-time request involves 2 PIs on this effort who focus on geacoustic inversion in deep water with complex seamount topography and acoustic scattering characterization of the biology and physics at the New England 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. On the other hand, the steep 3D topography and corrugated basalt seafloor will also impact low-frequency acoustic propagation. In particular, one expects acoustic scattering, diffraction and shadowing created by the seamounts. Those will create heavy 3D propagation effects, which are affected not only by the bathymetry, but also by the geoacoustic properties of the sub-bottom. The importance of seafloor geoacoustic properties to understand and predict acoustic propagation over seamounts has been emphasized during the 2004 ONR Basin Acoustic Seamount Scattering Experiment (BASSEX), but their experimental estimation stays an open question. 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 (geological, physical and biological) 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. A coordinated seamount acoustic experiment will be conducted involving high resolution, synoptic measurements of physical and biological processes using a combination of shipboard broadband backscatter, CTD and ADCP measurements, AUV-based sonar and oceanographic measurements, a novel towed vehicle with broadband split-beam arrays spanning 1-450 kHz (Deep-See), a towed low-frequency acoustic source (J15-3), a towed airgun (Sercel GI-210) and a moored Biosonar for long time scale observations 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. Further, low-frequency acoustic mapping of the Atlantis II seamount flank will be performed using the airgun and long-range 3D acoustic propagation experiments will be performed using the J15-3 source (those will rely on acoustic receiver mooring experiments deployed during a previous cruise).