Deep-water ambient sound over the Atlantis II seamounts in the Northwest Atlantica).

Ambient sound was continuously recorded for 52 days by three synchronized, single-hydrophone, near-bottom receivers. The receivers were moored at depths of 2573, 2994, and 4443 m on flanks and in a trough between the edifices of the Atlantis II seamounts. The data reveal the power spectra and intermittency of the ambient sound intensity in a 13-octave frequency band from 0.

Observation of exceptionally strong near-bottom flows over the Atlantis II Seamounts in the northwest Atlantic.

Knowledge of near-bottom ocean current velocities and especially their extreme values is necessary to understand geomorphology of the seafloor and composition of benthic biological communities and quantify mechanical energy dissipation by bottom drag. Direct measurements of near-bottom currents in deep ocean remain scarce because of logistical challenges. Here, we report the results of flow velocity and pressure fluctuation measurements at three sites with depths of 2573-4443 m in the area where the Gulf Stream interacts with the New England Seamounts.

Passive acoustic characterization of sub-seasonal sound speed variations in a coastal ocean.

Acoustic noise interferometry is applied to retrieve empirical Green's functions (EGFs) from the ambient and shipping noise data acquired in the Shallow Water 2006 experiment on the continental shelf off New Jersey. Despite strong internal wave-induced perturbations of the sound speed in water, EGFs are found on 31 acoustic paths by cross-correlating the noise recorded on a single hydrophone with noise on the hydrophones of a horizontal linear array about 3.6 km away.

Passive geoacoustic inversion in the Mid-Atlantic Bight in the presence of strong water column variability.

Empirical Green's functions are obtained for 31 paths in a highly dynamic coastal ocean by cross-correlation of ambient and shipping noise recorded in the Shallow Water 2006 experiment on a horizontal line array and a single hydrophone about 3600 m from the array. Using time warping, group speeds of three low-order normal modes are passively measured in the 10-110 Hz frequency band and inverted for geoacoustic parameters of the seabed. It is demonstrated that, despite very strong sound speed variations caused by nonlinear internal waves, noise interferometry can be successfully used to acoustically characterize the seafloor on a continental shelf.

Characterizing the seabed in the Straits of Florida by using acoustic noise interferometry and time warping.

Interferometry of ambient and shipping noise in the ocean provides a way to estimate physical parameters of the seafloor and the water column in an environmentally friendly manner without employing any controlled sound sources. With noise interferometry, two-point cross-correlation functions of noise serve as the probing signals and replace the Green's function measured in active acoustic remote sensing. The amount of environmental information that can be obtained with passive remote sensing and the robustness of the estimates of the seafloor parameters increase when contributions of individual normal modes are resolved in the noise cross-correlation function.

Normal mode dispersion and time warping in the coastal ocean.

Simple, analytically solvable models of normal mode propagation in the coastal ocean are developed and applied to study the effect of the seafloor bathymetry on modal travel times. Within the adiabatic approximation, horizontal inhomogeneity of the waveguide is found to change the modal dispersion curves in a way that helps separation of the modal components of the acoustic field using the time-warping transform. It is shown that moderate seafloor slopes can lead to surprisingly large errors in retrieved geoacoustic parameters and cause a positive bias in bottom sound speed estimates if horizontal refraction is ignored.