Noise interferometry is the process by which approximations to acoustic Green's functions, which describe sound propagation between two locations, are estimated by cross-correlating time series of ambient noise measured at those locations. Noise-interferometry-based approximations to Green's functions can be used as the basis for a variety of inversion algorithms, thereby providing a purely passive alternative to active-source ocean acoustic remote sensing. In this paper we give an overview of results from noise interferometry experiments conducted in the Florida Straits at 100 m depth in December 2012, and at 600 m depth in September/October 2013. Under good conditions for noise interferometry, estimates of cross-correlation functions are shown to allow one to perform advanced phase-coherent signal processing techniques to perform waveform inversions, estimate currents by exploiting non-reciprocity, perform time-reversal/back-propagation calculations and investigate modal dispersion using time-warping techniques. Conditions which are favourable for noise interferometry are identified and discussed.
Month-long time series of broadband coherent measurements of channel pulse responses in the Florida Straits allow for estimation of signal coherence under a great variety of signal parameters and environmental conditions. Two 32-element arrays, one vertical and another horizontal (bottomed) along the path of propagation allow for comparison of spatial coherency and single phone temporal coherencies. The transmitted signals cover 5 octaves from 100 through 3200 Hz. Coherencies for single resolved SRBR arrivals are compared with those for unresolved multipath BRB focused arrivals. Many factors are at play including the complication of coherent reception from nearby shipping and multipath interference. However, the time series are long enough to sort out and explain most relations to the environmental variability. Vertical and horizontal coherence lengths are compared over a wide range of frequencies. Generally, SBRB paths are found to be far more stable and coherent than RBR paths especially at higher frequencies suggesting that sound-speed variability near turning RBR rays/modes is more destructive to coherency. The loss of signal coherency for RBR paths is accompanied by a significant loss of signal intensity—as much as 10 to 15 dB.
With the ever-growing interest in satellite remote sensing, direct observations of short wave characteristics are needed along coastal margins. These zones are characterized by a diversity of physical processes that can affect sea surface topography. Here we present connections made between ocean wave spectral shape and wind forcing in coastal waters using polarimetric slope sensing and eddy covariance methods; this is based on data collected in the vicinity of the mouth of the Columbia River (MCR) on the Oregon-Washington border. These results provide insights into the behavior of short waves in coastal environments under variable wind forcing; this characterization of wave spectra is an important step towards improving the use of radar remote sensing to sample these dynamic coastal waters. High wavenumber spectral peaks are found to appear for U10 > 6 m/s but vanish for τ > 0.1 N/m2, indicating a stark difference between how wind speed and wind stress are related to the short-scale structure of the ocean surface. Near-capillary regime spectral shape is found to be less steep than in past observations and to show no discernable sensitivity to wind forcing.
On the New Jersey continental shelf ambient sound levels were recorded during tropical storm Ernesto that produced wind speeds up to 40knots in early September 2006. The seabed at the position of the acoustic measurements can be approximately described as coarse sand. Differences between the ambient noise levels for the New Jersey shelf measurements and deep water reference measurements are modeled using both normal mode and ray methods. The analysis is consistent with a nonlinear frequency dependent seabed attenuation for the New Jersey site.
Group speed of sound in moving fluids depends on the propagation direction, which breaks acoustic reciprocity. Acoustic nonreciprocity provides a means to measure fluid motion. Using nonreciprocity, one can measure fluid velocities that may be small compared to uncertainties in sound speed. Interferometry of diffuse acoustic noise, with receivers replacing the transceivers employed in active techniques, offers a simple, low-cost means of measuring nonreciprocity. Here, the feasibility of using passive measurements of acoustic nonreciprocity to estimate current velocity in the ocean is experimentally demonstrated for the first time. Estimates of depth-averaged flow velocity are retrieved from cross-correlations of low-frequency noise recorded in the Straits of Florida by near-bottom hydrophones separated by 5 and 10 km.
A tri-axial array of acoustic hydrophones has been installed off the South Florida coast in 145 m of water. The installation is one of a number of projects that have begun making use of the new South Florida Ocean Measurement Center facilities in Dania, FL. The arrays are linked to shore via a fiber-optic cable and powered from shore with a separate insulated copper cable. Two of the arms of the array are deployed on the bottom to the east and south of the origin. The third array is a sub-surface vertical mooring which includes a pressure sensor to monitor the depth of the top buoy. Each axis contains 32 hydrophones spaced in a nonlinear distribution to facilitate acoustic coherence measurements. Other sensor configurations are possible, including the replacement of hydrophones with other instruments such as current meters or temperature, pressure, or conductivity sensors. Small projectors are placed at the base of the vertical array and at the ends of the horizontal arrays, making it possible to determine the location of each sensor in real time via time-of-flight measurements. Computers running the Linux operating system control the projectors and process data from each leg of the array. Each of the three computers is connected via fiber-optic lines to a Fast Ethernet switch box and networked to shore. The computers can be accessed via TCP/IP connections and programmed from shore to suit new missions. A wireless link to the Nova Southeastern University network and the Internet makes it possible to access the system and conduct experiments from anywhere in the world. The current mission of this system is to study the coherence of shallow-water acoustic propagation. However, knowledge gained here will facilitate a variety of future projects-which include active and passive sonars, ambient noise study, high-frequency acoustic monitoring of fish migration, and acoustic tomographic monitoring of the Gulf Stream. Excellent data were obtained from several experiments. At the present time, only the vertical array is working but plans to restore the two horizontal arrays are actively pursued.
The Office of Naval Research is planning an underwater system of acoustic and environmental sensors, termed the Acoustic Observatory, at a site located off of Dania, Florida. It will enable researchers to make acoustic measurements and evaluate signal processing techniques in an ocean environment with downward refracting propagation, low loss bottom, and high levels of vessel traffic noise. As part of the site selection and evaluation process, vessel traffic was recorded using a state-of-the-art logging radar for a period of one week concurrent with recording of omnidirectional noise on hydrophones at two locations in water depths of 145 and 262 meters. Data were processed to provide a characterization of noise and traffic statistics and patterns relative to the Acoustic Observatory site. Differences in noise between the two sites, and correlations between vessel traffic and noise are presented.
An autonomous source was moored at ranges of 10 and then 20 km from a vertical receiver array with 32 elements in a depth of 145 m of water. M sequences were transmitted for 28 days at six center frequencies from 100 to 3200 in one octave increments. Arrivals and paths are identified with models and then fluctuation statistics, coherence, and predictability are examined in a parameter space of frequency, range, and receiver depth. A group of refracted-bottom-reflected (RBR) modes/rays has nearly equal group velocities and tends to focus in time and depth forming intense arrivals especially at the depth of the transmitted. A second group of surface reflected bottom reflected (SRBR) modes produce arrivals that fan out in time. Coastal areas inside western boundary currents have exceptionally variable sound speed fields owing to dynamical effects such as meanders, shelf waves, eddies, coastal upwelling and energetic internal waves and tides. Sound speed fluctuations are observed to be an order greater than the deep ocean. Very large changes in mean sound speed profiles and extreme gradients occur at subinertial periods. Also, potential energy of the internal wave field varies with the same longer periods as do statistical properties of observed acoustic signals.
Abstract : LONG-TERM GOALS. The long term goals motivating the establishment of the High Resolution Air-Sea Interaction DRI (Hi- Res DRI) are the determination of how well ship-based radars can measure the phase-resolved surface wave field (PRSWF), testing the skill of highly-nonlinear numerical surface wave models to predict the evolution of the PRSWF, and the incorporation of ocean wave effects into models of the Marine Atmospheric Boundary Layer (MABL). During the Planning Year our principal task will be to participate in meetings, discussions and planning required to formulating an experimental plan for the High Resolution Air-Sea Interaction experiment. OBJECTIVES. a) Radar imaging of the wave field and translation of the images into snapshots of elevation maps of the surface using both satellite and ship or floating platform based sensors. b) Measurements of the wave field and the atmosphere above the wave field using ASIS buoys to generate data sets that could be compared to wave models and radar both in the spectral and phase-resolved domains, and leads to a better understanding how the wind profile, fluxes, coefficient of drag vary with respect to differing wind and wave conditions. c) Measurements of breaking using both acoustic and microwave means as well as in-situ sensors locally.
Two-point cross-correlations function (CCF) of diffuse acoustic noise approximates the Green’s function, which describes deterministic sound propagation between the two measurement points. Similarity between CCFs and Green’s functions motivates application to acoustic noise interferometry of the techniques that were originally developed for remote sensing using broadband, coherent compact sources. Here, time reversal is applied to CCFs of the ambient and shipping noise measured in 100 meter-deep water in the Straits of Florida. Noise was recorded continuously for about six days at three points near the seafloor by pairs of hydrophones separated by 5.0, 9.8, and 14.8 km. In numerical simulations, a strong focusing occurs in the vicinity of one hydrophone when the Green’s function is back-propagated from the other hydrophone, with the position and strength of the focus being sensitive to density, sound speed, and attenuation coefficient in the bottom. Values of these parameters in the experiment are estimated by optimizing focusing of the back-propagated CCFs. The results are consistent with the values of the seafloor parameters evaluated independently by other means.
