Ocean Acoustic Tomography Research Articles

Double-Capon and double-MUSICAL for arrival separation and observable estimation in an acoustic waveguide

Recent developments in shallow water ocean acoustic tomography propose the use of an original configuration composed of two source-receiver vertical arrays and wideband sources. The recording space thus has three dimensions, with two spatial dimensions and the frequency dimension. Using this recording space, it is possible to build a three-dimensional (3D) estimation space that gives access to the three observables associated with the acoustic arrivals: the direction of departure, the direction of arrivals, and the time of arrival. The main interest of this 3D estimation space is its capability for the separation of acoustic arrivals that usually interfere in the recording space, due to multipath propagation. A 3D estimator called double beamforming has already been developed, although it has limited resolution. In this study, the new 3D high-resolution estimators of double Capon and double MUSICAL are proposed to achieve this task. The ocean acoustic tomography configuration allows a single recording realization to estimate the cross-spectral data matrix, which is necessary to build high-resolution estimators. 3D smoothing techniques are thus proposed to increase the rank of the matrix. The estimators developed are validated on real data recorded in an ultrasonic tank, and their detection performances are compared to existing 2D and 3D methods.

Passive ocean acoustic tomography

The possibility to apply natural acoustic ocean noise in the ocean and noise of distant vessels as sounding signals in order to determine the physical parameters of a water layer is considered in this paper. We developed the methods making it possible to suppress the non-diffuse components of noise produced, e.g., by local vessels and to account for hydrophone motion. These methods are applied to the noise records obtained in the course of a year-long experiment on long-range sound propagation in the Pacific Ocean. We confirmed experimentally our theoretical predictions as to the possibility of retrieving deterministic acoustic ray travel times in a nonuniform environment from a mutual correlation function of imperfectly diffuse (gradually anisotropic and spatially nonuniform) noise without invoking any data on its source. We performed passive measurements of sound velocity in the ocean with a relative error of about 0.1% by correlation of noise fields recorded with vertical aerials. This accuracy approaches that needed for oceanological applications. Further investigations are necessary to study the feasibility of passive acoustic tomography and thermometry in the ocean at distances of tens and hundreds of kilometers and the possibility to use simpler arrays not equipped with hydrophone positioning systems.

High frequency ocean acoustic tomography observation at coastal estuary areas

Ocean acoustic tomography (OAT) technique can obtain oceanographic information and has been received a lot of interest. High frequency OAT (in few kHz range) can be used for small and confined areas such as estuaries and bays with complicated hydrological conditions. In this study, we investigate the application of the high-frequency reciprocal transmission OAT to assess the temperature and current in Xiamen sea area using computer simulations and sea experiments. Based on the temperature data obtained from remote sensing, high frequency OAT is employed to reconstruct two-dimensional temperature, sound speed, and current field of the 1.2 km × 1.2 km region. The results show that increasing the number of acoustic stations decreases the travel-time errors of high frequency OAT; however, excessively increasing stations can not significantly improve the inversion accuracy. Furthermore, this method has also been examined by a sea experiment on monitoring the current and water temperature of Wuyuan Bay. High frequency OAT might provide an effective method on temperature and current observation at coastal estuary areas. (This work was financially supported by the National Science Foundation of China (Grant No. 11174240) and the Fundamental Research Funds for the Central Universities (2011121010))

Inversion of temperature vertical structure by ocean acoustic tomography data in the Luzon Strait

Luzon Strait is the key channel which connects the western Pacific and the South China Sea(SCS). It is very important to monitor the exchanges of materials and energy between the western Pacific and the SCS, which are caused by the Kuroshio and eddy's variability. The Kuroshio can intrude into the SCS through the Luzon Strait in various manners and significantly affects the oceanic variability of the SCS. A deep-sea acoustic tomography experiment was conducted at the northern part of the Luzon Strait during April 2008 to October 2008. Three 800Hz acoustic systems were deployed at depth about 800m on the subsurface mooring line, and the distance between the systems was about 40km. The four-month reciprocal acoustic transmission data between a pair systems were successfully obtained. The three groups of acoustic arrival peak were distinctly separated. The temperature vertical structure along the transmission line are inversed by using the Matched-Mode Processing method. The inversion results are in good agreement with the temperature field observations and HYCOM results. The 17-day and 23-day period signals are found in the inversion result, which correspond to the frequency of occurrences of cold eddy in the Luzon Strait. (Work supported by NBRPC 2007CB411803, NSFC 41176033 and ONR)

Automatic detection of the number of raypaths in colored noise using short-length samples

In ocean acoustic tomography (OAT) (especially in shallow water where raypaths are mixed), knowledge of the number of raypath is crucial for inversion algorithm. In this paper, a noise-whitening exponential fitting test (NWEFT) is presented in this context for detecting the number of raypaths. Classically, two suggested approaches are the Akaike information criterion (AIC) and the minimum description length (MDL). Based on ideal assumption of ergodic Gaussian random processes and white Gaussian noise, MDL is shown to be asymptotically consistent, whereas the AIC tends to overestimate the order of model. However, these assumptions could not be fulfilled in practical case of OAT. In order to be adapted for real case of OAT, noise-whitening processing is applied as first step. Then, NWEFT bases on the fact that the profile of the ordered eigenvalues fits an exponential law for short-length samples of white Gaussian noise. The number of raypaths could be detected when a mismatch occurs between observed profile and exponential model. The fact that NWFET works on short-length samples is very important as a long duration of the received signal in OAT is unavailable. Its performance is studied with synthetic and real data set and compared with classical algorithms.

Performance bounds on geoacoustic inversions

Lower bounds on the mean square error for parameter estimates using geoinversion methods are developed in terms of the tomographic version of the Cramer Rao bound. Two approaches are considered: (i) an active, coherent source with known waveform and time synchronization as in ocean acoustic tomography (OAT) and (ii) a passive, broadband source with known spectrum but with no time synchronization as in matched field tomography (MFT). Both approaches assume the receiver to be an array, e.g., vertical line array or towed horizontal line array (HLA). The formulation requires specifying the Green's functions connecting source and the sensors of the receiving array. The bounds demonstrate (i) OAT performance depends upon the mean square group speed spread of modes and (ii) MFT performance depends upon the phase speed spread where both are integrated across the band of the respective sources. The bound also indicates the coupling among parameter estimates. Uncorrelated estimates are usually desirable for efficient parameterization formulations. An example using a Pekeris waveguide with three unknown parameters: the ocean and bottom sound speeds and the depth. The extension to Bayesian problems where a joint a priori probability is given. This permits the evaluation enabled by the observed data.

Double multiple signal characterization active large band (D-MUSICAL) for extraction of observable in acoustic tomography

In the ocean, the acoustic propagation is characterized by different arrivals. Each arrival is characterized by its observables: amplitude, arrival time, emission, and reception angles. To achieve ocean acoustic tomography, a method to separate arrivals and extract observables has been developed. Two vertical arrays are used: a source and a receive array. Until now, the double-beamforming algorithm has been used to extract observables. It allows to go from the initial (time–source depth–receiver depth) domain to the beamformed domain (time–source angle–receive angle). This algorithm is efficient thanks to the 3-D domain of estimation but shows resolution limitation in the angle domain. To improve wave separation, we develop and apply a high-resolution algorithm based on MUSIC principle, in the same 3-D domain. The proposed method, called Double-MUSICAL, is the extension on the emission angle of the 2-D MUSIC Acitve Large band method (MUSICAL). Its principle is to separate the recording space onto the two orthogonal signal and noise spaces. We present results on real data recorded in an ultrasonic tank and show that the developed method better detects and separates the first arrivals than double beamforming.

Tracking tomographic arrivals.

Ocean acoustic tomography records an acoustic pattern of peaks that is interpreted in terms of ray arrivals. Before peaks can be tracked in the time-evolving environment, they must be associated with the correct ray path. Scattering by range-dependent ocean structure is a complicating factor. Fine scale structure limits the time and bandwidth coherence of an arrival and thus a single peak per ray path becomes multiple fading peaks. Associating the peak data with the ray model is often done by hand, but this has two drawbacks. First, a human is required to look at the data precluding processing the data in situ. Second, there is no guarantee that the analysis is repeatable. To address these concerns, three popular methods from radar data processing, the nearest neighbor standard filter, the probabalistic data association filter, and Viterbi data association, are proposed to solve the automatic data association problem. The peak arrival time, angle, and amplitude are clues used to construct a scoring function based on the likelihood ratio. A Kalman filter is used to restrict the expected arrival pattern to shifts and stretching between transmissions. A comparison of these methods with actual tomographic data will be presented.

Spatial coherence of acoustic reverberation and its possible use for characterization of the environment.

Cross‐correlation function of diffuse noise fields generated by random sound sources distributed on a surface or in a volume contains valuable information about the propagation environment. It was recently demonstrated [Godin et al. Geophys. Res. Lett. 37, L13605 (2010)] that this information can be successfully retrieved and used for passive ocean acoustic tomography. However, long averaging times are typically required for underwater noise to become sufficiently diffuse and for coherent features to become pronounced. In a waveguide with rough boundaries, one might expect that waves scattered by random roughness and waves that would be generated by random sources distributed on the boundaries have similar statistics. This paper investigates the possibility of using rough surface reverberation in an underwater waveguide, instead of the ambient noise, as a source of information about physical properties of the water column. Two‐point correlation function of the surface‐scattered waves is calculated in the ...

The sound of climate change

It came as a great shock in the 1960s that the oceans, like the atmosphere, have an active weather (i.e. ocean storms are called eddies). The traditional expedition mode of individual research vessels making independent measurements was no longer adequate. Ocean Acoustic Tomography was developed in direct response to the ‘eddie revolution’. Sound travels faster in warmer water; acoustic waves transmitted from ship to ship give information about the temperature and currents in the intervening waters. The transmission scale has widened over the years from 100 to 1000 to 10 000 km, approaching the antipodal scale of Ewing’s 1960 transmission from Perth (Australia) to Bermuda. In 1991 we successfully transmitted from a source ship on Heard Island in the Indian Ocean to receiver ships in the north and south Atlantic and Pacific Oceans. Brian Dushaw is planning to repeat Ewing’s experiment; he expects a reduction in travel time of approximately 10 s as confirmation of global ocean warming over the last 50 yr.Sea level rose 15 cm in the 20th century. The rate has since doubled; values up to 2 m by 2100 are now being quoted. To make accurate predictions we must understand the melting processes of continental ice sheets. Floating ice sheets from Antarctic and Greenland glaciers cover huge ocean caverns that have not to date been accessible to observation. We propose probing these caverns with sound waves to study the ocean dynamics at the underside of the floating ice sheets: a daring venture.

The 2010 Crafoord Prize awarded to Walter Munk

It came as a great shock in the 1960s that the oceans, like the atmosphere, have an active weather (i.e. ocean storms are called eddies). The traditional expedition mode of individual research vessels making independent measurements was no longer adequate. Ocean Acoustic Tomography was developed in direct response to the ‘eddie revolution’. Sound travels faster in warmer water; acoustic waves transmitted from ship to ship give information about the temperature and currents in the <em>intervening</em> waters. The transmission scale has widened over the years from 100 to 1000 to 10 000 km, approaching the antipodal scale of Ewing’s 1960 transmission from Perth (Australia) to Bermuda. In 1991 we successfully transmitted from a source ship on Heard Island in the Indian Ocean to receiver ships in the north and south Atlantic and Pacific Oceans. Brian Dushaw is planning to repeat Ewing’s experiment; he expects a reduction in travel time of approximately 10 s as confirmation of global ocean warming over the last 50 yr. Sea level rose 15 cm in the 20th century. The rate has since doubled; values up to 2 m by 2100 are now being quoted. To make accurate predictions we must understand the melting processes of continental ice sheets. Floating ice sheets from Antarctic and Greenland glaciers cover huge ocean caverns that have not to date been accessible to observation. We propose probing these caverns with sound waves to study the ocean dynamics at the underside of the floating ice sheets: a daring venture.

Inversion for Sound Speed Profile by Using a Bottom Mounted Horizontal Line Array in Shallow Water

Ocean acoustic tomography is an appealing technique for remote monitoring of the ocean environment. In shallow water, matched field processing (MFP) with a vertical line array is one of the widely used methods for inverting the sound speed profile (SSP) of water column. The approach adopted is to invert the SSP with a bottom mounted horizontal line array (HLA) based on MFP. Empirical orthonormal functions are used to express the SSP, and perturbation theory is used in the forward sound field calculation. This inversion method is applied to the data measured in a shallow water acoustic experiment performed in 2003. Successful results show that the bottom mounted HLA is able to estimate the SSP. One of the most important advantages of the inversion method with bottom mounted HLA is that the bottom mounted HLA can keep a stable array shape and is safe in a relatively long period.

Ocean tomography with acoustic daylight

Ambient noise in the ocean provides acoustic illumination, which can be used, similarly to daylight in the atmosphere, to visualize objects and characterize the environment. It has been shown theoretically that deterministic travel times between any two points in a moving or motionless, inhomogeneous, time‐independent medium can be retrieved from the cross‐correlation function of diffuse acoustic noise recorded at the two points, without a detailed knowledge of the noise field's sources or properties. In this paper, techniques are developed to account for receiver motion and suppress contributions due to powerful transient localized noise sources, such as nearby shipping, in order to enhance noise diffusivity. The data‐processing techniques are applied to ambient noise recordings of opportunity, which were obtained as a by‐product of a long‐range sound propagation experiment in the Pacific Ocean. The feasibility of passive ocean acoustic tomography with ambient noise recorded at two vertical line arrays is demonstrated successfully.

Long‐term acoustic tomography measurement of ocean currents at the northern part of the Luzon Strait

An ocean acoustic tomography (OAT) experiment was conducted during May to September 2008 in the Luzon Strait where the Kuroshio sporadically intrudes and strong internal tides and waves occur. Although the reciprocal sound transmissions between two acoustic stations were remarkably disturbed by these waves, ocean current signals proportional to the travel time differences, were successfully retrieved by low‐pass filtering out the high frequency wave components under these conditions. The drift of internal clock occurring during the observation is corrected through the procedure that the travel time differences for rays passing the upper 500 m layer are determined relative to those for rays passing near the underwater sound channel (USC) axis of depth about 1000 m. The observed currents relative to currents near the USC axis agree with the moored upward looking ADCP (acoustic Doppler current profiler) data. This study suggests a potential ability of OAT for the operational measurement of ocean currents.

Ocean acoustic tomography: Live from the Philippine Sea.

In this live interactive video session, scientists onboard the R/V Roger Revelle in the Philippine Sea off the coast of Taiwan describe the basic concept of sound propagation in the ocean in the context of an ongoing acoustic mooring deployment. Six acoustic source moorings and a distributed hydrophone array will be deployed during the course of the month-long cruise. The scientists will give an introduction to how research is carried out at sea and will describe basic concepts and advantages of using acoustics as a tool to learn about the ocean environment. They will show examples of oceanographic instrumentation, including conductivity-temperature-depth sensors, acoustic sources, and hydrophone receivers, and will describe the process of deploying acoustic moorings and acquiring oceanographic data. The girl scouts will learn how sound is used to map the 6-km deep ocean floor as well as how tomographic moorings can be used to sense the ocean interior. There will be the opportunity to interact with scientists on the research ship and ask questions about ocean acoustics or life at sea. [Field work supported by ONR.]

Sound transmission through a fluctuating ocean (1979) and ocean acoustic tomography (1995): An intertwined history.

Early attempts at understanding scintillations in sound transmission through the ocean assumed that ocean fine structure is homogeneous and isotropic. It is, in fact, dominated by internal waves, which are neither homogeneous nor isotropic. Sound Transmission Through a Fluctuating Ocean (Flatté et al., Cambridge University Press, Cambridge, England 1979) combined recently developed internal-wave models with path-integral methods to predict the fluctuations of resolved acoustic multipaths. At that time, Ocean Acoustic Tomography (Munk et al., Cambridge University Press, Cambridge, England 1995), which uses ray travel times to determine large-scale ocean structure, had just been proposed. The wideband acoustic sources and receivers required to make travel-time measurements for tomography also provided the technology needed to quantify acoustic fluctuations. Conversely, internal-wave-induced scattering limits travel-time measurement precision. Tomography experiments at 25-km range provided some of the earliest tests of path-integral predictions. Subsequent tomography experiments provided data at ever-increasing ranges and decreasing frequencies. The Acoustic Thermometry of Ocean Climate project made measurements at megameter ranges with 1400-m vertical line array receivers. The next step is a modular distributed VLA capable of spanning the full water column that is under development to enable separation of acoustic modes using spatial filtering and to fully characterize deep-water acoustic time fronts.

Travel-time sensitivity kernels versus diffraction patterns obtained through double beam-forming in shallow water

In recent years, the use of sensitivity kernels for tomographic purposes has been frequently discussed in the literature. Sensitivity kernels of different observables (e.g., amplitude, travel-time, and polarization for seismic waves) have been proposed, and relationships between adjoint formulation, time-reversal theory, and sensitivity kernels have been developed. In the present study, travel-time sensitivity kernels (TSKs) are derived for two source-receiver arrays in an acoustic waveguide. More precisely, the TSKs are combined with a double time-delay beam-forming algorithm performed on two source-receiver arrays to isolate and identify each eigenray of the multipath propagation between a source-receiver pair in the acoustic waveguide. A relationship is then obtained between TSKs and diffraction theory. It appears that the spatial shapes of TSKs are equivalent to the gradients of the combined direction patterns of the source and receiver arrays. In the finite-frequency regimes, the combination of TSKs and double beam-forming both simplifies the calculation of TSK and increases the domain of validity for ray theory in shallow-water ocean acoustic tomography.

Statistical signal characterization in ocean acoustic tomography and geoacoustic inversions.

A performance study of a statistical characterization of an underwater acoustic signal in relation to geoacoustic inversion or tomography problems is presented. The method of characterization has been presented by Taroudakis et al. [JASA 119, 1396–1405 (2006)] and is based on the use of an appropriate distribution law which describes the statistics of the sub‐band coefficients of the signal wavelet transform. The method has been applied with synthetic data simulating tomographic experiments with low‐frequency sources in shallow environments for the estimation of the water column and bottom properties. In this work the inversion procedure incorporating a genetic algorithm is applied in shallow water environments with simulated noisy data to assess the performance of the method and its limitations under realistic conditions.

Using short curved vertical arrays in ocean acoustic tomography

We consider the possibility of reconstructing the ocean mode structure based on measurements with short (not covering the entire ocean waveguide) arrays curved in an unknown way by ocean currents, which corresponds to the conditions of a real experiment. We have developed an algorithm for estimating the arrival times of various mode signals by solving a system of linear equations. The mode reconstruction accuracy as a function of the array length and profile curvature is investigated. The cases of using both the ocean noise field and deterministic signals as a sound source are considered. Comparison with the commonly used mode filtering method is made.

Remote sensing of Tyrrhenian shallow waters using the adjoint of a full-field acoustic propagation model

Over the past two years the capabilities of adjoint-based inversion have been explored for applications both in geoacoustic inversion and shallow-water acoustic tomography. Starting from case studies based on the principles of optimal control theory the performance of adjoint modeling has been constantly improved to more realistic scenarios ever since. In particular, the concept of semi-automatic adjoint generation has been introduced for inversion of satellite ocean colour imagery and geoacoustic characterization of seabed properties. In the framework of a Maritime Rapid Environmental Assessment sea trial (MREA/BP07) that was conducted in the same area south of the island of Elba as the earlier Yellow Shark experiment (YS94), this paper examines the original YS94 geoacoustic data and the recent MREA07 oceanographic data to demonstrate adjoint-based acoustic monitoring of environmental parameters in Mediterranean shallow waters. First, the adjoint-generated environmental gradients are validated for an application in geoacoustic inversion where the bottom geoacoustic parameters of the layered seabed are determined from long range acoustic propagation data in the water column. Then, for the application in ocean acoustic tomography, the temporal variability of MREA/BP07 sound-speed field data is analyzed in terms of empirical orthogonal functions and the adjoint-based approach is used to track the time-varying sound speed profile of the experimental transect.