Depth-dependent Sound Speed Profile Research Articles

Bottom Attenuation Coefficient Inversion Based on the Modal Phase Difference Between Pressure and Vertical Velocity from a Single Vector Sensor

An inversion scheme based on time-warping is presented for estimating the attenuation coefficient of a sediment bottom using a single vector sensor, restricted to shallow water and using low-frequency impulsive sources. The attenuation information is extracted from the modal phase difference between pressure and vertical velocity. The method is derived from Pekeris waveguide theoretical equations and the eigen values are obtained using the normal mode model Kraken. Some changes are made to the time-warping process to mitigate the inherent interference between adjacent modes, which improves the phase extraction capabilities. Results are presented for a two-layer, homogeneous environment using the RAM propagation model for depth-dependent sound speed profile simulations. This version of RAM was updated to provide radial and vertical velocities. For additional generality, the technique is evaluated in the presence of white noise.

Inversion of head waves in ocean acoustic ambient noise.

The virtual head wave is produced through cross-correlation processing of signals containing the real, acoustic head wave. The virtual head wave has the same phase speed as the head wave, but the travel time is offset, thus the term virtual. The virtual head wave, like the real head wave, propagates in a direction corresponding to the seabed critical angle. The virtual head wave travel time varies with array depth and water column depth. However, in a refracting environment, the travel time is also dependent on the depth-dependent sound speed profile. Previously, the virtual head wave was shown as observable from measurements of ocean ambient noise, and the arrival angle was used to estimate the seabed sound speed. By also using the virtual head wave travel times, it is possible to invert for array depth and water column depth. The previous analysis was limited to the assumption of a Pekeris waveguide, which is a special case of the more realistic refracting waveguide. In this paper, the virtual head wave and the inversion method are considered in environments having refracting sound speeds. The theoretical framework and the inversion method are presented along with illustrative simulations and application to the Boundary'03 data.

Underwater Target Localization and Synchronization for a Distributed SIMO Sonar with an Isogradient SSP and Uncertainties in Receiver Locations

A distributed single-input multiple-output (SIMO) sonar system is composed of a sound source and multiple underwater receivers. It provides an important framework for underwater target localization. However, underwater hostile environments bring more challenges for underwater target localization than terrestrial target localization, such as the difficulties of synchronizing all the underwater receiver clocks, the varying underwater sound speed and the uncertainties of the locations of the underwater receivers. In this paper, we take the sound speed variation, the time synchronization and the uncertainties of the receiver locations into account, and propose the underwater target localization and synchronization (UTLS) algorithm for the distributed SIMO sonar system. In the distributed SIMO sonar system, the receivers are organized in a star topology, where the information fusion is carried out in the central receiver (CR). All the receivers are not synchronized and their positions are known with uncertainties. Moreover, the underwater sound speed is approximately modeled by a depth-dependent sound speed profile (SSP). We evaluate our proposed UTLS algorithm by comparing it with several benchmark algorithms via numerical simulations. The simulation results reveal the superiority of our proposed UTLS algorithm.

Underwater acoustic localization of sperm whales with a pair of hydrophones

An iterative Bayesian approach using a pair of hydrophones was applied for the localization of vocalizing (click producing) sperm whales off the southern coast of Crete in the Eastern Mediterranean Sea in June 2015. The localization method relies on the time difference between direct and surface reflected arrivals at each hydrophone as well as between the direct arrivals at the two hydrophones, and on the knowledge of the hydrophones depths, whereas it accounts for refraction due to a depth-dependent sound-speed profile. The method provides range and depth estimates as well as estimates for the localization uncertainty reflecting measurement errors and environmental uncertainties. Further, by assuming a simple model for the array geometry bearing estimates were obtained. The inherent left-right ambiguity in bearing estimation can be resolved by changing the array orientation through maneuvering. The localization results were verified through encounters with the sperm whales when they ascended to the surface. [Work supported by EU-Greece through the Aristeia-II program and the NSRF 2014-2020/PERAN project.]

Stochastic matched-field localization of an acoustic source based on principles of Riemannian geometry.

Passive localization of acoustic sources is treated within a geometric framework where non-Euclidean distance measures are computed between a cross-spectral density estimate of received data on a vertical array and a set of stochastic replica steering matrices, rather than traditional replica steering vectors. A processing scheme involving matrix-matrix comparisons where steering matrices, as functions of the replica source coordinates, naturally incorporate environmental variability or uncertainty provides a general framework for considering the acoustic inverse source problem in an ocean waveguide. Within this context a subset of matched-field processors is examined, based on recent advances in the application of non-Euclidean geometry to statistical classification of data feature clusters. The matrices are interpreted abstractly as points in a Riemannian manifold, and an appropriately defined distance measure between pairs of matrices on this manifold defines a matched-field processor for estimating source location. Acoustic simulations are performed for a waveguide comprising both a depth-dependent sound-speed profile perturbed by linear internal gravity waves and a depth-correlated surface noise field, providing an example of the viability of this approach to passive source localization in the presence of sound-speed variability.

Target Localization for a Distributed SIMO Sonar With an Isogradient Sound Speed Profile

A distributed single-input multiple-output (SIMO) sonar system makes use of a sound source and a distributed star receiver network to localize targets. Compared with the terrestrial target localization, there are more challenges for underwater target localization. The underwater sound speed varies with depth, temperature, and salinity. As a result, the propagation path of the acoustic signal is not a straight line. It makes existing straight-line based distance measurements less accurate. Furthermore, time synchronization, which is closely related to time-based localization, has to be carried out because of the absence of GPS and clock differences of underwater devices. In this paper, we take the sound speed variation and time synchronization into account and propose two underwater target localization algorithms, namely, the space-alternating generalized expectation maximization-based underwater target localization (SAGE-UTL) and the nonlinear weighted least squares based underwater target localization (NWLS-UTL), for the distributed SIMO sonar system. The SAGE-UTL algorithm is designed for the situation, where the central receiver of the star receiver network needs to remain silent for its own safety. On the other hand, the NWLS-UTL algorithm is designed for the other situation, where the silence of the central receiver is not required. We assume that all the receivers are not synchronized, and the speed variation can be approximately modeled by a depth-dependent sound speed profile. Our proposed algorithms can jointly achieve target localization and time synchronization. We evaluate the SAGE-UTL and the NWLS-UTL algorithms with several numerical simulations. The results show good performance of our proposed algorithms compared with the Cramer–Rao bound and the tailored benchmark algorithms, such as the approximate nonlinear least squares algorithm and the approximate space-alternating generalized expectation maximization based underwater target localization algorithm.

Linearized two-hydrophone localization of a pulsed acoustic source in the presence of refraction: Theory and simulations

This paper develops an efficient three-dimensional localization method for transient acoustic sources, with uncertainty estimation, based on time differences between direct and surface-reflected arrivals at two hydrophones. The localization method accounts for refraction caused by a depth-dependent sound-speed profile using a ray-theoretic approach for calculating eigenray travel times and partial derivatives. Further, the method provides localization error estimates accounting for uncertainties of the arrival times and hydrophone locations, as well as for depth-dependent uncertainties in the sound-speed profile. In the first of two steps, source depth and range to each hydrophone are estimated using an iterative, linearized Gauss-Markov inversion scheme. In the second step, the estimated source ranges are combined with the hydrophone locations to obtain the source location in the horizontal. Localization performance is analyzed in a simulation study, and the linearized localization estimates and uncertainties are validated by comparison with a fully nonlinear (but numerically intensive) Markov-chain Monte Carlo inversion.

Bayesian linearized two-hydrophone localization of a pulsed acoustic source

A three-dimensional localization method for transient acoustic sources is developed, based on time differences between direct and surface-reflected arrivals at two hydrophones. The method accounts for refraction caused by a depth-dependent sound-speed profile using a ray-theoretic approach, and, further, it provides localization error estimates accounting for uncertainties of the arrival times and hydrophone locations, as well as for depth-dependent uncertainties in the sound-speed profile. In the first of two steps, source depth and range to each hydrophone are estimated using an iterative, linearized Gauss-Markov inversion scheme. In the second step, the estimated source ranges are combined with the hydrophone locations to obtain the source location in the horizontal. Localization performance is analyzed in a simulation study, and the linearized localization estimates and uncertainties are validated by comparison with a fully-nonlinear (but numerically intensive) Markov-chain Monte Carlo inversion. [Wor...

Theoretical analysis of warping operators for non-ideal shallow water waveguides.

Signals propagating in waveguides can be decomposed into normal modes that exhibit dispersive characteristics. Based on the dispersion analysis, the warping transformation can be used to improve the modal separability. Different from the warping transformation defined using an ideal waveguide model, an improved warping operator is presented in this paper based on the beam-displacement ray-mode (BDRM) theory, which can be adapted to low-frequency signals in a general shallow water waveguide. For the sake of obtaining the warping operators for the general waveguides, the dispersion formula is first derived. The approximate dispersion relation can be achieved with adequate degree of accuracy for the waveguides with depth-dependent sound speed profiles (SSPs) and acoustic bottoms. Performance and accuracy of the derived formulas for the dispersion curves are evaluated by comparing with the numerical results. The derived warping operators are applied to simulations, which show that the non-linear dispersion structures can be well compensated by the proposed warping operators.

The effects of sound in the marine environment workbench: A simulation tool to predict the impact of anthropogenic sound on marine mammals

The Effects of Sound in the Marine Environment (ESME) Workbench is a software tool designed to predict the impact of anthropogenic sounds on marine mammals. The ESME Workbench (http://esme.bu.edu) allows the user to use site-specific environmental data such as bathymetry and sound-speed profiles to predict sound propagation in a wide range of scenarios and to record the sound exposures received by virtual animals. The acoustic propagation models use range-dependent depth profiles and depth dependent sound speed profiles to compute the received sound level for simulated animal from each simulated source. The propagation models use bottom and sea surface characteristics to account for losses that occur during reflection at these boundaries. Sound sources are specified through parameters such as source location, frequency, intensity, and beam pattern. The animal behavior is simulated using the 3 MB animal movement model. We will provide hands-on demonstrations at the meeting for those interested in learning more about the ESME Workbench. [Funded by ONR.]

Modeling Propagation of Seismic Airgun Sounds and the Effects on Fish Behavior

High activity of seismic surveying in Norwegian waters has caused concerns about the impact the acoustic noise from the seismic airguns may have on marine life. There is evidence that this noise can cause reactions on the behavior of the fish resulting in reduced catches. To mitigate the problem and the conflict of interest between the fishing industry and the seismic exploration interest, the Norwegian Petroleum Directorate (NPD) commissioned SINTEF Information and Communication Technology (ICT, Trondheim, Norway) and the Department of Biology, University of Oslo (Oslo, Norway) to develop an acoustic-biological model to predict the impact of seismic noise on the fish population. The ultimate goal is to develop an acoustic-biological model to use in the design and planning of seismic surveys such that the disturbance to fishing interest is minimized. This acoustic module of the model is based on ray theory and can deal with range-dependent bathymetry and depth-dependent sound-speed profiles. The bottom is modeled as a sedimentary fluid layer over a solid elastic rock and the model requires the thickness and seismoacoustic properties of the sediments layer and the rock with compressional speed, shear speed, and absorption. The model simulates the total sound field, both in the time domain and in the frequency domain, out to very large distances. Calculated sound exposure levels are compared with startle response levels for cod. Preliminary conclusions indicate a required distance in the range of 5-10 km, but dependent on the depth and the season. In additions, under certain conditions, there will appear regions with hot spots where the sound level is significantly higher due to caustics and focusing of sound. Modeled results are compared with results obtained from a joint seismoacoustic survey conducted in summer 2009 at Vesterålen-Lofoten area (Nordland VII). In this experiment, signals were recorded at fixed hydrophone positions as the seismic vessel approached from a maximum distance of 30 km toward the receiving positions. The same situation was modeled using available geological and oceanographic information as input to the acoustic model. The agreement between the real and recorded signals and the model results is good. This indicates that in the future acoustic-biological models may be used in the design and planning of seismic surveys such that the disturbance to fishing is minimized.

Accurate Ranging in a Stratified Underwater Medium with Multiple Isogradient Sound Speed Profile Layers

Most of the available localization algorithms require pair-wise ranging between the nodes. The more accurate the ranging, the more precise the localization. In this paper, the problem of accurate ranging between sensor nodes in an underwater environment is considered. It is assumed that the underwater medium is composed of layers with an isogradient depth-dependent sound speed profile (SSP). In order to do the range estimation, we first show how the pair-wise time of flight (ToF) measurement between the nodes is related to the nodes’ positions. Then, based on this relation, we propose a novel ranging algorithm for an underwater acoustic sensor network (UASN). The proposed algorithm is based on the ToF measurement of the first arrival ray which may be reflected from the surface or the seabed. Through several simulations we show that the algorithm performs superb, and meets the Cramér Rao bound (CRB) for different values of the measurement noise power.

Estimates of the water column sound-speed field in three dimensions using a linearized perturbative technique

A perturbative inversion scheme [S. D. Rajan etal., J. Acoust. Soc. Am. 82, 998–1017 (1987)] is applied to estimate the water-column sound-speed field in a three-dimensional volume of the shallow ocean. The input data to the inversion are estimates of modal travel time data calculated from measurements from a distributed network of sources and receivers. The differences in the data are used as the basis for updating, or perturbing, a background model through the inversion algorithm to arrive at a solution. According to this technique, the horizontal plane is divided into a grid of range-independent regions and an estimate of the depth-dependent sound-speed profile within each region is obtained. The linearized perturbative technique is particularly well-suited for the three-dimensional inverse problem as the solution typically obtained in less than 20 iterations. During this talk, the method will be validated using synthetic data which are representative of oceanographic range-dependence in shallow water environments. The effect of the density and distribution of sources and receivers on the resolution of the inversion results will be discussed. [Work supported by ARL:UT IRD.]

Calibration of a passive acoustic target scattered data from the Gulf of Maine.

The scattered field from long vertical cylindrical targets, such as BBN targets, can be calculated by direct application of Green’s theorem and making use of appropriate boundary conditions. In a recent paper submitted to J. Acoust. Soc. Am., the authors show that the scattered levels from BBN-type targets in ocean waveguides with depth-dependent sound speed profile are influenced by the refractive nature of the incident field over the target depth extent. In the current work, the scattering model for vertically extended cylindrical (VEC) targets in range-dependent ocean waveguides is validated with experimental data from the 2006 ONR-sponsored ocean acoustic waveguide remote sensing (OAWRS) experiment in the Gulf of Maine. In order to take into account the scintillation in the measured scattered intensity caused by spatial and temporal fluctuations of the ocean waveguide, a stochastic approach is adopted. Monte Carlo simulations of the scattered field statistics are calculated by implementing the VEC model in a range-dependent environment randomized by internal waves. The simulated scattered field statistics for different source-receiver-target configurations is compared to the scattered returns from BBN targets measured during the 2006 Gulf of Maine experiment. Results show good agreement between the simulated and experimental scattered field statistics.

Track of a sperm whale from delays between direct and surface-reflected clicks

One dataset made available for the 2nd international workshop on detection and localization of marine mammals using passive acoustics featured a single sperm whale recorded for 25 min on five widely spaced, bottom-mounted hydrophones in the Tongue of the Ocean. In this paper, we track the whale using a model-based method that relies on the difference in arrival times along direct and surface-reflected propagation paths. Four receivers are required to estimate positions in three-dimensions. Details of the method are presented, and tracks are estimated using an isospeed and a depth-dependent sound speed profile (SSP). Depth estimates for the isospeed SSP are about 50–100 m shallower than for the depth-dependent SSP, and horizontal positions are similar. Performance estimates indicate that the depth-dependent SSP results are more accurate, with estimated depths of 650–760 m and average vertical and horizontal swim speeds (not accounting for current) of 0.30 m/s and 2.40 m/s, respectively.

Three-dimensional passive acoustic tracking of sperm whales (Physeter macrocephalus) in ray-refracting environments

A wide-aperture towed passive acoustic array is used to obtain ranges and depths of acoustically active sperm whales in the Gulf of Mexico in June 2004, by extending a technique previously reported [Thode, J. Acoust. Soc. Am. 116, 245-253 (2004)] to explicitly account for ray-refraction effects arising from a depth-dependent sound speed profile. Under this expanded approach, three quantities are measured from an impulsive sound: the time difference between direct-path arrivals on a forward and rear subarray, the time difference between the direct and surface-reflected paths on the rear subarray, and the acoustic bearing measured on the rear subarray. These quantities, combined with independent measurements of hydrophone depths and cable inclination, are converted into range-depth position fixes by implementing an efficient numerical procedure that uses a ray-tracing code to account for ray-refraction effects caused by depth-dependent sound speed profiles. Analytic expressions that assume a constant waterborne sound speed are also derived. Foraging depths of various sperm whales over 10 days in June, 2004 are estimated using the numerical technique.

Time domain analysis of normal mode, parabolic, and ray solutions of the wave equation

Estimation of sound speed and temperature fields from tomographic data requires knowledge of the accuracy of the forward model. For this reason, time domain results using three models derived from the acoustic wave equation are compared here. The models chosen for the analysis are normal modes (NM) and the parabolic equation (PE) and ray approximations. (The NM and PE time series are obtained via Fourier synthesis). The analysis is done using a depth-dependent sound-speed profile where only refracted and surface-reflected/refracted energy is allowed to propagate. Because the NM representation is theoretically exact in this environment, it is used as the comparative base. Travel-time sequences generated using the PE and ray approximations are compared to those generated using NM. For the angular aperature analyzed (±9 to ±16.5 deg), phase errors in the PE models investigated limit reliable predictions of travel time to distances of about 1000 km. However, the ray approximation is found to be adequate for estimating arrival times at ranges out to 4000 km for frequencies of 100 Hz and above.

Acoustic propagation in a refracting ocean waveguide with an irregular interface

A method is derived for computing the acoustic field in a cylindrically symmetric ocean with an irregular water—sediment interface and a general depth-dependent sound-speed profile. The approach is based on a stepwise-coupled mode solution which was originally developed for a homogeneous water column. The extension to a general sound-speed profile is carried out with the aid of Galerkin's method. Numerical examples are given for a shallow-water waveguide with an upward-refracting sound-speed profile. The first example shows that propagation loss is significantly greater in a rough-bottomed waveguide than in a smooth-bottomed waveguide. In the second example, it is shown that upward refraction reduces the effect of bottom roughness on sound propagation by reducing the insonification of the rough water—sediment interface.