Water Column Sound Speed Profile Research Articles

Optimized constraints for the linearized geoacoustic inverse problem

A geoacoustic inversion scheme to estimate the depth-dependent sound speed characteristics of the shallow-water waveguide is presented. The approach is based on the linearized perturbative technique developed by Rajan et al. [J. Acoust. Soc. Am. 82, 998-1017 (1987)]. This method is applied by assuming a background starting model for the environment that includes both the water column and the seabed. Typically, the water column properties are assumed to be known and held fixed in the inversion. Successful application of the perturbative inverse technique lies in handling issues of stability and uniqueness associated with solving a discrete ill-posed problem. Conventionally, such problems are regularized, a procedure which results in a smooth solution. Past applications of this inverse technique have been restricted to cases for which the water column sound speed profile was known and sound speed in the seabed could be approximated by a smooth profile. In this work, constraints that are better suited to specific aspects of the geoacoustic inverse problem are applied. These techniques expand on the original application of the perturbative inverse technique by including the water column sound speed profile in the solution and by allowing for discontinuities in the seabed sound speed profile.

Inversion for sound speed profiles in the northern of South China Sea

This paper represents the water column sound speed profiles (SSPs) by the empirical orthogonal functions (EOFs) and inverts them by the matched field method using the acoustic and oceanographic data simultaneously collected in the northern of South China Sea during a shallow water experiment in June 2009. The water column data show evident spatial and temporal variations caused by the internal waves. The EOFs are extracted from the long time water temperature data continuously measured by a temperature sensor array. Using a two-dimensional advective frozen-ocean model, the simulation of the inversions for the range-dependent shallow-water environment is performed and the results show that the inverted sound speed profile is consistent with the mean one along the acoustic signal propagation path. Eventually, the wide-band source (WBS) signals recorded by a vertical line array during the experiment are used to invert the average sound speed profiles, and the results agree well with the experimental measurements, demonstrating that the matched field method is valid and stable.

Application of an acoustic inversion technique to infer the presence of oil from anomalies in the water column sound speed profile.

This work attempts to exploit the difference in sound speed between oil and seawater to determine the location of large underwater oil plumes. Two perturbative inversion schemes, both based on the work of Rajan, et al. [JASA 82, 998–1017 (1987)] will be presented. Both methods can be used to estimate sound‐speed in the water column as a function of range and depth. The first technique uses horizontal wavenumber data which are estimated from the complex pressure field of a narrow band signal. The inversion result provides an estimate of sound speed profile along a two‐dimensional slice through the environment. The method is demonstrated using data from the Shallow Water 2006 (SW06) experiment. The second technique utilizes modal travel time data. When applied in a multi‐sensor deployment, the outputs of this inversion scheme provide a three‐dimensional characterization of the coastal environment. Furthermore, because sound propagates at a much faster speed than oil spill spreads, this technique can be used to track changes in the size and shape of the oil plume over time. [Work supported by ARL:UT IR&D]

Geoacoustic Inversion Using Backpropagation

This paper presents inversion results of the 2006 Shallow Water Experiment (SW06) data measured on a vertical line array. A low-frequency (100-900 Hz) chirp source was towed along two tracks (circle, straight line) at 30-m depth. For the inversions, a three-step optimization scheme is applied to the data using very fast simulated reannealing (VFSR). The objective function is defined by the energy of the backpropagated signal from the array to the source. At each step, water-column sound-speed profile (SSP), experimental geometry, and geoacoustic parameters are inverted successively. An environmental model is employed consisting of a linear segmented SSP in the water column, a sediment layer, and a half-space. The geometric parameter inversion results show good agreement with in situ measurements. Finally, the estimated geoacoustic parameters show that the experimental site near the vertical line array (VLA) is fairly homogeneous in bottom properties consisting of a 21-m-thick sediment layer with sound speed of around 1600 m/s over a hard basement whose sound speed is approximately 1750 m/s.

Geoacoustic and source tracking using particle filtering: Experimental results

A particle filtering (PF) approach is presented for performing sequential geoacoustic inversion of a complex ocean acoustic environment using a moving acoustic source. This approach treats both the environmental parameters [e.g., water column sound speed profile (SSP), water depth, sediment and bottom parameters] at the source location and the source parameters (e.g., source depth, range and speed) as unknown random variables that evolve as the source moves. This allows real-time updating of the environment and accurate tracking of the moving source. As a sequential Monte Carlo technique that operates on nonlinear systems with non-Gaussian probability densities, the PF is an ideal algorithm to perform tracking of environmental and source parameters, and their uncertainties via the evolving posterior probability densities. The approach is demonstrated on both simulated data in a shallow water environment with a sloping bottom and experimental data collected during the SWellEx-96 experiment.

Inversion for range-dependent water column sound speed profiles on the New Jersey shelf using a linearized perturbative method

The environment of the New Jersey shelf is characterized by high spatial and temporal variability of water column properties caused by intrusions of warm, salty water from the continental slope. These intrusions cause fluctuations in the water column sound speed profile which can have significant effects on acoustic propagation in shallow water. In this work, a linearized perturbative inverse technique is applied to estimate range-dependent water column sound speed profiles. This method utilizes estimates of horizontal wave numbers to determine sound speed as a function of depth. This technique is appropriate for the range-dependent shallow-water environment as horizontal wave numbers can be measured semilocally (1-2 km aperture) and their values are a direct measurement of the local environmental parameters. Difficulty is encountered in application of the perturbative inverse technique because the wave number data are insensitive to some portions of the waveguide and, as a result, the solution can deviate wildly from true values. This issue is addressed by application of approximate equality constraints which force the solution to be close to likely values at prescribed locations.

Modeling marine mammal sound exposure levels due to ship traffic noise.

Noise due to ship traffic is a pervasive component of the global and local ocean noise from 10 to 500 Hz, but little is known of its effects on marine mammals because such effects depend on the sound exposure levels at a given location. The approach presented here consists of modeling the sound field due to various spatial distributions of noise sources located 5–10 m below the sea surface. We approximate a three‐dimensional sound field by computing the sound propagation from each source along a prescribed number of two‐dimensional vertical radial slices of a range‐dependent environment, which includes bathymetry, seafloor geoacoustic properties, water column sound speed profiles, and sea surface roughness. Propagation results from all sources are incoherently summed to form the overall sound exposure map. By varying the density of ships in a given area we can simulate the effects of shipping lanes and assess how the characteristics of the underlying seafloor alter the results.

Simultaneous inversion of seabed and water column sound speed profiles in range-dependent shallow water environments.

It is well known that the properties of the water column and seabed affect acoustic propagation in shallow water. As a result, numerous inversion schemes have been developed to estimate environmental properties. Many of these algorithms address the problem of estimating properties of the seabed alone, while assuming properties of the water column are known. An example of such an inversion scheme is perturbative inversion which uses modal wavenumbers to obtain sound speed in the seabed as a function of depth [Rajan et al., (1987)]. The inversion algorithm involves solving an ill-posed problem, with regularization used to stabilize the solution, resulting in a smoothed version of the true sound speed profile. To simultaneously estimate water column and seabed properties, qualitative regularization is used to resolve the discontinuity in the sound speed profile at the seafloor and approximate equality constraints are used to constrain the solution in portions of the water column for which the data alone are insufficient. The primary advantage of simultaneously inverting for water column and sediment sound speed profiles is that poor knowledge of a priori information about water column is not aliased into the solution for the seabed. [Work supported by NDSEG and ONR.]

Estimating geoacoustic properties of marine sediment on the New Jersey Continental Shelf from broadband signals.

This paper presents geoacoustic inversions of broadband signals collected in the Shallow Water 2006 Experiments off the coast of New Jersey. An L-shaped array was deployed on the top of a sand ridge, in 70 m of water. The acoustic source was maintained at a distance of 190 m from the vertical leg of the L-shape array, and lowered from 10 to 60 m in 10-m intervals in the experiment. Two sets of chirps, low frequency (100–900 Hz) and midfrequency (1100–2900 Hz), were transmitted with approximately the same experimental geometry. The water column sound speed profiles were measured at the source position. This study examines the sediment information content from different frequency bands recorded on the vertical leg of the L-shape array. Because the chirps have different temporal resolutions and energy penetrabilities in the sediment, the received signals exhibit different bottom structures at different frequency bands. Travel time geoacoustic inversions are carried out at both frequency bands using signals from the resolvable reflections from the sediment layers. The variation in the oceanic sound speed profile is parametrized in terms of its characteristics and included in the inversion. [Work supported by ONR Ocean Acoustics.]

Inversion for range-dependent water column sound speed profiles in shallow water.

Spatial and temporal variability of the sound speed field in the water column can have a significant impact on acoustic propagation. It is difficult or sometimes impractical to measure fine scale variations in water column properties over an acoustic propagation path. When measurements are not available, water column properties must be approximated. In past work, inverse methods based on acoustic tomography and matched-field processing have been used to estimate mean water column sound speed profiles. The drawback of these methods is their inability to capture fluctuations in the water column sound speed profile. In this work, a perturbative scheme using modal wave numbers is used to obtain range-dependent features of the water column. Based on the work of Rajan et al. (1987), a technique has been developed to estimate water column sound speed profiles using historical data to constrain the inversion. This constraint allows for a robust inversion; the result is accurate both when the inputs are noisy and when the bottom is poorly known. The new technique is demonstrated using data from the Shallow Water 2006 (SW06) experiment. Inversion results are compared to in situ measurements from the towed CTD chain. [Work supported by NDSEG and ONR.]

Time domain geoacoustic inversion using back-propagation on an L-shaped array.

This paper presents inversion of Shallow Water 06 experimental data measured on an L‐shaped array (SWAMI 32). The array was deployed in 70‐m water depth and consists of an equally spaced 10‐element vertical line array (VLA) and a 256‐m‐long bottom moored 20‐element horizontal line array (HLA). A mid‐frequency (1100–2900‐Hz) chirp source was towed at 35‐m depth along a circular track around the VLA with radius 190 m. Time domain inversions using VLA, HLA, and both arrays were carried out and results compared. For the inversions, a multi‐step optimization scheme is applied to the data using very fast simulated reannealing. The objective function is defined by the power of the back‐propagated signal from the array to the source. At each step, water column sound speed profile, experimental geometry, and geoacoustic parameters are inverted successively. Accurate HLA positions were essential for the HLA and the HLA+VLA inversions. Finally, the inversion results were compared with other results near the site. [Work supported by ONR.]

Variability of the water column sound speed profile and its effect on acoustic propagation during the Shallow Water 2006 Experiment.

Water column sound speed variability can have a significant effect on acoustic propagation. However, measurements of water column properties, either by ship based systems or moored instruments, are often neither spatially nor temporally colocated with acoustic measurements. Variability of the water column over the acoustic propagation path must be approximated by extrapolation. During SW06, a low‐frequency sound source, broadcasting pure tones, was towed along radials from a vertical array comprised of both hydrophones and oceanographic sensors. Concurrently, and from the same ship, a towed CTD chain was used to measure the water column as a function of both time and space. Changes in sound speed as high as 15 m/s were recorded over a 5‐km aperture. The CTD chain, in combination with environmental sensors on the VLA, allowed for continuous monitoring of the water column sound speed profile at both the source and receiver locations. It is shown that horizontal wavenumber predictions for the acoustic field are biased by the sound speed measured at the source compared to at the receiver. In addition, predictions of the acoustic field are improved when the range dependence of the sound speed profile is considered. [Work supported by NDSEG and ONR.]

Short range geoacoustic inversion with a vertical line array

During the SW06 (Shallow Water 06) experiments, short range geoacoustic inversion experiments were carried out at a source‐receiver range of 230 m to determine the sea bed properties at fine spatial scales. Acoustic signals were collected at 16 hydrophones on a vertical line array from source depths of 15‐65 m in 10 m intervals. For this experimental geometry, the angular coverage for reflectivity versus grazing angle is 7‐25°. This paper combines the information extracted from low‐frequency (100‐900 Hz) and mid‐frequency (1500‐4500 Hz) LFM signals transmitted from the same source position to estimate multi‐layered geoacoustic models. The model parameterization was based on the number of resolved sub‐bottom reflections in the data. An inversion approach using adaptive simplex differential evolution was applied to the data to invert for the sub‐bottom sound speeds and layer thicknesses. The temporal variation of the water column sound speed profile was included in the inversion. The estimated sound speeds ...

Tools and techniques to measure environmental parameters that improve mid-frequency sonar performance predictions

The performance of mid-frequency active sonar systems in littoral waters depends upon several environmental factors and their variability in space and time. These factors include seabed scattering strength, bottom loss, and the water column sound speed profiles. DRDC Atlantic has developed experimental apparatus and techniques to measure these environmental parameters in order to enhance the accuracy of sonar performance estimates in the mid-frequency band, 1 to 10 kHz. Experiments have been conducted during sea-trials on the Scotian Shelf and Strataform area on the New Jersey Shelf. In some instances, these experiments have been in conjunction with international research partners to provide an inter-comparison between experimental techniques and/or a comparison of rapid environmental assessment techniques with more traditional, and time-consuming, methods. In this talk, we will demonstrate that predictions of sonar system performance improve when the environmental parameters obtained with these techniques are used, rather than parameters from archival sources.

Linear and nonlinear measures of ocean acoustic environmental sensitivity

This letter defines linear, linearized, and nonlinear measures of environmental sensitivity for ocean acoustic propagation that account for realistic uncertainties in various environmental parameters (water-column sound-speed profile and seabed geoacoustic properties). Simple interpretations of sensitivity are based on the implicit assumption of a linear relationship between parameter sensitivity and parameter uncertainty. This assumption is examined by comparing the three sensitivity measures over a range of parameter uncertainties about the actual assumed environmental uncertainty. Sensitivity range and depth dependencies are illustrated for realistic geoacoustic uncertainties and oceanographic variability of the sound-speed profile.

Adaptive sampling based on the waveguide invariant for determining environmental provinces

The waveguide invariant (related to the striations seen in range-frequency plots of sonar data) can be used as a tool to determine provinces in adaptive sampling schemes used to plan the placement of environmental measurements. The quantity from which the waveguide invariant is derived is used to demark spatial regions over which the environment can be considered to be range and bearing independent from an acoustic point of view. This allows significant speedup in acoustic propagation calculations. It also provides guidance for the placement of environmental measurements—particularly water column sound speed profile measurements. Results from the application of this technique to simulated data and to data from the Sea of Japan are shown and are compared to results of another method based on the adiabatic condition. [This work was supported by ONR with technical guidance provided by the Naval Research Laboratory, Stennis Space Center, MS.]

Environmental uncertainty in ocean acoustic source localization

Localization of an acoustic source in the ocean is often limited by lack ofknowledge of the physical properties of the environment, such as seabedgeoacoustic parameters and water-column sound-speed profile (SSP). Quantifyingenvironmental uncertainties and how they transfer to uncertainties for sourcelocalization represent important problems that are addressed in this paperusing Bayesian inference theory. Metropolis Gibbs’ sampling is appliedto estimate the uncertainties for environmental inversion in the form ofmarginal probability distributions, covariances and credibility intervals.Heat-bath sampling is applied to source localization with environmentaluncertainties, with the resulting localization uncertainty quantified interms of the joint marginal probability distribution for source range anddepth (i.e. a probability ambiguity surface). Localization uncertainties areexamined as a function of uncertainties in geoacoustic parameters and SSP.

Source localization in a time-varying ocean waveguide.

One of the most stringent impairments in matched-field processing is the impact of missing or erroneous environmental information on the final source location estimate. This problem is known in the literature as model mismatch and is strongly frequency dependent. Another unavoidable factor that contributes to model mismatch is the natural time and spatial variability of the ocean waveguide. As a consequence, most of the experimental results obtained to date focus on short source-receiver ranges (usually <5 km), stationary sources, reduced time windows and frequencies generally below 600 Hz. This paper shows that MFP source localization can be made robust to time-space environmental mismatch if the parameters responsible for the mismatch are clearly identified, properly modeled and (time-)adaptively estimated by a focalization procedure prior to MFP source localization. The data acquired during the ADVENT'99 sea trial at 2, 5, and 10 km source-receiver ranges and in two frequency bands, below and above 600 Hz, provided an excellent opportunity to test the proposed techniques. The results indicate that an adequate parametrization of the waveguide is effective up to 10 km range in both frequency bands achieving a precise localization during the whole recording of the 5 km track, and most of the 10 km track. It is shown that the increasing MFP dependence on erroneous environmental information in the higher frequency and at longer ranges can only be accounted for by including a time dependent modeling of the water column sound speed profile.

Comparison of robust matched-field processing algorithms in a real, shallow water environment

In recent years, a number of matched-field processing (MFP) algorithms have been proposed to mitigate the ever-present problem of mismatch. This paper compares the source localization performance of six processors—Bartlett, Midpoint, Minimum Variance (MV) Distortionless Response, White Noise Constrained, MV with Neighboring Location Constraints, and MV with Environmental Perturbation Constraints—in a real, shallow water environment. MFP was performed on an 84.4-m aperture, 16-element subset of a MPL vertical line array deployed from the R/P FLIP west of Point Loma in approximately 200-m water. A continuous wave source, emitting ten tonals from 53–197 Hz, was towed over both range-independent and range-dependent tracks at source ranges of 1–7 km and at source depths of 60–130 m. First, MFP results utilizing a detailed geoacoustic model of the region, water column sound speed profiles measured during the experiment, and a priori knowledge of array tilt, demonstrate general processor characteristics. Second, MFP results which assume: (1) reasonable though further mismatched geoacoustic parameters; (2) water column sound speed profiles based upon historical data; and (3) no a priori knowledge of array tilt, illustrate processor performance under various mismatch conditions likely to be encountered in realistic applications.