Range-dependent Shallow Water Environments Research Articles

Effect of oceanography on broadband acoustic wave propagation in a range dependent shallow water environment

To investigate the effects of oceanographic variability on broadband acoustic wave propagation in shallow water environments, two experiments were carried out on the New England continental shelf region in 2017 and 2022, respectively. Towed sources were used to transmit chirp waveforms over the frequency ranges of 0.5—1.25 kHz and 1.5 – 4.0 kHz on the same acoustic tracks. The broadband acoustic transmissions were received by vertical acoustic arrays with multiple hydrophones. In 2017, the sound speed in the water column was isovelocity while in 2022 the presence of a duct at the depth of 20 m and upward refracting conditions reduced the interaction of acoustic transmissions with the seafloor compared to the experiment in 2017. Experimental observations show the importance of the water column on bottom interacting broadband waveforms causing challenges for the inversion algorithms. In this paper, we present acoustic and oceanographic results showing how the variability of sound speed could change the characteristics of the broadband transmission. Modeled results versus experimental data are discussed. [Work supported by ONR.]

Broadband acoustic wave propagation in temporally variable range dependent shallow water environment

Broadband acoustic wave propagation in shallow water regions involves interactions with the sea surface, sea bottom, and the water column. The topic of shallow water acoustics in the presence of variable water column has been of interest in recent years. Experimental observations have shown the importance of the effects due to the boundaries and the sound speed profile in the water column. However, more work needs to be done to establish ground truth in numerical modeling for temporally variable water column in a range dependent environment. In this paper we examine the broadband propagation on the shelf as well as the range dependent shelf break regions to depict how temporal variability of sound speed could change the dispersion characteristics of the broadband signal. Comparison between modeled and experimental data are provided. [Work supported by ONR OA program.]

A fast multi-layer boundary element method for direct numerical simulation of sound propagation in shallow water environments

Direct three-dimensional (3D) numerical simulations of acoustic fields in range-dependent shallow water environments remains a challenge due to environmental complexities and large computational cost. We develop an efficient 3D boundary element method (BEM) to solve the Helmholtz equation for shallow water acoustic propagation, which utilizes a Pre-corrected Fast Fourier Transform (PFFT) approach to reduce the computational effort from O(N2∼3) to O(Nlog⁡N) where N is the total number of boundary unknowns. To account for inhomogeneous media, the method allows for arbitrary number of coupled multi-layer BEM sub-domains. With O(Nlog⁡N) efficiency and the use of massively parallel high-performance computing platforms, we are able to conduct multi-layer 3D direct numerical simulations of low-mid frequency acoustics over kilometer ranges. We perform extensive validations of the method and provide two shallow water waveguide examples benchmarked against theoretical solutions. To illustrate the efficacy and usefulness of the PFFT-BEM method, we perform 3D large-scale direct numerical simulations to assess the performance of two established canonical models: axisymmetric coupled mode model for 3D seamount; and Kirchhoff approximation and perturbation theory for 3D rough surface scattering.

Effects of time dispersion on echo, reverberation, and echo to reverberation ratio in a range-dependent Pekeris waveguide

In shallow water, active sonar performance is typically limited by reverberation, making the prediction of target echo and reverberation, and their ratio, an important part of sonar performance prediction. In range-dependent shallow water environments, these quantities are often calculated without considering the effect of dispersion. The effect of time dispersion is considered taking examples for a range-dependent bathymetry from the 2010 Weston Memorial Workshop. Using the analytical method of [M. A. Ainslie and D. D. Ellis (in press), IEEE Journal of Oceanic Engineering], combined with normal mode predictions [D. D. Ellis (1995). The Journal of the Acoustical Society of America, 97(5), 2804-2814], neglect of time dispersion is found to result in an error of up to 16 dB in the echo level for a short CW pulse (duration 3 ms). The effect of 3D geometry is considered, and results for a cylindrically symmetric bathymetry are shown to differ by up to 14 dB from the corresponding results with a Cartesian symmetry. The difficulties associated with modeling an LFM pulse are discussed.

Echo, Reverberation, and Echo-to-Reverberation Ratio for a Short Pulse in a Range-Dependent Pekeris Waveguide

In shallow water, active sonar performance is typically limited by reverberation, making the prediction of target echo, reverberation, and echo-to-reverberation ratio an important part of sonar performance prediction. In range-dependent shallow-water environments, the echo is often calculated without considering the effect of dispersion, and reverberation predictions are often limited to Lambert's rule for seabed scattering. In this paper, analytical formulae are derived for the echo, including the effect of time dispersion, and for reverberation for non-Lambert scattering laws. More specifically, the method for calculating the short-pulse echo intensity developed by Harrison and Nielsen is extended to a range-dependent bathymetry. For reverberation, the Zhou–Harrison method is also generalized to cover an arbitrary power law for the seabed scattering coefficient, combined with a range-dependent bathymetry. These formulae are applied to range-dependent test cases from an international workshop, and compared with predictions using a normal mode sum. Neglect of time dispersion is found to result in an error of up to 16 dB. Results for a cylindrically symmetric bathymetry differ by up to 14 dB from the corresponding results with a Cartesian symmetry.

Frequency invariability of acoustic field and passive source range estimation in shallow water

The frequency invariability of the warped modal signal and the warped signal autocorrelation function in shallow water is discussed. A method is proposed for passive source-range estimation based on the frequency invariability and warping transform of signal autocorrelation function received by a single hydrophone in a range-independent or weak range-dependent shallow water environment. In the method, a guided source with a known range is employed to provide the crucial and relative invariant scaled features. The experimental data in shallow water with an iso-speed profile and a fluctuated thermocline are used to verify this approach. The relative errors of the source range estimation are basically less than 10%.

Assessment of geoacoustic inversion methods

Sound transmission in shallow water is strongly affected by the physical and acoustic properties of the ocean bottom. Over the past decade, sophisticated methods have been developed for estimating parameters of geoacoustic models that account for the interaction of sound with the bottom. The performance of the methods has been compared in benchmarking exercises for range-independent and range-dependent shallow water environments using simulated data. This paper extends the comparison of geoacoustic inversion methods to assess performance using data from experiments at sites where the ocean bottom environment was well known from independent ground truth information. There are several aspects to performance assessment. The comparison presented here shows the accuracy of estimates from various inversion methods compared to the ground truth data about the ocean bottom sediments. The methods that are compared include matched field inversion; perturbation techniques based on modal waveumber estimation; bottom loss measurements; travel time tomography; and in situ physical measurement. The assessment shows overall consistency from all the methods.

A moment-based technique to palliate pervasive clutter while preserving objects of interest

High false alarm rates are a persistent problem for ASW active sonars. This talk describes a recently developed sonar clutter characterization and control method, the “Poisson-Rayleigh Inspired Method” (PRIM). The closed-form PRIM is based on NRL's two-parameter Poisson-Rayleigh (P-R) model that, like the popular K-type model, provides a physical context for relating the characteristics of data distributions to scatterer attributes (density and relative strength). However, with its extra degree of freedom, the P-R model offers the potential to exploit more information through higher-order (4th and 6th) data moments, and thus do a better job of characterizing clutter. The technique is demonstrated on normalized, broadband sonar clutter data collected in two range-dependent shallow-water environments, one bottom dominated and one fish dominated. The data results suggest that in contrast to the popular K-distribution, the power of the discrete-scatterer component in the P-R distribution can provide feature information that is largely independent of the peak signal-to-noise ratio of an echo, implying a potential for improved rejection of clutter relative to target-like objects. [Work supported by ONR.]

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.

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.

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.]

BROADBAND SIGNAL SIMULATION IN SHALLOW WATER

Today's minimum requirements for ocean acoustic models are to be able to simulate broadband signal transmissions in 2D varying environments with an acceptable computational effort. Standard approaches comprise ray, normal mode and parabolic equation techniques. In this paper we compare the performance of four broadband models [Formula: see text] on a set of shallow-water test environments with propagation out to 10 km and a maximum signal bandwidth of 10–1000 Hz. It is shown that a computationally efficient modal approach as implemented in the [Formula: see text] model is much faster than standard, less optimized models such as [Formula: see text] and [Formula: see text]. However, the handling of range dependency in the adiabatic approximation is not always sufficiently accurate, and it is suggested that a mode coupling approach be adopted in [Formula: see text]. Moreover, the interpolation of modal properties in range could lead to a further significant speed-up of mode calculations in range-dependent environments. It is concluded that coupled modes with wavenumber interpolation in both frequency and range remain the most promising wave modeling approach for broadband signal simulations in range-dependent shallow water environments. At higher frequencies (> 1 kHz) there is currently no alternative to rays as a practical signal simulation tool.

Range-dependent geoacoustic inversion: results from the inversion techniques workshop

Model-based geoacoustic inversion in range-dependent underwater environments is a challenging task constrained by data quality (synthetic or measured) and propagation-model efficiency and accuracy. The Inversion Techniques Workshop (ITW), held in Gulfport, MS, May 15-18, 2001, was organized for the acoustics community to present state-of-the-art numerical geoacoustic inversion capabilities in range-dependent shallow-water environments. The organizers defined five range-dependent test cases (three synthetic and two experimental cases). Two of the synthetic cases were adopted for geoacoustic inversion in this paper. The first test case (TC1) is a monotonic down-slope bathymetry problem and the adiabatic normal-mode model PROSIM was applied for the inversion in this case. The second test case (TC3) is a flat-bottom case with an intrusion. The forward model used in this case was RAMGEO. The global optimization package SAGA was used for geoacoustic inversion of the synthetically generated reference solutions for TC1 and TC3. In general, the geoacoustic inversion results are in good agreement with the true solutions provided by the organizers. The results obtained demonstrate the feasibility of performing geoacoustic inversion in synthetic range-dependent shallow-water environments. However, results show that the propagation model choice in the inversion is strongly dependent on the specific range-dependent environment.

Guest editorial special issue on geoacoustic inversion in range-dependent shallow-water environments

Guest editorial special issue on geoacoustic inversion in range-dependent shallow-water environments

Matched Field Tomographic Inversion to Determine Range Dependent Geoacoustic Properties

The paper describes a new broadband tomographic matched field method for estimating the geoacoustic properties of a range-dependent shallow water environment. This method is designed for a multiple acoustic element configuration (several sources and vertical arrays deployed in an ocean region) in order to estimate the range and cross-range properties of the sediment over the region. The synthetic pressure fields (replicas) for the tomographic inversion are computed using a ray model. A linear processor operating in the frequency domain is used to quantify the match between replica and measured fields. This processor is based on coherent summations over frequencies and receiver pairs. The method is demonstrated for a geoacoustic ocean model simulating the environment of the Haro Strait experiment. The area is divided into cells in which the geoacoustic properties are range independent but can vary from one cell to another. The layer thickness can vary within a cell. Results are presented for the estimation of the compressional velocity and sediment layer thickness in the ideal case of a noise free synthetic data set.

MATCHED FIELD TOMOGRAPHIC INVERSION TO DETERMINE RANGE DEPENDENT GEOACOUSTIC PROPERTIES

The paper describes a new broadband tomographic matched field method for estimating the geoacoustic properties of a range-dependent shallow water environment. This method is designed for a multiple acoustic element configuration (several sources and vertical arrays deployed in an ocean region) in order to estimate the range and cross-range properties of the sediment over the region. The synthetic pressure fields (replicas) for the tomographic inversion are computed using a ray model. A linear processor operating in the frequency domain is used to quantify the match between replica and measured fields. This processor is based on coherent summations over frequencies and receiver pairs. The method is demonstrated for a geoacoustic ocean model simulating the environment of the Haro Strait experiment. The area is divided into cells in which the geoacoustic properties are range independent but can vary from one cell to another. The layer thickness can vary within a cell. Results are presented for the estimation of the compressional velocity and sediment layer thickness in the ideal case of a noise free synthetic data set.

Tomographic inversion of geoacoustic properties in a range-dependent shallow-water environment.

This paper presents a matched-field tomographic method to estimate the geoacoustic properties of the ocean bottom for a range-dependent medium in shallow water. The inversion method has been developed in order to interpret experimental data from the Haro Strait PRIMER sea trial. This experiment was carried out in June '96 and used low-frequency broadband signals that were received on three vertical line arrays. Inversion of the data is particularly difficult because of the complex bathymetry of the Haro Strait experimental site. For this inversion, a range-dependent ray code was developed to solve the forward problem, allowing an arbitrarily layered bottom environment. The inversion scheme is based on modeling the propagation time and the amplitude of the recorded data, and a simple new cost function is proposed. The signal ray paths are identified automatically using a simple process that compares calculated and measured travel times. Data from multiple source positions are used to invert the range dependence of the geoacoustic model. The environment is separated into segments, and within each segment the inversion is carried out layer by layer for a multilayer geoacoustic model. Starting with the topmost layer, the range-dependent thickness and sound speed are estimated via a Monte Carlo method. Inversion results are presented for synthetic and experimental data from the Haro Strait sea trial.

Multipath compensation in shallow water environments using a virtual receiver

An acoustic technique for compensation of signal distortion due to propagation is developed in analogy with an astronomical technique in which light from a bright “guide star” is used to correct atmospheric aberration of weaker objects that are nearby in the angular sense. The acoustic technique investigated here uses a vertical array to receive both the signal from a broadband “guide source” and an unknown “objective source” which propagates over a partially shared path. The algorithm is a spatial–temporal cross correlation of the two signals and is termed a “virtual receiver” as the output approximates the signal that the unknown source would produce at the location of the guide source. By strategically locating the guide source, many of the distorting effects of an unknown propagation region can be removed, including mode coupling and multipaths. The virtual receiver can also be combined with matched field processing techniques to estimate the unknown source location. Results are given for a variety of numerical experiments in both the time and frequency domains. The numerical simulations are used to illustrate the virtual receiver algorithm in range-dependent shallow water environments exhibiting lossy propagation and mode coupling.

Mode coupling and mode stripping in propagation across the range-dependent Atlantic Generating Station (AGS) site.

The complicated patterns of modal structure and attenuation observed in sound propagation through a range-dependent shallow-water environment are used to investigate and visualize the mechanisms of mode coupling and stripping. Broadband experiments have been carried out at the geologically known AGS site in the past few years. This presentation reports on a recent experiment which provides significantly more acoustic data along different propagation tracks and, consequently, permits high resolution of physical processes in range. A mode-filtering technique is used to construct modal structure from the data, while modal attenuation is obtained from spectral ratios. The evolution of modal structure is determined as a function of frequency and range. This is examined in view of environmental parameters along a selected propagation track. Numericalsimulations are carried out using interpolations of the known geoacoustic environment and employing the KRAKEN normal mode and RAM parabolic equation models. Results from these simulations are compared with the experimental data.

Matched-field processing in a range-dependent shallow water environment in the Northeast Pacific Ocean

This paper describes matched-field processing (MFP) of data collected in shallow water off the western coast of Vancouver Island in the Northeast Pacific Ocean. The data were collected from a vertical line array (VLA) as part of the PACIFIC SHELF trial carried out on the continental shelf and slope during September 1993, sensors in the 16-element VLA were evenly spaced at depths between 90 and 315 m, while the sound source was towed along radial paths or arcs. In this paper, we present results of the analysis of data from a continuous wave (CW) source which was towed downslope at a depth of 30 m in water from 150 to 375 m deep, in order to model the range-dependence of the acoustic propagation efficiently, the replica fields were calculated using the adiabatic normal mode approximation. This approximation was considered appropriate for the bottom slopes of the environment. Using sparse bathymetric data, a water sound speed profile and estimates of bottom properties, MFP correlations on individual ambiguity surfaces were found to be greater than 0.9 for the strongest signals. On account of environmental mismatch, the source position could not be determined unambiguously from most of the ambiguity surfaces even at high signal-to-noise ratios. Nevertheless, when an efficient linear tracker was applied to the ambiguity surfaces to find tracks, the source track was recovered at both low and high signal-to-noise ratios, this tracker performs the analysis at a constant depth and reports the track with the highest estimated track signal-to-noise ratio.