Geoacoustic Inversion Methods Research Articles

On the Dependence of Acoustic Modes on Media Parameters in the Pekeris-Airy Waveguide

A waveguide consisting of an isospeed water layer overlying a halfspace with a sound speed gradient is considered. The dependence of horizontal wavenumbers of normal modes in such a waveguide on the sound frequency, water depth and geoacoustic parameters of the bottom are investigated. It is shown that these dependencies are described by ordinary differential equations that can be solved numerically at very low computational cost providing a substantial increase in the efficiency of broadband modelling of sound propagation. The explicit formulae for the derivatives of horizontal wavenumbers with respect to the bottom parameters can be also used in dispersion-based geoacoustic inversion methods. This approach can be extended to the case of a waveguide consisting of several layers of Airy-type media. We also propose a new and concise proof that the spectrum of the Sturm-Liouville problem from which the modes are found for this waveguide is purely discrete.

Meso-scale seabed quantification with geoacoustic inversion

Knowledge of sub-seabed geoacoustic properties, for example depth dependent sound speed and porosity, is of importance for a variety of applications. Here, we present a semi-automated geoacoustic inversion method for autonomous underwater vehicle data that objectively adapts model inference to seabed structure. Through parallelized trans-dimensional Bayesian inference, we infer seabed properties along a 12 km survey track on the scale of about 10 cm and 50 m in the vertical and horizontal, respectively. The inferred seabed properties include sound speed, attenuation, density, and porosity as a function of depth from acoustic reflection coefficient data. Parameter uncertainties are quantified, and the seabed properties agree closely with core samples at two control points and the layering structure with an independent sub-bottom seismic survey. Recovering high resolution seabed properties over large areas is shown to be feasible, which could become an important tool for marine industries, navies and oceanic research organizations.

Inference of geoacoustic model parameters from acoustic field data: Perspectives on Geoacoustic Inversion

Estimation of parameters of geoacoustic models from acoustic field data has been a central research theme in acoustical oceanography and ocean acoustics. During the past several decades, highly efficient numerical inversion techniques have been developed that provide model parameter estimates and their uncertainties based on statistical inference methods. However, the methods are model-based and the inversions are prone to errors related to model mismatch. In any event, the inversions can generate only effective models of the true structure of the ocean bottom, which is generally highly variable over relatively small spatial scales in range and depth. There are also questions about the theory for modelling sound propagation in porous sediment media that raise doubt about the validity of inversion results. In most inversions, a visco-elastic theory is used, but is this the most appropriate propagation model? Another question is about the impact of neglecting shear waves in geoacoustic models. Most inversions assume a fluid model of the ocean bottom. This paper revisits issues that have raised questions about limitations of geoacoustic inversion methods, and discusses the impact of various mitigation measures that have been applied. The paper concludes with musings about new inversion techniques based on machine learning.

Estimation of seabed properties from ocean ambient noise in shallow water: A review

Ocean ambient noise is the sound persisting in the ocean, which includes contributions from natural and anthropogenic sources, such as rain, wind, shipping, and marine mammals. Multiple interactions of the sound with the surface and bottom in shallow water affect the spatial characteristics of ambient noise. Thus, the spatial noise properties, coherence, and directionality of ambient noise depend on the characteristics of the shallow water environment, such as depth, sound speed profile, and seabed sediment properties. Acoustic properties of seafloor sediments can be extracted from ambient noise based on these noise properties using various types of noise data and geo-acoustic inversion methods. This paper presents a review of literature concerning investigations into available methods, the implementations of these methods, and their verification and validation. The efficacy and the applicability of these inversion methods for a wide range of ocean noise data are also examined in this review.

Joint geoacoustic inversion based on Pearson correlation coefficient constraints.

This Letter proposes a joint geoacoustic inversion method for modal group velocity dispersion and amplitudes of waveform by incorporating a Pearson correlation constraint. Numerical simulations show that this joint inversion leads to improved geoacoustic inversion performance with smaller uncertainties compared to separate inversion methods when applied to data from a single receiver. Additionally, the effective use of the Wasserstein metric from optimal transport theory is explored and compared to the more-common L2 norm misfit measure. The Letter also presents a qualitative representation of joint inversion convergence obtained through multiple independent runs of genetic algorithms. The algorithm is applied to simulated data.

Geoacoustic inversion using very-low-frequency modal interference characteristics.

This paper presents a modal-based geoacoustic inversion method adapted for a very-low-frequency leaky waveguide. It is applied to air gun data collected by a seismic streamer during the multi-channel seismic exploration experiment in the South Yellow Sea. The inversion is carried out by filtering the waterborne and bottom-trapped mode pairs from the received signal and comparing the modal interference features (waveguide invariant) to replica fields. The effective seabed models are inferred at two positions, and the two-way-travel time of basement interface reflected waves calculated using these models exhibit good agreement with geological exploration results.

Geoacoustic Inversion Using Modal Dispersion Curves

This paper presents a broadband geoacoustic inversion method based on matching the estimated dispersion curves extracted from low frequency pulse signal by warping transform and the simulated dispersion curves. The single receiver broadband inversion of the geoacoustic parameters is implemented using the first three orders of the dispersion curves, and the results are assessed using the parameter distribution and propagation loss deviation. The simulation outcomes demonstrate the method’s broad applicability for inverting geoacoustic parameters and the warping transform’s relative accuracy for dispersion curves estimation in waveguide environments with significant vertical sound speed modification.

Application of Dual-Source Modal Dispersion and Variational Bayesian Monte Carlo Method for Local Geoacoustic Inversion in Weakly Range-Dependent Shallow Water

Application of Dual-Source Modal Dispersion and Variational Bayesian Monte Carlo Method for Local Geoacoustic Inversion in Weakly Range-Dependent Shallow Water

Inversion of Sound Speed and Thickness of High-Speed Sediment Using Interference Structure in Shadow Zone

The geoacoustic parameter acquisition in the deep sea is of great significance to the research of ocean acoustics. This paper found that the interference structure of the shadow zone induced by the reflection of the high-speed sediment layer could be simply described by the grazing angle of the surface-bottom reflection from the theory of ray acoustics, when the source and receiver depth makes the grazing angle of the surface-bottom reflection consistent with that of the bottom-surface reflection. On this basis, a geoacoustic parameter inversion method by spatial position matching of interference fringes in the shadow zone was proposed, and an interference fringe extraction method was designed based on the maximum between-class variance algorithm in this paper. After extracting the results by the stripe coordinates in the simulation environment, the density was obtained by assuming the base sound speed as an empirical value and combining with Hamilton’s empirical formula, and the sediment sound speed and thickness were inverted by the grid search method. Those inversion results were compared with the multi-dimensional inversion results of the genetic algorithm. The simulation results showed that the fringe extraction method proposed in this paper could effectively extract the interference fringes formed by the reflection of the high-speed sediment in the shadow zone, and compared with the multi-dimensional optimization process, the relatively accurate inversion results of the sound speed and thickness of high-speed sediment could be obtained more effectively and quickly by taking the spatial position of the interference fringe as the cost function of the matching parameter combined with the grid search method in this paper.

Trans-dimensional geoacoustic inversion on a range-dependent track: Using chirp subbottom survey data as prior information for seabed layering

Geoacoustic inversion methods can be broadly divided into two categories. Fixed-dimensional methods are based on the assumption of a known environmental parametrization, including the number of seabed layers. Trans-dimensional (trans-D) methods estimate the environmental parametrization as part of the inverse problem. Although trans-D methods are very powerful, they have barely been applied in range-dependent geoacoustic inversion, and fully range-dependent trans-D inversion is a challenging problem. To mitigate this issue, we propose to use chirp subbottom survey data as prior information within a trans-D inversion method. To do so, the method considers the seabed as an unknown number of homogeneous sediment layers; the number of layers and geoacoustic properties within layers are constant over range, but layer interface depths are range dependent, with the lateral variation informed by the two-way travel-times to reflectors from the chirp survey. The method was successfully applied for single hydrophone geoacoustic inversion using data collected during the 2017 Seabed Characterization Experiment. [Work supported by the Office of Naval Research.]

Overview of deep water acoustics

<p indent=0mm>The aim of research on ocean acoustics is to reveal the phenomenon and physical mechanism of sound propagation in complex marine environments, which is the basis of the high technology and application of underwater acoustics. From the view of the sound field in deep ocean and SONAR application, the progress of ocean acoustic theories and experiments by developed countries has been reviewed firstly. The history of basic research in China is introduced briefly. In 1997 and 2012, the progress of ocean acoustics in China has been summarized at the First and Third International Conference on Shallow Water Acoustics, respectively. In recent years, with the development of maritime strategy, research on ocean acoustics in China is moving from shallow water to deep water. Some progress on deep water acoustics has been made based on the funding from the Global Change and Air-sea Interaction Project and the National Natural Science Foundation of China. The scientific research achievements on deep water acoustics are reviewed from the aspects of acoustic experimental technology, sound propagation in complex ocean environments, sound field spatial correlation, characteristics of ambient noise and application on ocean tomography, ocean reverberation, geoacoustic inversion, and source localization. The distributed sound signal recorders are developed which can work at the maximal depth of <sc>12000 m.</sc> Experiment data from the South China Sea, the Pacific Ocean and the Indian Ocean surveys have been recorded with them. Three-dimensional propagation phenomena with the presence of seamount and trench-slope basin have been observed and explained by ray theory. Characteristics of spatial correlation of sound field for different ocean environments have been investigated. The sound field fluctuation and the probability distribution of the transmission losses (TLs) caused by internal waves, mesoscale eddy, surface waves and bottom roughness are systematically studied. Longitudinal and vertical correlations within the direct zone and convergence zone are relatively high for the main energy coming from the rays reflected by sound channel. Then long-time ambient noise measurements with typhoon passing by the submerged buoys have been done. The noise model for the South China Sea has been developed. A passive ocean tomography method from ambient noise is proposed to estimate sound speed profile. A geoacoustic inversion method for deep water environments is proposed by using the TLs in the shadow zone, which is very sensitive to bottom parameters. By using the relationship of arrival angle and source ranges, three source localization methods have been proposed, which can estimate the source depth and range at the same time with high precision. Finally, the key research directions of deep water acoustics are given, such as characteristics of sound field in typical ocean environments, effects of ocean environments on SONAR performance, weak signal enhancement methods based on the interference of multi-path arrivals, and environment measurements and target detection methods with unmanned platforms. It has reference meaning for future work of deep water acoustics.

Transdimensional Geoacoustic Inversion Using Prior Information on Range-Dependent Seabed Layering

This article proposes a transdimensional (trans-D) geoacoustic inversion method adapted to range-dependent (RD) propagation tracks based on prior information from a high-resolution seismic survey. Most trans-D inversions to date model the seabed as a stack of range-independent homogeneous layers, with unknown geoacoustic parameters and an unknown number of layers. The proposed method models the seabed as an unknown number of homogeneous sediment layers with an RD thickness structure and applies an adiabatic normal-mode model to predict acoustic propagation. To do so, the method extrapolates trans-D seabed models proposed at the receiver position over the range of the propagation track using reflector-interface information from a seismic survey. The method is applied successfully to modal time–frequency dispersion data collected over an RD track during the 2017 Seabed Characterization Experiment (SBCEX).

An overview of seabed characterization experiment

An overview of the Spring 2021 component of the Seabed Characterization Experiment will be presented. The project is focused on the acoustics of fine-grained ocean bottom sediments. The study site is the New England Shelf, south of Martha’s Vineyard, MA, USA. The goals of the project are (1) understand the physical mechanisms that control acoustic propagation in fine-grained sediments, (2) quantify uncertainties in the estimation of seabed parameters, (3) correlate the observed horizontal variations in the acoustical field with the measured horizontal variability of the seabed and (4) assess the performance of the resulting geoacoustic models, and inversion and statistical inference methods. These goals have been pursued by obtaining direct measurements and/or inferring the values of the following parameters over a wide frequency band: compressional wave speed and attenuation, shear wave speed and attenuation, and seabed layering and gradients. Oceanographic data were collected to support both forward and inverse modeling efforts. A major part of this effort was completed in 2017 and a follow-on effort is planned for 2021 and 2022. This presentation provides an overview of the project, a summary of past results, and a discussion of the most recent work. [Work supported by ONR.]

An Inversion Method for Geoacoustic Parameters of Multilayer Seabed in Shallow Water

To solve the problem of obtaining geoacoustic parameters of multi-layer seabed in shallow water, a geoacoustic inversion method based on Bayesian theory is studied in this paper. In this study, the seabed is assumed to be a horizontal layered elastic medium, and the error function is established according to the sound pressure field in the water layer and bayesian theory. The global optimization algorithm is used to obtain the three target parameters of wave velocity, density and wave velocity attenuation in each layer of the seabed. The designed optimization method can correct the parameters in the process of finding the solution according to the physical characteristics of the upper and lower layers. The simulation results show that this method greatly improves the stability and accuracy of the inversion results for the multi-layer seabed model.

Quantitative metrics to experimentally benchmark geoacoustic inversion methods

Geoacoustic inversion schemes are complex methods that combine various experimental designs (e.g., arrays or single receivers, fixed or moving sources,…), different data processing methods (e.g., tracking ray arrivals, filtering modes, estimating spectral matrices,…) and different inversion methods (e.g., brute force grid search, stochastic optimization,…). Because of this complexity and variability, geoacoustic inversion methods are difficult to benchmark. Although simulated benchmarks are possible, experimental benchmarks are more difficult since (1) experimental ground truth is never fully available and (2) different methods may sense different features/scales of the seabed. Nonetheless, assessing the performance of geoacoustic inversion methods is of paramount importance, since the resulting geoacoustic models are used to predict acoustic field for various applications, including Navy operations or noise pollution monitoring. In this presentation, we propose quantitative metrics to assess the experimental performances of inversion methods. The proposed metrics allow inter-comparison of various geoacoustic models, but also comparison of predicted acoustic field with experimental (independently collected) data. The metrics are successfully applied on a subset of data collected during the Shallow Water 2006 experiment.

Synchrosqueezing transform for geoacoustic inversion with air-gun source in the East China Sea

This paper presents a geoacoustic inversion method combining with warping transform and synchrosqueezing transform to improve the precision of extracting modal dispersion curves in shallow water waveguide. Dispersive characteristic is often used to invert geoacoustic parameters, while the dispersion curves are hard to be extracted in short range. Warping transform can compensate the nonlinearity of received signal but the frequency resolution is still inaccurate. Synchrosqueezing transform sharpens the frequency resolution of warped modes, which makes the extracted dispersion curves more accurate. The comparison of simulation shows that the result using combined method is closer to the preset value when implementing geoacoustic inversion of dispersion characteristics. Furthermore, this method is used to invert the sea bottom parameters of a shallow water experiment in the East China Sea. The Bayesian method and insitu measurement are used to verify the reliability of inversion results.

Bayesian Inversion for Geoacoustic Parameters in Shallow Sea.

Geoacoustic parameter inversion is a crucial issue in underwater acoustic research for shallow sea environments and has increasingly become popular in the recent past. This paper investigates the geoacoustic parameters in a shallow sea environment using a single-receiver geoacoustic inversion method based on Bayesian theory. In this context, the seabed is regarded as an elastic medium, the acoustic pressure at different positions under low-frequency is chosen as the study object, and the theoretical prediction value of the acoustic pressure is described by the Fast Field Method (FFM). The cost function between the measured and modeled acoustic fields is established under the assumption of Gaussian data errors using Bayesian methodology. The Bayesian inversion method enables the inference of the seabed geoacoustic parameters from the experimental data, including the optimal estimates of these parameters, such as density, sound speed and sound speed attenuation, and quantitative uncertainty estimates. The optimization is carried out by simulated annealing (SA), and the Posterior Probability Density (PPD) is given as the inversion result based on the Gibbs Sampler (GS) algorithm. Inversion results of the experimental data are in good agreement with both measured values and estimates from Genetic Algorithm (GA) inversion result in the same environment. Furthermore, the results also indicate that the sound speed and density in the seabed have fewer uncertainties and are more sensitive to acoustic pressure than the sound speed attenuation. The sea noise could increase the variance of PPD, which has less influence on the sensitive parameters. The mean value of PPD could still reflect the true values of geoacoustic parameters in simulation.

Nonlinear time-warping made simple: A step-by-step tutorial on underwater acoustic modal separation with a single hydrophone.

Classical ocean acoustic experiments involve the use of synchronized arrays of sensors. However, the need to cover large areas and/or the use of small robotic platforms has evoked interest in single-hydrophone processing methods for localizing a source or characterizing the propagation environment. One such processing method is "warping," a non-linear, physics-based signal processing tool dedicated to decomposing multipath features of low-frequency transient signals (frequency f < 500 Hz), after their propagation through shallow water (depth D < 200 m) and their reception on a distant single hydrophone (range r > 1 km). Since its introduction to the underwater acoustics community in 2010, warping has been adopted in the ocean acoustics literature, mostly as a pre-processing method for single receiver geoacoustic inversion. Warping also has potential applications in other specialties, including bioacoustics; however, the technique can be daunting to many potential users unfamiliar with its intricacies. Consequently, this tutorial article covers basic warping theory, presents simulation examples, and provides practical experimental strategies. Accompanying supplementary material provides matlab code and simulated and experimental datasets for easy implementation of warping on both impulsive and frequency-modulated signals from both biotic and man-made sources. This combined material should provide interested readers with user-friendly resources for implementing warping methods into their own research.

An Experimental Benchmark for Geoacoustic Inversion Methods

Over the past 25 years, there has been significant research activity in development and application of methods for inverting acoustical field data to estimate parameters of geoacoustic models of the ocean bottom. Although the performance of various geoacoustic inversion methods has been benchmarked on simulated data, their performance with experimental data remains an open question. This article constitutes the first attempt of an experimental benchmark of geoacoustic inversion methods. To do so, the article focuses on data from experiments carried out at a common site during the Shallow Water 2006 (SW06) experiment. The contribution of the article is twofold. First, the article provides an overview of experimental inversion methods and results obtained with SW06 data. Second, the article proposes and uses quantitative metrics to assess the experimental performance of inversion methods. From a sonar performance point of view, the benchmark shows that no particular geoacoustic inversion method is definitely better than any other of the ones that were tested. All the inversion methods generated adequate sound-speed profiles, but only a few methods estimated attenuation and density. Also, acoustical field prediction performance drastically reduces with range for all geoacoustic models, and this performance loss dominates over intermodel variability. Overall, the benchmark covers the two main objectives of geoacoustic inversion: obtaining geophysical information about the seabed, and/or predicting acoustic propagation in a given area.

Inferring low-frequency compressional sound speed and attenuation in muddy sediments from long-range broadband sound propagation

While researchers have successfully applied many geo-acoustic inversion methods, involving normal mode analysis or matched field processing, to invert seabed parameters in sandy bottoms, inferring low-frequency compressional sound speed and attenuation in mud is still a challenging problem, especially for a marine sediment where a mud layer overlies a sandy bottom. Recent studies show that there is an ambiguity between the low-frequency attenuation of mud and sand in three different inversion algorithms based on (1) modal amplitude, (2) transmission loss, and (3) spatial coherence measurements [JASA 143, 1798 (2018)]. An increase (decrease) of mud attenuation can be compensated by decreasing (increasing) the attenuation in the sandy basement. In this paper, an inversion method combining these three inversion algorithms and statistical inference techniques (e.g., Bayesian-Maximum Entropy, [JASA 138, 3563–3575 (2015)]) is utilized to analyze the long-range broadband acoustic signals measured by several vertical and horizontal line arrays during the Seabed Characterization Experiment 2017 conducted in the New England Mud Patch. Reliable estimates of the low-frequency mud properties are obtained by removing the ambiguity. Finally, the inverted results are compared with historical experimental data and theoretical predictions. [Work supported by ONR Ocean Acoustics.]While researchers have successfully applied many geo-acoustic inversion methods, involving normal mode analysis or matched field processing, to invert seabed parameters in sandy bottoms, inferring low-frequency compressional sound speed and attenuation in mud is still a challenging problem, especially for a marine sediment where a mud layer overlies a sandy bottom. Recent studies show that there is an ambiguity between the low-frequency attenuation of mud and sand in three different inversion algorithms based on (1) modal amplitude, (2) transmission loss, and (3) spatial coherence measurements [JASA 143, 1798 (2018)]. An increase (decrease) of mud attenuation can be compensated by decreasing (increasing) the attenuation in the sandy basement. In this paper, an inversion method combining these three inversion algorithms and statistical inference techniques (e.g., Bayesian-Maximum Entropy, [JASA 138, 3563–3575 (2015)]) is utilized to analyze the long-range broadband acoustic signals measured by several vert...