Geoacoustic Inversion Methods Research Articles

The Airy phase of explosive sounds in shallow water.

The Airy phase is identified in the received signals from explosive charges deployed in a shallow water acoustic experiment conducted in the New England Mudpatch region during the spring of 2017. Measured and modeled time-frequency dispersion curves are compared and a geoacoustic sensitivity study utilizing marginal probability distributions for the sound speed in five sediment layers is performed. The analysis suggests that inclusion of the Airy phase frequency and arrival time in a geoacoustic-inversion method could lower the uncertainty of sound speed parameter estimation in a multi-layer sediment as compared to methods that do not include the Airy phase structure.

Hybrid geoacoustic inversion method and its application to different sediments

Bottom acoustic parameters have large effects on sound propagation both in shallow water and in deep water with an incomplete sound channel. A hybrid geoacoustic inversion method is proposed to invert for sound speed, density and attenuation coefficient based on the facts that the bottom acoustic parameters have different sensitivities to sound field at different ranges. The hybrid inversion method is applied to inversion for bottom parameter with different sediments in the Yellow Sea, East China Sea, and South China Sea and verified by using the core sampling measurements. The relationships between the sediments types and the bottom acoustic parameters are given. The differences of the sediment acoustic parameters between sandy silty(or silty sand) and clay(or mud) sediment at low frequency are discussed in the end. [Work supported by the National Natural Science Foundation of China under Grant No. 10434012 and Grant No. 41561144006.]

An experimental benchmark for geoacoustic inversion methods

Over the past 25 years, many methods have been developed for estimating parameters of geoacoustic models of the ocean bottom from acoustic field data. The methods involved model-based comparisons between experimental data and calculated fields or field quantities. Benchmarking workshops based on simulated field data confirmed the ability of the methods for inverting geoacoustic model parameters for range-independent and range-dependent ocean environments. Although there are many published examples of geoacoustic inversions in various different ocean environments, the methods have not been tested in an experimental benchmark. This paper describes the design of an experimental benchmark for geoacoustic inversion methods, and presents an analysis of the performance of the methods with experimental data. The benchmark involves application of different inversion methods to data spanning a frequency band from 50to 2500 Hz that were obtained at a single well-surveyed ocean bottom site. The performance metric involves comparisons of: estimated geoacoustic profiles against the ground truth information of the site; modal wavenumbers for the inverted models; and transmission loss calculations based on the estimated models against measured data. The analysis shows that the methods are capable of estimating realistic geoacoustic profiles, with sound speed the most reliable estimated model parameter.

Geoacoustic Inversion Using Time Reversal of Ocean Noise**Supported by the National Natural Science Foundation of China under Grant Nos 11434012 and 41561144006.

We present a passive geoacoustic inversion method using two hydrophones, which combines noise interferometry and time reversal mirror (TRM) techniques. Numerical simulations are firstly performed, in which strong focusing occurs in the vicinity of one hydrophone when Green’s function (GF) is back-propagated from the other hydrophone, with the position and strength of the focus being sensitive to sound speed and density in the bottom. We next extract the GF from the noise cross-correlation function measured by two hydrophones with 8025-m distance in the Shallow Water ’06 experiment. After realizing the TRM process, sound speed and density in the bottom are inverted by optimizing focusing of the back-propagated GF. The passive inversion method is inherently environmentally friendly and low-cost.

Geoacoustic inversion using low frequency broadband acoustic measurements from L-shaped arrays in the Shallow Water 2006 Experiment.

When using geoacoustic inversion methods, one objective function may not result in a unique solution of the inversion problem because of the ambiguity among the unknown parameters. This paper utilizes acoustic normal mode dispersion curves, mode shapes, and modal-based longitudinal horizontal coherence to define a three-objective optimization problem for geoacoustic parameter estimation. This inversion scheme is applied to long-range combustive sound source data obtained from L-shaped arrays deployed on the New Jersey continental shelf in the summer of 2006. Based on the sub-bottom layering structure from the Compressed High-Intensity Radiated Pulse reflection survey at the experimental site, a two-layer (sand ridge overlaying a half-space basement) range-independent sediment model is utilized. The ambiguities of the sound speed, density, and depth of the sand ridge layer are partially removed by minimizing these objective functions. The inverted seabed sound speed over a frequency range of 15-170 Hz is comparable to the ones from direct measurements and other inversion methods in the same general area. The inverted seabed attenuation shows a nonlinear frequency dependence expressed as αb=0.26f1.55(dB/m) from 50 to 500 Hz or αb=0.32f1.65(dB/m) from 50 to 250 Hz, where f is in kHz.

Bayesian Geoacoustic Inversion With the Image Source Method

This paper develops a Bayesian approach to the image source method (ISM) for efficient inversion of seabed reflection data to estimate geoacoustic parameters and uncertainties. Based on the representation of layered seafloor-reflected signals by image sources, ISM is a very efficient method which provides the local sound-speed profile (SSP) of the sediment structure. It is a two-step method: first, the image sources are detected and localized from the recorded signals, and second, from these locations, the thickness and sound speed of each sediment layer are estimated from the Snell–Descartes law of refraction. This work focuses on the definition and construction of the image sources with a distinction between real and virtual image sources which has consequences on the uncertainties of ISM. The localization of the image sources is performed within a Bayesian formulation based on sampling the posterior probability density to estimate the median SSP and uncertainties. The algorithm is tested first on synthetic data, with results in excellent agreement with Bayesian travel-time inversion but a much lower computational cost. Finally, the Bayesian ISM is applied to at-sea data measured in the Scattering And ReverberAtion from the sea Bottom (SCARAB) experiment, which took place near Elba Island off the west coast of Italy in 1998, and the resultant sediment SSP agrees well with previous results of other geoacoustic inversion methods.

Geoacoustic Inversion Based on Modal Dispersion Curve for Range-Dependent Environment**Supported by the National Natural Science Foundation of China under Grant No 11174235, and the Fundamental Research Funds for the central Universities of Ministry of Education of China under Grant No 3102014JC02010301.

A geoacoustic inversion method is proposed based on the modal dispersion curve of two-wideband explosive signals for range-dependent environment. It is applied to the wideband explosive sound source data from the South China Sea in 2012. The travel time differences of different modes at various frequencies and distances are extracted by warping transform. The mean bottom acoustic parameters are inverted by matching the theoretical modal time differences to that of the experimental data. The inversion results are validated by using other explosive signals at different distances.

Image source detection for geoacoustic inversion by the Teager-Kaiser energy operator.

This letter presents an improvement of the image source method for geoacoustic inversion. The algorithm is based on the Teager-Kaiser energy operator which amplifies the discontinuities in signals while the soft transitions are reduced. This property is exploited for accurate detection of time arrivals and thus for location of the image sources. The effectiveness of the method is shown on both synthetic and real data and the inversion results are, overall, in good agreement with ground truth and other inversion results with a significant reduction of computation time.

On the inversion of sediment density profile by the image source method

Image Source Method is a recently developed method for geoacoustic inversion. Under the Born approximation, the reflection of a spherical wave above a stratified seafloor is modeled as the simultaneous emission of image sources, symmetric of the real one relatively to the layers. These image sources are detected and located by the use of Teager-Kaiser Energy Operator (TKEO), which amplifies sudden changes in signal amplitudes, and by a triangulation scheme. Time and arrival angle of the recorded signals coming from the image sources are the input values of the inversion algorithm which gives the thickness and sound speed of each detected sediment layer by the use of Snell-Descartes laws. The objective of the present work is to extend this method to the inversion of the sediment density profile. To this end, one supplementary data is required. Experimental amplitudes corresponding to arrival times of the reflected signals, detected by TKEO, are compared to theoretical ones computed by a numerical evaluation of the Sommerfeld integral. Having first inverted sound speed profile and neglecting absorption coefficient, density is the only remaining parameter and thus can be obtained. The effectiveness of this method is tested on both synthetic and real data.

Compressive geoacoustic inversion using ambient noise

Surface generated ambient noise can be used to infer sediment properties. Here, a passive geoacoustic inversion method that uses noise recorded by a drifting vertical array is adopted. The array is steered using beamforming to compute the noise arriving at the array from various directions. This information is used in two different ways: Coherently (cross-correlation of upward/downward propagating noise using a minimum variance distortionless response fathometer), and incoherently (bottom loss vs frequency and angle using a conventional beamformer) to obtain the bottom properties. Compressive sensing is used to invert for the number of sediment layer interfaces and their depths using coherent passive fathometry. Then the incoherent bottom loss estimate is used to refine the sediment thickness, sound speed, density, and attenuation values. Compressive sensing fathometry enables automatic determination of the number of interfaces. It also tightens the sediment thickness priors for the incoherent bottom loss inversion which reduces the search space. The method is demonstrated on drifting array data collected during the Boundary 2003 experiment.

Synthetic aperture geoacoustic inversion in the presence of radial acceleration dynamics

Traditionally matched-field geoacoustic inversion experiments sampled the acoustic field on long arrays and require powerful transmissions in order to reduce parameter uncertainty. However, single-hydrophone based geoacoustic inversion methods are attractive compared to the ones using long arrays. A low signal to noise ratio (SNR), single-receiver, broadband, and frequency coherent matched-field inversion that exploits coherently repeated transmissions to improve estimation of the geoacoustic parameters have been previously proposed. The long observation time creates a synthetic aperture due to relative source-receiver motion. This paper extends broadband synthetic aperture geoacoustic inversion to cases where the velocity of the source/receiver changes. In addition, a pulse-by-pulse coherent processing using the Bayesian approach is proposed. The method is demonstrated with low SNR, 100–900 Hz LFM data from the Shallow Water 2006 experiment.

An improved broadband matched field processor for geoacoustic inversion (L)

The approach proposed here requires that for each parameter combination in a search space only a minimum processor value (over frequencies) be retained. Thus all frequencies considered at that point will have processor values at least as great as the final value. Consequently, (1) the frequency most sensitive to a parameter dominates the search automatically and (2) resolution has improved with major sidelobe reduction. The method is intended primarily for geoacoustic inversion methods (where the signal-to-noise levels are high).

Broadband synthetic aperture matched field geoacoustic inversion with a single hydrophone

Traditionally matched-field geoacoustic inversion experiments sampled the acoustic field on long arrays and require powerful transmissions in order to reduce parameter uncertainty. However, single-hydrophone based geoacoustic inversion methods exist. Practically, these methods are attractive compared to the ones using long arrays. In a single hydrophone setup, spatial diversity is traded off for frequency diversity; the source is broadband. This paper uses single-hydrophone frequency coherent matched-field inversion and exploits extended-duration coherent transmissions (multiple LFM chirps) to increase signal to noise ratio. As a result, the overall signal becomes Doppler/motion intolerant. But Doppler can be modeled by including source and hydrophone horizontal motions. To correlate well with the measured field across a receiver trajectory and to incorporate a transmission across source trajectory, Doppler in waveguide and normal mode theory are applied. The method is demonstrated with 100-900 Hz LFM SW06 data with low signal to noise ratio.

Hybrid geoacoustic inversion scheme with an equivalent seabed model

Acoustic propagation in shallow water is greatly influenced by the properties of the bottom. The purpose of geoacoustic inversion is estimation of ocean bottom acoustic parameters such as sediment sound-speeds, densities, and attenuations from measured acoustic fields. It is a helpful supplement to direct measurements. Especially, geoacoustic inversion could give low frequency attenuation, which cannot be measured by coring the sediment. Therefore, it has been paid much attention in recent years. A hybrid geoacoustic inversion scheme, which combines several inversion methods together to invert for the bottom parameters, has been proposed based on the fact that the bottom acoustic parameters have different sensitivities to the different physical parameters of acoustic field. This inversion scheme could avoid the problem of the multiple solutions, which are often companied with some geoacoustic inversion methods. The validity of the inversion scheme is verified in a series of sea experiments at different sites. The inverted bottom parameters could be used to forecast the sound propagations and distinguish the atlas marked bottom type quite well. The mapping relation between the sediment types and the acoustic parameters are also given. [Work supported by the National Natural Science Foundation of China under Grant No. 10974218 and Grant No. 10734100]

Hybrid geoacoustic inversion scheme with an equivalent seabed model

Acoustic propagation in shallow water is greatly influenced by the properties of the bottom. The purpose of geoacoustic inversion is estimation of ocean bottom acoustic parameters such as sediment sound speeds, densities, and attenuations from measured acoustic fields. Especially, geoacoustic inversion could give low frequency attenuation, which cannot be measured by coring the sediment. Therefore, it has been paid much attention in recent years. A hybrid geoacoustic inversion scheme, which combines several inversion methods together to invert for the bottom parameters, has been proposed based on the fact that the bottom acoustic parameters have different sensitivities to the different physical parameters of acoustic field. This inversion scheme could avoid the problem of the multiple solutions, which are often companied with some geoacoustic inversion methods. The validity of the inversion scheme is verified in a series of sea experiments at different sites. In the experiment, six different sediment types: Fine Sand, Silty Sand, Sand Silty, Sand-Silty-Clay, Silty Clay and Mud, are included in an area in the Yellow Sea. The inverted bottom parameters could distinguish the atlas marked bottom type quite well. [Work supported by the National Natural Science Foundation of China under Grant Nos. 11074269 and 10734100.]

Inversion of elastic bottom parameters from reflection data

In this paper, the wave reflection at the interface between sea water and the half-space elastic bottom is considered and the sensitivity of reflection coefficient to the geoacoustic parameters, such as bottom density, P-wave velocity and its attenuation, S-wave velocity and its attenuation, has also been analyzed. In order to establish a geoacoustic inversion method with the ocean bottom reflection coefficient, a simulated experiment is carried out in the laboratory tank, where a PVC plate is used as the elastic bottom. In the experiment, a high-frequency underwater sound wave is transmitted by a source at fixed position and received using a hydrophone at different position with equal interval. By processing the measurement reflected signals at different receiving positions, the reflection coefficient for different incident angles can be obtained, with which the simulated bottom parameters have been inversed. An inversion method based on the sound transmission loss in water is also accomplished. These two inversion results are compared with the measurement result obtained according to the time delay for received multipath signals which reflect/refract at the liquid/elastic or solid/liquid interface of the PVC plate and the feasibility and reliability of the inversion from reflection data are discussed at last.

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.

Sequential geoacoustic inversion at the continental shelfbreak

Environmental parameters can have large spatial and temporal variability in shelfbreak regions. The capability of sequential Bayesian filters in tracking this variation is investigated. Particle filtering (PF) is used to extract the environmental parameters and their uncertainties. The method tracks the environment with fewer particles relative to conventional geoacoustic inversion methods using successive independent inversions. As an example, data from the Shallow Water 2006 Experiment are processed. The PF approach first is used to track the source and the environment with little spatial variation just northwest of the shelfbreak. Then the strongly range-dependent shelfbreak region is analyzed and the PF results are compared to previous geoacoustic inversion studies from the region.

Passive geoacoustic inversion in a dispersive waveguide

This study introduces a single-receiver geoacoustic inversion method adapted to shallow water and low frequency sources. Because of the single receiver context, most existing methods are based on the time-frequency (TF) analysis of the received signal, and their practical applications are restricted to impulsive sources. The proposed method is different and allows for geoacoustic inversion using unknown frequency-modulated sources. To perform the inversion, the modes are first filtered from the received signal using advanced TF analysis. Then, the filtered modes are processed in the frequency domain using a new transformation called modal reversal. This transformation, parametrized using environmental information, undoes dispersion for a given mode. When environment is well known, dispersion is perfectly compensated and all the reversed modes are in phase. This is not the case when modal reversal is ill-parametrized. Consequently, modal reversal can be used as the core of an original inversion scheme. Inversion results are obtain through a specific cost function adding up the reversed modes. Source/receiver range and source frequency-law are obtained as a by-product of the geoacoustic inversion. The method can thus be adapted to the study of low-frequency calls of marine mammals in shallow water.

Range aliasing in frequency coherent geoacoustic inversion

This paper discusses the effects of frequency selection on source localization and geoacoustic inversion methods that use frequency coherent objective functions. Matched-field processors based on frequency-coherent objective functions often have rapidly fluctuating range ambiguity surfaces. Insufficient sampling in frequency domain results in range aliasing terms that affect geoacoustic inversion. Range aliasing and its effects on source localization and environmental parameter inversion are demonstrated on data collected during the MAPEX2000 experiment. Guidance for frequency selection to avoid range aliasing is provided.