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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.

In this paper, broadband airgun data from the 1995 Shallow Water Acoustics in Random Media (SWARM'95) experiment are analyzed to study the influence of internal waves on sound propagation and modal behavior, and the subsequent effects on geoacoustic inversion. Modal behavior and fluctuation due to sound intensity focusing and defocusing, when the leading front of internal waves is parallel to and approaches or passes through the acoustic propagation track, are presented. A modal filtering technique, the time-warping transform, is applied to the airgun data to resolve dispersed modes. The extracted modes are used to estimate seabed model parameters by geoacoustic inversion. Two scenarios, defocusing and focusing periods containing 14 events, are selected for study. Seven data sets for each type (defocusing or focusing) are analyzed to extract dispersion curves and carry out geoacoustic inversion. The inversion results for each type of the events are similar with a small standard deviation from the mean. However, the mean values and standard deviations for the defocusing and focusing types are quite different. The impacts of internal waves on inversion results are presented and discussed.

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.

International audience; This paper presents single receiver geoacoustic inversion of two independent data sets recorded during the 2017 seabed characterization experiment on the New England Mud Patch. In the experimental area, the water depth is around 70 m, and the seabed is characterized by an upper layer of fine grained sediments with clay (i.e., mud). The first data set considered in this paper is a combustive sound source signal, and the second is a chirp emitted by a J15 source. These two data sets provide differing information on the geoacoustic properties of the seabed, as a result of their differing frequency content, and the dispersion properties of the environment. For both data sets, source/receiver range is about 7 km, and modal time-frequency dispersion curves are estimated using warping. Estimated dispersion curves are then used as input data for a Bayesian trans-dimensional inversion algorithm. Subbottom layering and geoacoustic parameters (sound speed and density) are thus inferred from the data. This paper highlights important properties of the mud, consistent with independent in situ measurements. It also demonstrates how information content differs for two data sets collected on reciprocal tracks, but with different acoustic sources and modal content. OAPA

This paper presents single receiver geoacoustic inversion of a combustive sound source signal, recorded during the 2017 Seabed Characterization Experiment on the New England Mud Patch, in an area where water depth is around 70 m. There are two important features in this study. First, it is shown that high-order modes can be resolved and estimated using warping (up to mode number 18 over the frequency band 20–440 Hz). However, it is not possible to determine mode numbers from the data, so that classical inversion methods that require mode identification cannot be applied. To solve this issue, an inversion algorithm that jointly estimates geoacoustic properties and identifies mode number is proposed. It is successfully applied on a range-dependent track, and provides a reliable range-average estimation of geoacoustic properties of the mud layer, an important feature of the seabed on the experimental area.This paper presents single receiver geoacoustic inversion of a combustive sound source signal, recorded during the 2017 Seabed Characterization Experiment on the New England Mud Patch, in an area where water depth is around 70 m. There are two important features in this study. First, it is shown that high-order modes can be resolved and estimated using warping (up to mode number 18 over the frequency band 20–440 Hz). However, it is not possible to determine mode numbers from the data, so that classical inversion methods that require mode identification cannot be applied. To solve this issue, an inversion algorithm that jointly estimates geoacoustic properties and identifies mode number is proposed. It is successfully applied on a range-dependent track, and provides a reliable range-average estimation of geoacoustic properties of the mud layer, an important feature of the seabed on the experimental area.

An important issue in understanding seabed geoacoustic properties at the site of the 2017 Seabed Characterization Experiment (SBCEX) on the New England Mud Patch is the extent to which gradients exist in geoacoustic properties (sound speed and density) over the upper mud layer. This paper applies Bayesian inversion and uncertainty quantification to modal dispersion data, resolved by warping analysis, to consider whether mud-layer gradients are required to fit the data. The inversion is based on a hybrid seabed-model parameterized in terms of an upper sediment layer with a general representation of smooth, continuous gradients in geoacoustic properties based on Bernstein-polynomial basis functions, above an unknown number of discrete layers formulated trans-dimensionally. The inversion results are compared to those from other acoustic data sets collected in the region as well as to nearby core measurements.

This paper presents geoacoustic inversion of modal dispersion data in the South China Sea using a single hydrophone and multiple impulsive sources at ranges from 5–100 km along a shallow-water track with slowly varying bathymetry. As a first step, the source waveform and bubble pulse are deconvolved from the recorded time series using a short-range source recording, corrected for the surface reflection. A time-frequency warping analysis is used to filter individual modes and obtain dispersion (arrival time as a function of frequency) data for three modes. Trans-dimensional Bayesian inversion is applied to the modal dispersion data, based on probabilistic sampling over an unknown number of seabed layers. Range-dependent inversion is considered, based on separating the environment into a sequence of range-independent sections, with frequency-dependent modal propagation times summed over segments. The inversion results are compared to core samples collected at sites along the survey line and to an independent headwave arrival-time analysis of the impulsive-source data.

This paper examines the impact of Mike’s research on geoacoustic inversion for characterization of sea bed sediments. First is his paper in 1987 that reported the use of vertical coherence of ambient ocean noise for estimating critical angles of sea bed sediment. Apart from the novel use of ambient noise as a sound source, his work opened the idea to make use of information about the ocean bottom contained in spatial phase relationships in vertical hydrophone array data. The practice was quickly adapted and widely applied as matched field inversion. However, the inversions were generally carried out using visco-elastic theory of sound propagation, an approach that is not the most appropriate for applications with porous sediment material. Mike’s next contribution addressed this issue in his series of papers starting around 1997 on the grain-shearing and viscous grain-shearing models of sound propagation in porous media. His theory provided a physical basis for the model parameters that are used to describe the interaction of sound with porous sediment media in geoacoustic inversions. Recent examples are shown that indicate how these innovations have become standard practice in geoacoustic inversions.This paper examines the impact of Mike’s research on geoacoustic inversion for characterization of sea bed sediments. First is his paper in 1987 that reported the use of vertical coherence of ambient ocean noise for estimating critical angles of sea bed sediment. Apart from the novel use of ambient noise as a sound source, his work opened the idea to make use of information about the ocean bottom contained in spatial phase relationships in vertical hydrophone array data. The practice was quickly adapted and widely applied as matched field inversion. However, the inversions were generally carried out using visco-elastic theory of sound propagation, an approach that is not the most appropriate for applications with porous sediment material. Mike’s next contribution addressed this issue in his series of papers starting around 1997 on the grain-shearing and viscous grain-shearing models of sound propagation in porous media. His theory provided a physical basis for the model parameters that are used to describ...

This paper describes a set of unconventional seismic experiments designed to characterize the subsurface hydrate distribution at Mississippi Canyon Block 798 in the northern Gulf of Mexico. The sea floor at the survey site lies within the hydrate stability zone and heat-flow data indicate that the zone may be 400 m thick. Sediments near the sea floor are characterized by relatively weak seismic reflections. It is possible that the weak reflectivity partially is due to dispersed distribution of hydrates within the fine-grained sediments. The data were acquired using a vertical hydrophone array deployed at the sea bottom. Two different approaches were used at the site; one used a conventional seismic source, and the other used the noise generated by the ship. The results from the experiments indicate that the vertical array performs well for high-resolution characterization of the gas-hydrate stability zone.

This paper presents trans-dimensional geoacoustic inversion of two low-frequency broadband signals recorded during the Seabed Characterization Experiment (SBCEX). The considered sources are a chirp emitted by a towed J15 source and an underwater impulse created by a combustive sound source (CSS). Corresponding received signals are recorded on single hydrophones, and the two source/receiver configurations are reciprocal, on the same track. Input data for inversion are modal time-frequency dispersion curves, estimated using warping. Inversion is then performed within a Bayesian framework. A trans-dimensional inverse algorithm is used to estimate the model parametrization (i.e., number of seabed layers), and the mode covariance matrices are estimated as a first-order autoregressive error process. Inversion results on the two reciprocal tracks are consistent with the modal information available in each dataset. Results are also consistent with what is known about the area.

This paper presents geoacoustic inversion of several low-frequency broadband signals recorded during the Seabed Characterization Experiment (SBCEX) that took place on the New England Mud Patch in March 2017. The considered sources are chirps emitted by a towed J15 source and underwater impulses created by MK64 explosions. These sources are low-frequency (f

This paper presents a method for estimating bottom geoacoustic properties especially the sediment attenuation from information contained in normal modes of a broadband signal. Propagating modes are resolved using the time-warping technique applied to signals from light bulb sound sources deployed at ranges of 5 and 7 km in the Shallow Water '06 experiment. A sequential inversion approach is designed that uses specific features of the acoustic data that are highly sensitive to specific geoacoustic model parameters. The first feature is the modal group speed, which is inverted for seabed sound speed, density, and sediment thickness. The second feature is the modal depth function for inverting receiver depths. The third feature is related to the modal coefficient spectra, and this is inverted for source depth and sediment attenuation. In each subsequent stage, estimates from the previous stage(s) are used as known values. The sequential inversion is stable and generates estimates for the geoacoustic model parameters that agree very well with results from other experiments carried out in the same region. Notably, the inversion obtains an estimated attenuation of 0.078 dB/λ in the band 120–180 Hz for the de-watered marine sediment characteristic of the continental shelf at the site.

Attenuation of sound in the seabed plays an important role in predicting transmission loss in shallow water waveguides. Methods to invert the attenuation from low-frequency acoustic field data include time-frequency techniques that make use of modal dispersion. Since modal separation improves as a sound signal that propagates to longer ranges, most of the inversion methods based on modal dispersion were carried out with long range data. Recently a time-warping signal processing technique was introduced that enables high resolution of modes at relatively short ranges. Time-warping involves an axis transformation that transforms the original time-frequency relationship of the modes to a new domain in which the modes are approximately tonal and are well resolved. This paper shows that the inversion can be carried out directly in the time-warped domain, and extends the work to estimate low-frequency seabed attenuation.

This paper describes results from geoacoustic inversion of low-frequency acoustic data recorded at a receiving array divided into two sections, a sparse bottom laid horizontal array (HLA) and a vertical array (VLA) deployed in shallow water. The data are from an experiment conducted by the Norwegian Defence Research Establishment (FFI) in the Barents Sea, using broadband explosives (shot) sources. A two-layer range-independent geoacoustic model, consistent with seismic profiles from the area, described the environment. Inversion for geoacoustic model parameters was carried out using a fast implementation of the hybrid adaptive simplex simulated annealing (ASSA) inversion algorithm, with replica fields computed by the ORCA normal mode code. Low-frequency (40-128 Hz) data from six shot sources at ranges 3-9 km from the array were considered. Estimates of sediment and substrate p-wave velocities and sediment thickness were found to be consistent between independent inversions of data from the two sections of the array

A multichannel seismic survey was carried out using the high‐resolution deep‐towed acoustics/geophysics system (DTAGS) to image the structure of deep‐sea gas hydrates on the continental slope off Vancouver Island and to determine the velocity profile of the hydrated sediments. The high‐frequency DTAGS data provide the means to estimate the frequency response of the bottom simulating reflector (BSR) that defines the base of the hydrate stability field in these sediments, over a broad frequency band from 15 to 650 Hz. The DTAGS sections resolved fine‐scale layering as thin as a few meters within the hydrated zone and below the BSR, and they revealed small‐scale faults and vertically oriented zones of very low acoustic reflectivity that may represent channels for upward migration of fluids or gas. Interval velocities determined from the DTAGS data indicate uniformly low values of about 1500 m/s to depths of 100 m below sea floor (mbsf), increasing to about 1850 m/s at the BSR (250 mbsf). The reflection from the BSR that is normally well defined in conventional low‐frequency seismic surveys is at least twenty times weaker at the high DTAGS frequencies. The reflection coefficient‐versus‐frequency data support a new model for the velocity profile at the BSR that consists of a thin, 4–8‐m layer at the BSR in which the velocity decreases by 250 m/s. The thin transition layer at the BSR implies relatively high methane flux rates of at least 1.5 mm/year.

International audience; An inversion scheme based on time-warping is presented for estimating seabed sound attenuation from modal dispersion of close-range single-hydrophone data. The dispersion information is extracted directly from the warped signal spectrum. Seabed sound speed and density are inverted from the modal group velocity curves, and the attenuation is inverted from the normalized modal amplitudes. The method is applied to experimental data collected in the Yellow Sea of China during the winter of 2002. The inverted sound speed and density are consistent with the sand-silt-clay sediment at the site, and the attenuation is nonlinear over the frequency band from 125-500 Hz.

International audience; In this paper, a method is described for inverting geoacoustic parameters of the seabed from short range field data recorded by single hydrophone. The original data in time domain are processed by a warping operator at first, and then the dispersion curve and the mode amplitude ratios are extracted separately from the warped data. The velocity and the density in the bottom are inverted from the dispersion curve, and the attenuation from the mode amplitude ratios, respectively. The performance of the method is examined using simulated data and then experimental data from the North Sea of China. The source used in the experiment was a small explosive charge that provided good signal to noise ratio over the frequency band from 200 Hz to 1 kHz. The depth of the water was about 30m, and the water sound speed was nearly constant with depth. The seabed geoacoustic parameters are inverted from the data received at different ranges from 2 to 14 km. The results from the different ranges are consistent with a simple half space model of the bottom. The seabed velocity is about 1600 m/s.

For single-beam acoustic seabed classification from ship-mounted echosounders, the beam footprint on the seafloor is often smaller than the spatial scale of seabed variability. In deep water, the beam footprint on the seafloor from a ship-mounted sounder is quite large, inhibiting mapping of seabed variability. To mitigate this problem, a high-resolution deep-sea acoustic seabed classification system, QTC DEEP, was developed by adapting and applying Quester Tangent Corporation (QTC) acoustic seabed classification technology, known as QTC VIEW Series 4, together with a pinger electronics package mounted together inside a pressure-housing on board a remotely operated vehicle. This paper presents the classification of seafloor echo return data from three sites on the British Columbia coast to demonstrate the utility of QTC DEEP .

A matched mode geoacoustic inversion is presented for broadband acoustic signals recorded in experiments at a shallow water region of the New England Bight. First the modal behavior of the waveguide in the presence of 3D effects due to the water column and bottom bathymetry is examined. Spatial measurements of the temperature and salinity profiles and bathymetry provide data for calculating 3D propagation of the broadband acoustic wave field. Frequency shift of the broadband signal along one of the source-receiver paths in the experiment is attributed to internal waves propagating in the water column. Geoacoustic model parameters of the seabed along a source-receiver track with constant bathymetry are estimated by matched mode processing through matching the phase of the mode signals recorded at a Vertical Line Array. A warping transform was applied to identify and isolate modes, and the filtered modes were inverse transformed back to the time domain to reconstruct the mode signals. The inversion results are compared with the results from other inversion methods to assess the performance of the inversion scheme for estimating reliable values of the geoacoustic parameters in the experiment region.

This volume contains the collection of papers from the second workshop on Experimental Acoustic Inversion Techniques for Exploration of the Shallow Water Environment. Acoustic techniques provide the most effective means for remote sensing of ocean and sea floor processes, and for probing the structure beneath the sea floor. No other energy propagates as efficiently in the ocean: radio waves and visible light are severely limited in range because the ocean is a highly conductive medium. However, sound from breaking waves and coastal shipping can be heard throughout the ocean, and marine mammals communicate acoustically over basin scale distances. The papers in this book indicate a high level of research interest that has generated significant progress in development and application of experimental acoustic inversion techniques. The applications span a broad scope in geosciences, from geophysical, biological and even geochemical research. The list includes: estimation of geotechnical properties of sea bed materials; navigation and mapping of the sea floor; fisheries, aquaculture and sea bed habitat assessment; monitoring of marine mammals; sediment transport; and investigation of natural geohazards in marine sediments. Audience This book is primarily intended for physicists and engineers working in underwater acoustics and oceanic engineering. It will also be of interest to marine biologists, geophysicists and oceanographers as potential users of the methodologies and techniques described in the book contributions. © 2006 Springer. All rights reserved.

Acoustic propagation experiments have been carried out in the Florida Straits with a multi‐frequency broadband source that transmitted M‐sequence pulses over a range of 10 km to a sparse‐filled vertical line array. The sound source was cycled in octaves from 100 Hz to 3200 Hz, transmitting each octave for one hour. This paper presents results of matched field inversions of the acoustic field data to estimate geoacoustic model parameters for the experimental site. The inversion is very sensitive to the sediment sound speed at the sea floor. The estimated value of 1560 m/s is consistent with fine‐grain calcareous sand material that is considered to represent ground truth for the site.

Estimation of geoacoustic models of the sea bed is an underlying research issue in understanding acoustic propagation in shallow water environments where the propagation is generally bottom limited. Inversion methods based on matched field processing have become widely used in applications with experimental data at various sites worldwide. Traditionally, the experiments have been carried out with controlled source geometries. This paper presents a new experimental approach that makes use of the noise radiated by passing ships as the sound source for the inversion. Ship noise data were measured on a 16-element vertical line array in shallow water off the west coast of Vancouver Island. The data were filtered into low (70-110 Hz) and high (170-290 Hz) frequency bands, and processed in an inversion algorithm based on back propagation of the spectral components of the noise signal. The geoacoustic model that generated the most accurate focus at the source location was taken as the best estimate. The band limited data allowed estimation separately of geoacoustic model parameters of the sea floor with the high frequencies, and then for the deeper layers using the low frequencies. The estimated model parameters compared well with ground truth data from a seismic survey and from sediment samples at the site

Marine gas hydrates are cage-like structures of water containing methane or some higher hydrocarbons that are stable under conditions of high pressures and low temperatures. The hydrate structures are formed in sediments of continental margins and are found worldwide. The stability zone extends to about 200 m beneath the sea floor, and hydrates exist in several different forms within the zone, from massive ice-like features at cold seeps on the sea floor to finely distributed deposits in sediment pores over extensive areas. The base of the stability zone is characterized by a strong acoustic impedance change from high velocity hydrated sediments above to low velocity gas below. This acoustic feature generates a strong signal in seismic surveys called the Bottom Simulating Reflector, and it is widely used as an indicator of the presence of hydrates. This paper reviews the current knowledge of hydrate systems from research carried out on the Cascadia Margin off the west coast of Vancouver Island, and in the...

The seven papers in this special issue deal with the effects of internal waves and internal tides on acoustic propagation and communication in shallow water, and geoacoustic characterization of the seabed.

This paper reports measurements of sound attenuation in marine sediments from two locations on the outer New Jersey continental shelf. At one site the sediment in the top 20 m is primarily sandy clay, while the other site includes a thin (3–5 m), over-lying layer of sand at the sea floor. The attenuation was inverted from close range, broadband data from light bulb implosions deployed at stations at the sites. The inversion method made use of the time-frequency dispersion information in signals received at single hydrophones. The signals were first processed by time warping to resolve the propagating modes at relatively close ranges (50–80 water depths). The inversion is carried out in two stages. The first stage inverted the sound speed and density by modeling the modal group velocities, and these estimates were used in the second stage to invert the attenuation from the modal amplitude ratios. The results provide estimates of low-frequency sound attenuation that can be compared to predictions from different models of sound propagation to assess the frequency dependence in the band from 100–500 Hz.

The concept of bottom loss has traditionally been used as a measure of acoustic reflectivity and transmission at the ocean bottom. The measure was generally derived experimentally from measurements of transmission loss versus range, using a simple model that assumed perfect reflection at the sea floor to process the transmission loss data of bottom reflected signal paths. More recently, the interaction of sound with the ocean bottom has been described in terms of a geoacoustic model that defines a physical structure of sound speed, density and attenuation in the material beneath the sea floor. The model parameters are inferred from acoustic field data or observables derived from the field using sophisticated inversion techniques. The estimated model can be compared with bottom loss measurements through calculations of the plane wave reflection coefficient. This paper illustrates comparisons of bottom loss measurements for low- and high-frequency bands (50 –20,000 Hz) and calculations from estimated geoacoustic profiles for deep and shallow water environments. Conditions are discussed that limit the performance of present day inversion techniques: rough interfaces on and below the sea floor, consolidated material that supports shear wave propagation and range variation of sub-bottom structure. [Work supported by ONR Ocean Acoustics Team.]

Submarine gas hydrates were detected over large areas in the Cascadia margin off the west coast of British Columbia by the presence of bottom simulating reflectors in seismic surveys. Initial estimates of gas quantity based on these data and results from an ocean drilling program leg were highly optimistic, and subsequently many different experiments were carried out to quantify the estimates. This paper presents results from an air gun survey with ocean bottom seismometers deployed around a hydrated cold vent to determine the hydrate concentration in the vicinity of the vent. Reflection travel time data from the survey were used in a linearized inversion to determine the velocity structure within the hydrate stability zone. Reflection amplitude data were analyzed to generate maps of the reflectivity at the sea floor and at the base of the stability zone. The travel time inversion revealed that the sediment velocities were very low, indicating low concentration of hydrate (less than 2% of sediment volume)...

This paper describes the results of preliminary acoustic sea bed classification surveys in three areas in the southern Gulf Islands of British Columbia to develop methods for mapping habitat of Pacific Sand Lance (PSL). Little is known about this important species, and much less is known about their use of subtidal burying habitat for overwintering. Grid surveys were run using a dual‐frequency (24 and 200 kHz) single beam echosounder, and the data were classified using conventional statistical segmentation procedures using QTC IMPACT. This type of seabed classification separates the seabed into self‐consistent regions called seabed classes based on acoustic diversity—primarily on the basis of echo shape. Sediment grab samples and video were also obtained to provide ground‐truth and labels for the acoustic seabed classes. The acoustic seabed classification surveys were successful in identifying subtidal sands with low fractions of fines (silts) and sand wave fields among a variety of seabed types identifie...

The goal in geoacoustic modeling is to develop realistic geophysical models of the ocean bottom that can be used in numerical calculations of the acoustic field in the ocean. The ocean bottom is assumed to be a layered structure of different types of sediment material that have been deposited over geological times. However, in shallow water the bottom is generally much more complex. The sediment material is variable on different spatial scales horizontally and is inhomogeneous in depth below the sea floor. Despite this complexity in realistic bottom environments, there has been considerable success using the simplified approach of a layered, range‐independent geology in low‐frequency (20–500 Hz) applications with inversion techniques that provide estimates of geoacoustic model parameters and their uncertainties. This paper reviews some of the most effective inversion techniques and compares their performance in estimating realistic and effective geoacoustic profiles in applications with data from the rece...

Marine sediment attenuation at low frequencies (under 5 kHz) is generally difficult to be directly measured by in situ probes embedded in the sediment, partly due to the very short propagation distances. An alternate experimental technique is to use single bottom bounce signals received by a vertical line array. The frequency dependence of the sediment attenuation is first obtained by comparing the amplitude differences of the sea floor reflection and the sub bottom layer reflection at different frequencies. The absolute attenuation is then obtained by using the previously estimated sound speed and layer thickness. Inherently there is uncertainty introduced in each stage of the attenuation estimation procedure. To evaluate the uncertainty of the attenuation estimate, the standard deviation of the signal fluctuation is mapped to the intermediate result first, and then Bayesian inversion results of the sound speed and the layer thickness are included in the final attenuation estimates. This uncertainty analysis is demonstrated by the estimation of the sediment attenuation from the low frequency chirp data collected in a variable water column environment in the Shallow Water 06 experiment. [Work supported by ONR Ocean Acoustics.]

Acoustic propagation experiments in shallow water over thinly-sedimented bedrock sometimes show high propagation loss, with resonant-like structure, in the 10 to 100 Hz band. This can be interpreted with the aid of plane-wave reflection models that associate high bottom reflection loss at certain combinations of frequency and grazing angle with shear-wave resonances in the sediment layer. In this paper, the seabed is modelled as a thin layer of elastic-solid sediment overlying a hard rock substrate, with plane interfaces at layer boundaries. The bottom reflection loss resonances are explained by both full wave-theoretical reflection loss calculations and a simplified coherent ray reflection loss model. For the ray model, two simplifying assumptions are applied: (1) there is no conversion of energy between compressional waves and shear waves at the water-sediment boundary, and (2) sediment shear waves are attenuated so strongly that they traverse the sediment layer once up and down only. The paper investigates the effect of sediment shear-wave attenuation on bottom reflection loss. We find that the resonance frequencies are controlled by the shear speed and sediment layer thickness, and that the amount of bottom reflection loss is sensitive to the shear-wave attenuation.

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 ...

This paper presents the geoacoustic inversion of broadband data to estimate the sea bed parameters of SWARM95 site. The data set used in this study contains air gun signals that were transmitted at the depths of 25 and 45 m, respectively, below the sea surface, while the source ship was maneuvering at small speed (towing); the signals then were collected at the ranges around 5 km by a vertical line array. A canonical geoacoustic model was generated first according to previous experimental research results, for the purpose of a parameter sensitivity study; then both hybrid optimization and Bayesian inversion techniques were applied to the real data. The data error covariance matrix is estimated to resolve the spatial correlated data error in Bayesian inversion approach. Since the ratio of range and water depth is rather large in this study, only the geoacoustic parameters at the top of the sediment are sensitive in the inversion. The optimization results show the consistency of sensitive geoacoustic/geomet...

The first stage of a seafloor observatory to monitor marine gas hydrates will be deployed in the Gulf of Mexico in the fall of 2005. One component of the monitoring station is a vertical line hydrophone array moored on the seafloor. This paper reports research to investigate using matched field inversion of sound generated by passing ships to detect changes in the geoacoustic parameters of the seabed that may indicate the occurrence of a gas hydrate dissociation event. First, synthetic multifrequency data were used to investigate the performance of the inversion method for estimating sediment sound speeds near the seafloor. The synthetic data were generated using the parabolic equation method for a geoacoustic environment that simulated the seabed at the site in Mississippi Canyon. The geoacoustic model consisted of a multilayered structure with relatively slow sound speeds to significant depths below the seafloor. The inversion based on normal mode replica fields indicated that performance strongly depends on using a realistic parametrization of the geoacoustic model. Experimental ship noise data were recorded along radial tracks in a preliminary deployment of the vertical array. Results are reported for estimating a geoacoustic model from the noise data.

The topic of depth compensation of single beam echo time series for seabed classification is fairly well studied. The effect of seabed slope has not been publicized. In applications for seabed classification, seabed slope is observed to be associated with classification inaccuracy and failure. In cases of higher slope, single beam bathymetry also becomes inaccurate. A survey of 2 fjords with extreme variation in slope is presented as a representative example and testing bed for investigating slope. The direct effect on seabed echoes is investigated and explained in reference to a simple model of beam‐echo geometry. Survey bathymetry is compensated for slope; inaccuracies of up to 5% of depth are corrected, however bottom picking accuracy is diminished in areas of slope and cannot be improved. Surveys of a gas hydrate site and a river will also be presented as applications of these ideas. Early results from BORIS model studies of methods for compensation of slope and depth may also be presented.

Plane‐wave reflection from a thin, elastic‐solid sediment layer over a hard basement is modeled by a simple model that coherently combines rays from only two ray families: a family of compressional waves that transit the sediment layer any number of times before returning to the water and a similar family that includes doubly converted compressional waves. (The latter are compressional waves that are converted to shear waves at the basement, make one or more round‐trip transits of the sediment layer, and are converted back into compressional waves at the basement.) The theory is applied to homogeneous sediments having low shear speeds, and it provides valuable insight into the physical processes governing reflection from several seabed environments of interest. The coherent sum of all the rays belonging to these families leads to an approximate closed‐form expression for the overall reflection coefficient that includes several characteristic features also simulated by full wave‐theoretical numerical models, such as bottom‐loss resonances related to sediment shear waves and large losses associated with the excitation of interface waves at the sediment/basement boundary. Calculated examples are provided.

Geoacoustic models have been estimated for the upper oceanic crust by inversion of bottom reflection loss versus angle data. The data were obtained in the PACIFIC ECHO experiments at thin‐sediment sites in the North Pacific where the age of the crust increased from 40 to 64 million years. The measurements were carried out using a horizontal line array and small explosive charges, and the reflection loss was determined from beamformed array data in order to resolve the specular component. The geoacoustic models consist of profiles of the density, and the compressional (P) and shear (S) wave speeds and attenuations of both the sediment and basalt crust layers. The inversion assumes either a simple homogeneous half‐space basement, or a layered model with constant sound‐speed gradients in the upper crust overlying a homogeneous basement. The results indicate that the P‐wave speed near the sea floor increases with increasing age of the oceanic crust from values of 2700 to about 3300 m/s. These values are signi...