Sediment Sound Speed Research Articles

High-frequency sediment sound speed and attenuation measurements during TREX13 (Target and Reverberation Experiment 2013) with a new portable velocimeter

During the Target and Reverberation Experiment 2013 (TREX13), high-frequency measurements of sediment sound speed and attenuation were collected throughout the experiment site. These measurements were performed using the INSEA, a diver-portable array of sources and receivers conceived and developed by French companies in collaboration with research institutions. During each deployment of the instrument, the INSEA was inserted 10–15 cm into the sediment and narrow-band pulses covering the 70 to 350 kHz range were transmitted through the sediment. The sound speed is determined from the time-of-flight and attenuation is determined from the amplitude ratio of the transmissions through the sediment and through the water. The variability of the TREX13 site made it possible to collect data in several different sediment types including mud, silty-sand, and sand sediments each with low to high concentrations of shells. In addition to the acoustic measurements, diver cores and shell samples were also collected. The sound speed and attenuation measured in these sediments are presented and discussed. [Work supported by DGA, ONRG, and SERDP.]

Range dependent sediment sound speed profile measurements using the image source method

This paper presents a range dependent sediment sound speed profile measurement obtained using the image source method. This technique is based on the analysis of the seafloor reflected acoustic wave as a collection of image sources which positions are linked with the thick-nesses and the sound speed of the sediment stack. The data used were acquired by the NURC in 2009 during the Clutter09 experiment. The equipment used was an autonomous undersea vehicle towing a 1600-3500 Hz frequency band source and a 32 m horizontal line array of 32 hydrophones at 12 m above the seabed. Under the assumption of locally range independent seabed properties, the moving horizontal array provides successive range independent sediment sound speed profiles along a track to obtain the range and depth dependent structure of the seafloor. Two key steps include recovery of the time-varying unknown array shape from the data and spatial filtering of the successive sound speed profiles. A comparison of the image source method result and seismic data along nearly the same 14 km track indicates that the seabed stratigraphy is correctly mapped by this method.

Quantifying the effects of roughness scattering on reflection loss measurements

Seafloor reflection loss and roughness measurements were taken at the Experimental Validation of Acoustic Modeling Techniques experiment in 2006. The magnitude and phase of the reflection loss was measured at frequencies from 5 to 80 kHz and grazing angles from 7° to 77°. Approximately 1500 samples were taken for each angle. The roughness was measured with a laser profiler. Geoacoustic parameters such as water and sediment sound speed and density were measured concurrently. The reflection loss data were compared with three models: A flat interface elastic model based on geoacoustic measurements; a flat interface poro-elastic model based on the Biot/Stoll model; and a rough interface model based on the measured interface roughness power spectrum. The data were most consistent with the poro-elastic model including scattering. The elastic model consistently predicted values for the reflection loss which were higher than measured. The data exhibited more variability than the model due to layering and fluctuations in the propagating medium.

A stochastic response surface formulation for the description of acoustic propagation through an uncertain internal wave field

A modeling and simulation study is performed in a littoral ocean waveguide subject to uncertainty in four quantities: source depth, tidal forcing, initial thermocline structure, and sediment sound speed. In this partially known shelf-break environment, tidal forcing over a density-stratified water column produces internal tides and solitary wave packets. The resulting uncertainty in the space-time oceanographic field is mapped into the sound speed distribution which, in turn, introduces uncertainty into the acoustic wave field. The latter is treated as a stochastic field whose intensity is described by a polynomial chaos expansion. The expansion coefficients are estimated through constrained multivariate linear regression, and an analysis of the chaos coefficients provides insight into the relative contribution of the uncertain acoustic and oceanographic quantities. Histograms of acoustic intensity are estimated and compared to a reference solution obtained through Latin Hypercube sampling. A sensitivity analysis is performed to illustrate the relative importance of the four contributions of incomplete information about the environment. The simulation methodology represents an end-to-end analysis approach including both oceanographic and acoustic field uncertainty where the latter is quantified using stochastic basis expansions in the form of a polynomial chaos representation.

High-frequency acoustic backscattering from a sand sediment: Experiments and data/model comparisons

In the Spring of 2012, high-frequency backscattering from a sandy sediment was measured in the Gulf of Mexico at the site of the upcoming, ONR-sponsored reverberation experiment. The measurements were made using an array of sources and receivers that collected data from 200 to 500 kHz and that could be rotated such that the incident grazing angles varied from 10 to 50 degrees. This array was used previously to measure scattering from a sand/mud sediment during the Sediment Acoustics Experiment 2004 (SAX04). To support data/model comparisons, the seabed roughness, sediment shell content, sediment sound speed, and sediment attenuation were also measured. For scattering below the critical grazing angle, sediment roughness is found to be the dominant scattering mechanism while above the critical angle, roughness scattering underpredicts the measured scattering strength. To understand the scattering strength at high grazing angles, scattering from shells and shell hash is considered. The measured scattering strengths and environmental properties at the experiment site are also compared to those made during SAX04. (Work supported by the US Office of Naval Research)

An inverse method for estimating sediment sound speed in the ocean

In this work, a new inverse method for estimating sediment sound-speed profiles is investigated. Stickler and Deift derived a trace formula for recovering a seiment sound-speed profile by simply using a reflection coefficient at very low frequencies. The method may be sensitive to noise and also involves computationally cumbersome calculations. In our approach, we first design a linear approximation for the trace formula based on the Born approximation, in order to reduce the computational cost. The stability of the modified inverse algorithm is then tested with synthetic noisy data. Finally, we look into ways for relaxing the limiting assumptions of the approach. [Work supported by ONR and the NSF CSUMS program.]

Geoacoustic Inversion of Mid-Frequency Bottom Loss Data in Shallow Water off the East Coast of Korea

Mid-frequency bottom loss measurements over a grazing angle range of 25 to 70° at frequencies of 6, 8, and 10 kHz were made off the east coast of Korea in October 2008. In this paper, geoacoustic parameters including the sediment sound speed, density, attenuation coefficient, and thickness of the surficial sediment layer are estimated via comparison between the measured bottom loss and a bottom reflection forward model based on the two-layered fluid sediment structure. A global search is performed over the search space of geoacoustic parameters by a genetic algorithm method, in which the weighted squared error between the data and the model predictions is used as an objective function to be minimized. The inversion result shows that a thin lower sound speed layer with a thickness of 0.4 m overlies the higher sound speed sediment. This result is consistent with marine geological observations conducted at the experimental site.

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.]

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]

Mid-frequency geoacoustic inversion using bottom loss data from the Shallow Water 2006 Experiment

Geoacoustic inversion work has typically been carried out at frequencies below 1 kHz, assuming flat, horizontally stratified bottom models. Despite the relevance to Navy sonar systems many of which operate at mid-frequencies (1-10 kHz), limited inversion work has been carried out in this frequency band. This paper is an effort to demonstrate the viability of geoacoustic inversion using bottom loss data between 2 and 5 kHz. The acoustic measurements were taken during the Shallow Water 2006 Experiment off the coast of New Jersey. A half-space bottom model, with three parameters density, compressional wave speed, and attenuation, was used for inversion by fitting the model to data in the least-square sense. Inverted sediment sound speed and attenuation were compared with direct measurements and with inversion results using different techniques carried out in SW06. Inverted results of the present work are consistent with other measurements, considering the known spatial variability in this area. The observations and modeling results demonstrate that forward scattering from topographical changes is important at mid-frequencies and should be taken into account in sound propagation predictions and geoacoustic inversion. To cope with fine-scale topographic variability, measurement technique such as averaging over tracks may be necessary.

Direct measurements of sediment sound speed at mid- to high-frequency in a sand sediment

Direct measurements of sediment sound speed were made near Panama City, Florida in April 2011. Considerable effort is being made to provide detailed environmental characterization for an upcoming mid-frequency sound propagation and reverberation experiment in 2013 and the measurements presented here are part of that effort. Two direct measurement systems are shown: one is called the Sediment Acoustic-speed Measurement System (SAMS) and the other is the attenuation array which was deployed in the Sediment Acoustics Experiment 2004. SAMS consists of ten fixed sources and one receiver. The receiver is driven into the seabed by a motor, which allows precise penetration depth up to 3 m with arbitrary step size. Measurements were made in the frequency range of 1 – 50 kHz. The attenuation array consists of two transmitters and two receivers mounted on a diver-deployable frame and can provide surficial sediment sound speed and attenuation to a depth of about 10 cm between 40 and 260 kHz. Sediment sound speeds obt...

Bottom loss measurements and their use in studying the physics of sound propagation in marine sediments

Bottom reflection loss measurements have been conducted for four decades and have provided key insights into the physics of propagation in marine sediments. The advantage of bottom loss as an analysis quantity is that, in principle, it completely isolates the role of the seabed and permits separation and identification of key physical mechanisms. For example, it was early deep water bottom loss measurements that first made it clear that strong positive sediment sound speed gradients existed and the concomitant surprising realization that the dominant seabed interacting path was the refracted and not reflected path over a wide angular and frequency band. In this talk, various discoveries from bottom loss measurements are discussed including the role of sediment layering, attenuation gradients, and shear waves.

Sediment sound speed and attenuation results from the sediment acoustics experiments in 1999 (SAX99) and 2004 (SAX04).

Sediment acoustics experiments in 1999 (SAX99) and 2004 (SAX04) were conducted in shallow water off the Florida Panhandle near Fort Walton Beach, Florida. Measurements were made of the frequency dependence of sound speed and attenuation in sandy sediments, supplemented by detailed environmental characterization. Following SAX99 laboratory measurements were conducted to further investigate the physical mechanisms contributing to the frequency dependences observed. An overview will be given of these sound speed and attenuation results, and the implications for sediment acoustics modeling will be discussed. [Work supported by ONR.]

An approximate form of the Rayleigh reflection loss and its phase: Application to reverberation calculation

A useful approximation to the Rayleigh reflection coefficient for two half-spaces composed of water over sediment is derived. This exhibits dependence on angle that may deviate considerably from linear in the interval between grazing and critical. It shows that the non-linearity can be expressed as a separate function that multiplies the linear loss coefficient. This non-linearity term depends only on sediment density and does not depend on sediment sound speed or volume absorption. The non-linearity term tends to unity, i.e., the reflection loss becomes effectively linear, when the density ratio is about 1.27. The reflection phase in the same approximation leads to the well-known "effective depth" and "lateral shift." A class of closed-form reverberation (and signal-to-reverberation) expressions has already been developed [C. H. Harrison, J. Acoust. Soc. Am. 114, 2744-2756 (2003); C. H. Harrison, J. Comput. Acoust. 13, 317-340 (2005); C. H. Harrison, IEEE J. Ocean. Eng. 30, 660-675 (2005)]. The findings of this paper enable one to convert these reverberation expressions from simple linear loss to more general reflecting environments. Correction curves are calculated in terms of sediment density. These curves are applied to a test case taken from a recent ONR-funded Reverberation Workshop.

Fine-Scale Volume Heterogeneity in a Mixed Sand/Mud Sediment off Fort Walton Beach, FL

As part of the effort to characterize the acoustic and physical properties of the seafloor during the high-frequency 2004 Sediment Acoustics Experiment (SAX04), fine-scale variability of sediment sound speed and density was measured in a medium quartz sand using diver cores and an in situ conductivity probe. This study has a goal of providing environmental input to high-frequency backscatter modeling efforts. Because the experiment was conducted immediately following exposure of the site to Hurricane Ivan, measurements revealed storm-generated sedimentary structure that included mud deposits and trapped sand pockets suspended in the mud. Fluctuations of sediment sound speed and density were measured downcore at 1- and 2-cm increments, respectively, with standard laboratory techniques. Sediment density was also measured on a very fine scale with an in situ conductivity probe [in situ measurement of porosity (IMP2)] and by means of computed tomography (CT) imaging of a diver core. Overlap between the locations of the diver cores and the conductivity probe measurements allowed an examination of multiple scales of sediment heterogeneity and a comparison of techniques. Sediment heterogeneity was characterized using estimates of covariance corresponding to an algebraic form for the power spectrum of fluctuations obtained from core, conductivity, and CT measurements. Correcting for sampling brings the power spectra for density fluctuations determined from the various measurements into agreement, and supports description of heterogeneity at the site over a wide range of scales by a power spectrum having a simple algebraic form.

Time-of-Flight Measurements of Acoustic Wave Speed in a Sandy Sediment at 0.6–20 kHz

There is considerable interest within the underwater acoustics community as to whether a fluid model or a poroelastic (Biot) model provides a more accurate representation of sandy sediments. One key metric used to determine this is the acoustic wave speed in the seabed, since the Biot model predicts a sound speed that is frequency dependent whereas the traditional fluid model assumes a sound speed that is constant with frequency. Results obtained during the 1999 Sediment Acoustics Experiment (SAX99) showed some evidence of sound-speed dispersion [IEEE J. Ocean. Eng., vol. 27, no. 3, pp. 413-428, 2002]. The results were consistent with Biot model predictions that employed inputs based on geophysical measurements made at the site. However, only a limited data set was obtained at frequencies from 1 to 10 kHz where the model exhibited its greatest sound-speed variation. Furthermore, these were relative-rather than absolute-measurements of sound-speed dispersion. During the SAX04 sea trial, conducted in autumn 2004 about a kilometer from the location of the SAX99 site, acoustic data were collected on receivers buried in the seabed using a pair of transmitters located within the seabed and a third located in the water column directly above the buried receivers. This source geometry enabled direct time-of-flight (TOF) measurements of acoustic wave speed along all three Cartesian axes. The results are normalized by the acoustic wave speed in the overlying water. Horizontal measurements yielded absolute dispersion estimates but the vertical data were limited to relative estimates due to uncertainty in the depths of the receivers. Results show dispersion within the error limits of the measurement with normalized sediment sound speed increasing from 1.05 at 600 Hz to 1.13 at 20 kHz. The frequency dependence of the measured sound-speed ratios reported on in this paper is in agreement with a simplified poroelastic model [J. Acoust. Soc. Amer., vol. 110, no. 5, pp. 2276-2281, 2001] evaluated using physical parameters measured nearby during SAX99, but the measured sound-speed ratios are about 3% lower than the model predicts; however, some of the vibracores taken at the SAX04 site indicate the presence of small mud inclusions at about 1-m depth, and model results using the oases seismoacoustic model indicate that the lower sound speeds are consistent with the presence of a thin muddy layer. In addition, sound speed along the vertical axis showed substantially greater variability with frequency than did the measurements along the horizontal axes. Results obtained from a simple numerical model indicate that the greater variability in the vertical direction can be explained by interference from reflected arrivals from a low-speed reflector at approximately 1-m depth. Using the porosity β as a free parameter, a best fit of the poroelastic model to the data is obtained for β = 0.425 . Although this is higher than the value of β = 0.385 measured in the sandy sediment during SAX99, heuristic arguments based on the self-consistent model results and the vibracores are presented to support the hypothesis that localized muddy inclusions at the experimental site increased the average porosity over the horizontal propagation paths and resulted in the lower sound-speed ratios.

Geoacoustic Inversion for the New Jersey Shelf: 3-D Sediment Model

A 3-D model of sediment sound speed for a 90-km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area on the New Jersey shelf was constructed by application of a geoacoustic inversion technique. This approach is based on a combination of seismic reflection measurements and a perturbative inversion scheme using horizontal wave number estimates. In a two-step process, seismic reflection measurements were used to identify depths at which discontinuities in the sound-speed profile (SSP) likely occur. Then, the perturbative inversion algorithm made use of this <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a priori</i> information by employing qualitative regularization, an optimal method for addressing stability and uniqueness issues associated with solving the ill-posed inversion problem that provides for resolution of the layered seabed structure.

Estimation of Geoacoustic Properties of Marine Sediment Using a Hybrid Differential Evolution Inversion Method

This paper reports the inversion of midfrequency (1500-4500 Hz) chirps from a short-range transmission experiment conducted on the New Jersey Continental Shelf during the 2006 Shallow Water Experiment (SW06). The source was held at different depths and the sound signals were recorded at a vertical line array to investigate the interactions with the sea bottom at various grazing angles. Strong reflections from the sediment layer were seen in the data for all of the sources. Due to the presence of complex microstructures in the thermocline of the oceanic sound-speed profile, fluctuations both in amplitude and arrival time of the direct path arrivals were observed. Time variation of the water-column environment was also evident during the source transmissions. To mitigate the effects of the ocean environment on the seabed property estimation, a multistage optimization inversion was employed. The sound speed and the experimental geometry were inverted first using only the travel times of the water-column arrivals. The bottom sound speed and the sediment layer thickness were then inverted by matching the travel times of the bottom and sub-bottom reflections. The average of the estimated values for the sediment sound speed is 1598 m/s, consistent with in situ measurements from other experiments in the vicinity.

A particle filtering approach for multipath arrival time estimation from acoustic time series.

Accurately estimating arrival times from acoustic time series in the ocean leads to a wealth of information on the geometry of the sound propagation environment and environmental parameters such as sound speed in sediments. In this work, a sequential Monte Carlo method is developed that treats the spatial variation of signal paths as moving targets, dynamically modeling their temporal location at spatially separated receiving phones. The spatial rather than the temporal sequential variability of the problem permits the application of smoothing techniques to the arrival time estimates, significantly reducing uncertainty. The results are compared to maximum likelihood estimates for the same test cases; the comparison demonstrates an advantage in using the proposed approach, which can be employed for reduction in uncertainty in arrival time estimation and, consequently, in geometric and geoacoustic inversion. It is further demonstrated how this method accurately extracts path amplitudes, which can then be employed for attenuation inversion. This method, suitable for environments with an unknown number of arriving paths, is also tested successfully on Haro Strait Primer Experiment and Shallow Water 06 data. [Work supported by ONR.]

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.]