Sediment Sound Speed Profiles Research Articles

Direct measurements of acoustic and physical properties in mud and comparison to sediment acoustics models

Sediment acoustics models relate seabed physical properties to acoustic wave speed and attenuation, and direct measurements of sediment acoustic and physical properties provide a means to test such models. We examine direct measurements from two field experiment sites with muddy sediments that contain different ratios of clay, silt, sand, and organic components, comparing the acoustic data with sediment acoustic model predictions using parameters obtained from cores as model inputs. In the New England Mud Patch, sediment sound speed profiles up to several meters below the seabed surface were measured using in situ acoustic probes mounted to a gravity corer. At a second site near Mobile Bay, in situ probe measurements of sound speed, attenuation, and shear speed were obtained in the upper 20 cm of sediment, and diver cores were collected and scanned with an acoustic core logger. Sediments from both sites were analyzed for porosity and grain size distribution. The applicability of various models to the mud found at each site will be discussed, as well as effects of infauna and organic matter, which were also measured, but are not explicitly included in the models. [Work sponsored by ONR.]

On acoustic reflection from a seabed exhibiting a non-uniform sound speed profile, with relevance to fine-grained sediments.

An analysis of the plane wave reflection coefficient of the seabed, R, is developed for two upward-refracting sediment sound speed profiles: the two-parameter linear and the three-parameter inverse-square, both extending to infinite depth. For the linear profile, it turns out that |R| = 1, representing total reflection for all grazing angles and all frequencies, signifying that in this special case, |R| is insensitive to the gradient. The implication is that if |R| is to return information about the shape of a profile, the gradient must change with depth, either smoothly through the presence of second- and/or higher-order depth derivatives or discontinuously at, say, an interface between sediment layers. The inverse-square is an example of a profile with a smoothly varying gradient, for which a general, closed-form expression for R is derived, valid for all grazing angles and all frequencies. When the sound speed ratio is less than unity, representative of a fine-grained sediment (mud), |R| exhibits two frequency regimes, designated high and low, separated by a transition frequency, fT. In each of these regimes, |R| exhibits a frequency-dependent angle of intromission, which exhibits high- and low-frequency limiting values, differing by approximately 3.5°, depending on the geo-acoustic parameters of the sediment.

In situ measurements of compressional and shear wave properties during gravity coring operations

Sediment cores provide valuable insight about the physical properties of the seabed, and laboratory measurements of sediment wave speed from cores are often considered “ground truth.” However, sound-speed estimates obtained from cores can be inaccurate due to changes in pressure, temperature, and mechanical properties of the sediment caused by the removal of the core from the seabed and its subsequent transport to the laboratory. To address these issues, the Acoustic Coring System (ACS) was developed. The system uses sets of transducers mounted below the penetrating tip of a sediment corer to make in situ measurements of geoacoustic properties as the corer penetrates the seabed. Compressional wave measurements are obtained with rod-mounted piezoelectric cylinders, and shear wave measurements are obtained with bender elements mounted in flat blades. The ACS was deployed during the environmental survey for ONR Seabed Characterization Experiment (SBCEX) in 2016 and in support of the ONR Canada Basin Acoustic Propagation Experiment (CANAPE) in 2017. In both cases, the system provided measurements of the sediment sound-speed profile as a function of depth within the seabed. On-going work focused on the measurement of shear wave speed profiles and compressional and shear wave attenuation will also be presented. [Work supported by ONR.]Sediment cores provide valuable insight about the physical properties of the seabed, and laboratory measurements of sediment wave speed from cores are often considered “ground truth.” However, sound-speed estimates obtained from cores can be inaccurate due to changes in pressure, temperature, and mechanical properties of the sediment caused by the removal of the core from the seabed and its subsequent transport to the laboratory. To address these issues, the Acoustic Coring System (ACS) was developed. The system uses sets of transducers mounted below the penetrating tip of a sediment corer to make in situ measurements of geoacoustic properties as the corer penetrates the seabed. Compressional wave measurements are obtained with rod-mounted piezoelectric cylinders, and shear wave measurements are obtained with bender elements mounted in flat blades. The ACS was deployed during the environmental survey for ONR Seabed Characterization Experiment (SBCEX) in 2016 and in support of the ONR Canada Basin Acoustic...

In Situ Measurements of Compressional Wave Speed During Gravity Coring Operations in the New England Mud Patch

This paper presents measurements of sediment sound-speed profiles measured in situ using the acoustic coring system (ACS). The reported measurements were obtained from seven gravity cores collected in the New England Mud Patch. The ACS uses two sets of transducers mounted below the penetrating tip of a sediment corer to make in situ measurements of geoacoustic properties as the corer penetrates the seabed. The in situ sound-speed profiles are interpreted in the context of stratigraphic layering measured by a seismic survey, and the measured sound-speed profiles are consistent with the geophysical description. The in situ sound-speed profiles measured by the ACS were compared to conventional measurements of sound speed on the recovered cores using the multisensor core logger (MSCL). The MSCL data displayed both random and systematic errors that were attributed to disturbance from the coring process and/or the handling of the sediments after collection. Finally, using porosity and grain size distributions measured from discrete samples of the cored sediments, the in situ sound-speed measurements were compared to empirical regressions based on independent data sets.

Path losses in sandy sediments with upward refracting sound speed profiles

In this work, receiver-to-receiver path losses calculated from measurements of underwater explosions in shallow water (water depth ∼15 m) off the coast of Virginia Beach, VA will be presented. Within the approximate frequency range of 40 to 70 Hz, these data have path losses that suggest energy increasing with range by up to 10 dB. Above this frequency range, these data show energy decreasing with range as expected. An analytical model [Frisk, George V., “Determination of sediment sound speed profiles using caustic range information,“ Bottom-Interacting Ocean Acoustics. Springer, US, 1980, 153–157] and numerical propagation models will be used to show that these path losses can be attributed to an upward refracting sound speed gradient in the sediment. This upward refraction of sound waves propagating in the seabed results in the formation of caustics that define shadow zone boundaries. Below the cutoff frequency of the waveguide, receivers in the shadow zone will exhibit lower sound levels than receivers positioned farther from the source and outside of the shadow zone. While frequencies above cutoff will also undergo refraction in the seabed, the observability of this effect will be masked by trapped modes propagating in the water.

Spherical wave scattering from rough surfaces and array processing: Application to sound-speed profile measurement uncertainty analysis.

A three-dimensional modeling of spherical wave reflection on layered media with rough interfaces is used to study the roughness influence in sediment sound-speed profile measurements. The configuration of the numerical experiment consists of a point source and a horizontal array of hydrophones. In addition, theoretical uncertainties on the angle of arrival and travel time of a spherical wave reflected over a slightly rough interface are derived by including the array processing in the Helmholtz-Kirchhoff integral. The sound-speed profile sensitivity to those parameters is used to compare theory and numerical experiments. The agreement is good and shows a surprisingly strong influence of the roughness on the sound-speed profile measurement.

A comparison between direct measurements and inference results from an acoustic propagation experiment in Currituck Sound

An acoustic propagation experiment was conducted in Currituck Sound to characterize low frequency propagation in a very shallow-water environment with water depths of only a few meters. Signals from a Combustive Sound Source (CSS) were recorded on bottom mounted geophones and a vertical array of hydrophones. The CSS produces a broadband signal with significant low frequency energy, and the analysis presented in this paper focuses on frequencies below 2.5 kHz. A statistical inference method was applied to obtain an estimate of the sediment sound-speed profile as a function of depth in the seabed. During the experiment, in situ measurements of compressional and shear wave speed and attenuation were also collected 30 cm below the sediment-water interface. Bimorph bender elements were employed to generate and receive horizontally polarized shear waves in the frequency range from 200 Hz to 1 kHz. Compressional waves were measured using cylindrically shaped piezoelectric elements operated in the 5 kHz to 100 kH...

Seabed property inversion—Sequential and direct approaches

The goal of this work is to improve our knowledge of the ocean medium using physics of sound propagation and statistical signal processing. Two of the approaches we discuss are based on global and local optimization combined with sequential filtering. A third one falls under the category of direct (“exact”) methods. The first two approaches are based on the extraction and the association between paths/modes and detected arrivals and also produce posterior probability density functions, characterizing the uncertainty structure of modal/multipath arrivals. These are then propagated backwards through an acoustic model for posterior probability density function calculation of environmental parameters. A third method extends Stickler’s exact inverse approach, employing measurements of a reflection coefficient at low frequencies to obtain the sediment sound speed profile. We develop an approximation based on interpolation and a regularization approach. The former improves on results of a previous method for sol...

Seafloor sound-speed profile and interface dip angle measurement by the image source method: Application to real data

The image source method characterizes the sediment sound-speed profile from seafloor reflection data with a low computational cost compared with inversion techniques. Recently, the method has been extended to treat non-parallel sediment layering. The method is applied to data from an autonomous underwater vehicle (AUV) towing a source (1600–3500 Hz) and an horizontal array of hydrophones. AUV reflection measurements were acquired every 3 m along 10 criss-cross lines over a 1km 2 area with evidently dipping layers. Mapping the along track sound-speed profiles in geographical coordinates results in a pseudo-3D (Nx2D) sediment structure characterization of the area down to several tens of meters in the sub-bottom. The sound speed profile agreement at crossing points is quite good.

Sensitivity analysis of the image source method to out-of-plane effects

In the context of seafloor characterization, the image source method is a technique to estimate the sediment sound-speed profile from broadband seafloor reflection data. Recently the method has been extended to treat non-parallel layering of the sediment stack. In using the method with measured data, the estimated sound-speed profiles are observed to exhibit fluctuations. These fluctuations may be partially due to violation of several assumptions: (1) the layer interfaces are smooth with respect to the wavelength and (2) out-of-plane effects are negligible. In order to better understand the impact these effects, the sensitivity of the image source method to roughness and out-of-plane effects are examined.

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.

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.]

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.]

Plane wave reflection coefficient from near field measurements

The plane wave reflection coefficient for all angles and frequencies is a unique descriptor of the acoustic properties of a seabed, but it is difficult to measure directly in shallow water. However, one can calculate the point receiver result by a plane wave expansion given the plane wave reflection coefficient. In this paper it is shown that it is also possible to perform the more useful inverse operation of calculating the plane wave reflection coefficient given point source and receiver measurements. This means that it is possible to make propagation predictions in the presence of complex bottom structure using short range reflection measurements but without ever determining the detailed geoacoustic parameters. Simulations show that the method works for multiple isospeed layers and also continuously varying sediment sound speed profiles. The feasibility of experimental determination of the plane wave reflection coefficient is addressed.

Caustic envelopes and cusp coordinates due to the reflection of a spherical wave from a layered sediment

A compilation of analytical formulas is presented, describing ray properties of a family of sediment sound speed profiles. The profiles are described as “generalized power law” profiles because they are generalizations of a similar family of simpler power law profiles considered in earlier work [M. A. Ainslie, J. Acoust. Soc. Am. 96, 2506–2515 (1994)]. Properties considered include ray paths, caustic envelopes, and cusp coordinates due to a point source in isovelocity water. For a few example cases the caustic shapes are illustrated by computing the pressure field using a normal mode program.

Tomographic Inversion for Geoacoustic Parameters in Shallow Water

This paper examines the linearized tomographic inversion of simulated data for a shallow water, multi-array, multi-source scenario. The environments represented include simulations of (1) highly idealized constant regions as well as (2) the Haro Strait Test of June 1996 which displays range, depth, and azimuthal variability, i.e., 3-D dependence on environmental parameters where these parameters can include water depths and multiple sediment sound-speed profiles, densities, depths, and attenuations. This tomographic inversion method is independent of the number of parameters to be determined. However, the method does assume that some inversion method (such as RIGS, simulated annealing, genetic algorithms, etc.) has already estimated range-independent average source-to-receiver environmental parameters. These average parameters are then input into the tomographic inversion which relies on a matrix of path-cell distances. The matrix condition number, Λ, is a determining feature for the inversion accuracy where Λ is a function of source and receiver distributions and their subsequent path distances through the region cells. Additionally, the accuracy of the input estimates for the average geoacoustic properties is also an important factor in the final 3-D tomographic inversion accuracy. Results using this (linearized) tomography inversion method show a potential for excellent error estimates (much less than 1%) for the environmental parameters assuming exact, idealized input values. Errors are still quite reasonable (well under 10%) if more realistic, i.e., erroneous, input values are assumed. This paper will conclude with a discussion of upcoming future directions.

TOMOGRAPHIC INVERSION FOR GEOACOUSTIC PARAMETERS IN SHALLOW WATER

This paper examines the linearized tomographic inversion of simulated data for a shallow water, multi-array, multi-source scenario. The environments represented include simulations of (1) highly idealized constant regions as well as (2) the Haro Strait Test of June 1996 which displays range, depth, and azimuthal variability, i.e., 3-D dependence on environmental parameters where these parameters can include water depths and multiple sediment sound-speed profiles, densities, depths, and attenuations. This tomographic inversion method is independent of the number of parameters to be determined. However, the method does assume that some inversion method (such as RIGS, simulated annealing, genetic algorithms, etc.) has already estimated range-independent average source-to-receiver environmental parameters. These average parameters are then input into the tomographic inversion which relies on a matrix of path-cell distances. The matrix condition number, Λ, is a determining feature for the inversion accuracy where Λ is a function of source and receiver distributions and their subsequent path distances through the region cells. Additionally, the accuracy of the input estimates for the average geoacoustic properties is also an important factor in the final 3-D tomographic inversion accuracy. Results using this (linearized) tomography inversion method show a potential for excellent error estimates (much less than 1%) for the environmental parameters assuming exact, idealized input values. Errors are still quite reasonable (well under 10%) if more realistic, i.e., erroneous, input values are assumed. This paper will conclude with a discussion of upcoming future directions.

Sediment tomography in the Middle Atlantic Bight

Sediment properties such as compressional speed and attenuation have been extracted using the modal dispersion of explosive signals in the Middle Atlantic Bight. Data collected during the Shelf Break PRIMER experiment are used to generate sediment sound speed profiles along various propagation paths. Compressional speeds are inverted using traveltime of modes 1 to 9 from 10 Hz to 225 Hz. Compressional attenuation is inverted using modal amplitude ratios. Gravity cores taken at the experimental site give direct mesurements of compressional speed and attenuation for comparison. A posteriori estimates of resolution are evaluated. Nonlinear inversion techniques based on a Genetic Algorithm and Levenberg–Marquardt methods are used. Methods for increasing the efficiency of the inversion algorithm are also investigated. [Work supported by ONR.]

Measurements and modeling of low-frequency bottom-scattering strengths from the Scotian Continental Rise

A series of low-frequency (190–310 Hz), bottom-backscattering strength measurements were made on the Scotian Continental Rise in August 1993 using combinations of short-duration cw and HFM signals. Grazing angles realized were between 5 and 30 deg. A characteristic of many sites were strong (on the order of 10 dB) oscillations with a grazing angle superimposed on generally Mackenzian backscatter. The oscillatory behavior is due to caustics within the sediment, i.e., a propagation effect. Previous modeling of this general area [P. D. Mourad and D. R. Jackson, J. Acoust. Soc. Am. 94, 344–358 (1993)] predicted such an oscillatory behavior but only for unrealistic sediment sound–speed profiles. An alternative bottom-scattering model is proposed with the goal of predicting the observed scatter using a more realistic geoacoustic model. Measurement/model comparisons will be presented for several representative sites. [Work supported by SPAWAR, NRL, and ONR.]

A model/data comparison for low-frequency bottom backscatter

This article describes a model for bottom backscattering strength applicable over the frequency range 100–1000 Hz. The model includes backscatter due to both a rough water/sediment interface and internal sediment inhomogeneities—i.e., sediment-volume scattering within an upward-refracting sediment. The net result is predictions for bottom backscattering strength that depend on the following: frequency, sound speed of the water near the water/sediment interface, density ratio, sound absorption within the sediment, near-surface sound-speed gradient within the sediment, two interfacial roughness parameters, and a parameter proportional to the scattering cross section of a unit volume of sediment. The sediment sound-speed profile is specified in terms of near-surface values for the sound-speed ratio and gradient. Salient features of the modeled backscatter strength include marked departures from Lambert’s law for sediments with sound speed ratios greater than 1; 1–5-dB oscillations as a function of grazing angle, due to the complex structure of the sound field in the sediment; the dominance of sediment-volume scattering over rough surface scattering for most of the grazing angles, frequencies, and sediment types considered; and, for soft sediments, a decrease by about 8–10 dB in the overall level of backscatter as frequency increases from 100 Hz to almost 1000 Hz.