Search

This study applies three classification methods exploiting the angular dependence of acoustic seafloor backscatter along with high resolution sub-bottom profiling for seafloor sediment characterization in the Eckernförde Bay, Baltic Sea Germany. This area is well suited for acoustic backscatter studies due to its shallowness, its smooth bathymetry and the presence of a wide range of sediment types. Backscatter data were acquired using a Seabeam1180 (180 kHz) multibeam echosounder and sub-bottom profiler data were recorded using a SES-2000 parametric sonar transmitting 6 and 12 kHz. The high density of seafloor soundings allowed extracting backscatter layers for five beam angles over a large part of the surveyed area. A Bayesian probability method was employed for sediment classification based on the backscatter variability at a single incidence angle, whereas Maximum Likelihood Classification (MLC) and Principal Components Analysis (PCA) were applied to the multi-angle layers. The Bayesian approach was used for identifying the optimum number of acoustic classes because cluster validation is carried out prior to class assignment and class outputs are ordinal categorical values. The method is based on the principle that backscatter values from a single incidence angle express a normal distribution for a particular sediment type. The resulting Bayesian classes were well correlated to median grain sizes and the percentage of coarse material. The MLC method uses angular response information from five layers of training areas extracted from the Bayesian classification map. The subsequent PCA analysis is based on the transformation of these five layers into two principal components that comprise most of the data variability. These principal components were clustered in five classes after running an external cluster validation test. In general both methods MLC and PCA, separated the various sediment types effectively, showing good agreement (kappa >0.7) with the Bayesian approach which also correlates well with ground truth data (r2 > 0.7). In addition, sub-bottom data were used in conjunction with the Bayesian classification results to characterize acoustic classes with respect to their geological and stratigraphic interpretation. The joined interpretation of seafloor and sub-seafloor data sets proved to be an efficient approach for a better understanding of seafloor backscatter patchiness and to discriminate acoustically similar classes in different geological/bathymetric settings. [ABSTRACT FROM AUTHOR]

An acoustic propagation experiment was conducted in Currituck Sound to characterize low-frequency propagation in a very-shallow-water estuarine environment. The water column properties were homogeneous over the study area, and the emphasis of this work is on understanding the propagation effects induced by the estuarine bed. During the experiment, low-frequency sound propagation measurements of waterborne sound and interface waves were acquired, and direct measurements of the compressional and shear wave properties were obtained at high frequencies. The propagation data consist of signals from a Combustive Sound Source recorded on bottom mounted geophones and a vertical line array of hydrophones. A statistical inference method was applied to obtain an estimate of the sediment compressional and shear wave speed profiles as a function of depth within the estuarine bed. The direct measurements were obtained in situ by inserting probes 30 cm into the sediment. Sediment acoustics models were fit to the high-frequency in situ data to enable comparison with the inferred low-frequency wave speeds. Overall, good agreement was found between the directly measured and inferred wave speeds for both the compressional and shear wave data. [ABSTRACT FROM AUTHOR]

The preceding paper in this series [Mantouka, Dogan, White, and Leighton, J. Acoust. Soc. Am. 140, 274-282 (2016)] presented a nonlinear model for acoustic propagation in gassy marine sediments, the baseline for which was established by Leighton [Geo. Res. Lett. 34, L17607 (2007)]. The current paper aims further advancement on those two studies by demonstrating the particular effects of the sediment rheology, the dispersion and dissipation of the first compressional wave, and the higher order re-scattering from other bubbles. Sediment rheology is included through the sediment porosity and the definition of the contact interfaces of bubbles with the solid grains and the pore water. The intrinsic attenuation and the dispersion of the compressional wave are incorporated using the effective fluid density model [Williams, J. Acoust. Soc. Am. 110, 2276-2281 (2001)] for the far field (fully water-saturated sediment). The multiple scattering from other bubbles is included using the method of Kargl [J. Acoust. Soc. Am. 11, 168-173 (2002)]. The overall nonlinear formulation is then reduced to the linear limit in order to compare with the linear theory of Anderson and Hampton [J. Acoust. Soc. Am. 67, 1890-1903 (1980)], and the results for the damping coefficients, the sound speed, and the attenuation are presented. [ABSTRACT FROM AUTHOR]

In order to characterize and map the seafloor sediments in the southern South China Sea, the physical and geoacoustic properties of the sediments were measured and analyzed at 21 stations. Sound speeds were measured using the coaxial differential distance measurement method on deck, and were corrected to in situ sound speeds using the correction-to-laboratory-velocity method. The results show that the distributions of sound speed ratio and acoustic impedance are closely related to the sediments physical properties. The study area can be divided into 3 geoacoustic provinces: the highest speed zone on the Sunda Shelf, and the intermediate and lowest speed zones on the continental slope and Nansha Trough. The correlations between the sound speed ratio and physical parameters fit linear relationships similarly or even better than the quadratic relationships derived in previous studies. This good fit may however be due to the limited range of data samples used in this study. Compared with the predicted sound speeds from Hamilton and Bachman's or Anderson's empirical equations, the measured values are smaller, which is probably due to different sedimentary environments and measurement methods. [ABSTRACT FROM AUTHOR]

Core sediment sampling has been conducted to investigate the physical and acoustic properties of inner shelf sediments in the South Sea, Korea. Thirty-two piston core samples were analyzed for sediment texture (grain size as well as, fractions of sand, silt, and clay), physical properties (porosity, water content, bulk density, grain density, and shear strength), and acoustic properties (compressional wave velocity and attenuation). The sediments in the study area consist mostly of homogenous mud. The study area can be divided into two different areas (south of Namhae Island and southeast of Naro Island) by texture, and physical and acoustic properties. The mean grain size generally decreases seaward from the mouth of Yeosu Sound and eastward in the southeast area of Naro Island. The compressional wave velocity decreases southeastward from 1500 to 1480 m/s and then increases to 1520 m/s due to relict sediments deposited in mid-shelf of the South Sea. Porosity, water content, and bulk density show an inverse pattern to velocity. The geoacoustic properties of the study area are generally similar to those of continental terrace sediment studied by Hamilton in the North Pacific. The area south of Namhae Island area matches Hamilton's model better than the area southeast of Naro Island. This result is believed to be related to the higher silt and sand contents as well as lower clay content than southeast of Naro Island. [ABSTRACT FROM AUTHOR]

The submarine in-situ environmental state was simulated to study the acoustic properties of seafloor sediments of the South China Sea in different temperature and pressure environments in this article with the application of the small-scale acoustic measurement system of seafloor sediments. Some conclusions could be drawn, including: (1) the velocity and center frequency of sound wave propagation of sediment are influenced by temperature more than by water pressure in different depth; (2) with the temperature rising, the sound velocity and the energy dissipation increase while the center frequency of sound wave propagation decreases; and (3) with the seawater pressure rising, the sound velocity varied little within 1% through a depth of 1500 m and the energy dissipation decreases large. So when correcting the laboratory measurement sound speed into the in-situ data, it should be noticed that the influence of temperature differing from the in-situ environment is significant and the influence of water pressure variation may be omitted. The variation of the acoustic attributes of the experimental sample showed greatly nonlinearity when the temperature changes, which might result from a lot of coarse sand and shell remains scattered in the measured sediment sample. Compared to the linear acoustic properties of reconstructed samples with an even structure it is maybe the reason for nonlinear variation of acoustic properties due to the uneven structure. Moreover, the correction method of Hamilton with the sound-speed ratio of sediment to bottom seawater should be used with caution due to the nonlinear variation character. [ABSTRACT FROM AUTHOR]

Several physics-based seabed geoacoustic models (including the Biot theory) predict that compressional wave attenuation α2 in sandy marine sediments approximately follows quadratic frequency dependence at low frequencies, i.e., α2≈kfn (dB/m), n=2. A recent paper on broadband geoacoustic inversions from low frequency (LF) field measurements, made at 20 locations around the world, has indicated that the frequency exponent of the effective sound attenuation n≈1.80 in a frequency band of 50-1000 Hz [Zhou et al., J. Acoust. Soc. Am. 125, 2847-2866 (2009)]. Carey and Pierce hypothesize that the discrepancy is due to the inversion models' neglect of shear wave effects [J. Acoust. Soc. Am. 124, EL271-EL277 (2008)]. The broadband geoacoustic inversions assume that the seabottom is an equivalent fluid and sound waves interact with the bottom at small grazing angles. The shear wave velocity and attenuation in the upper layer of ocean bottoms are estimated from the LF field-inverted effective bottom attenuations using a near-grazing bottom reflection expression for the equivalent fluid model, derived by Zhang and Tindle [J. Acoust. Soc. Am. 98, 3391-3396 (1995)]. The resultant shear wave velocity and attenuation are consistent with the SAX99 measurement at 25 Hz and 1000 Hz. The results are helpful for the analysis of shear wave effects on long-range sound propagation in shallow water. [ABSTRACT FROM AUTHOR]

The large velocity dispersion recently reported could be explained by a gap stiffness model incorporated into the Biot model (the BIMGS model) proposed by the author. However, at high frequencies, some measured results have been reported for negative velocity dispersion and attenuation proportional to the first to fourth power of frequency. In this study, first, it is shown that the results of velocity dispersion and attenuation calculated using the BIMGS model are consistent with the results measured in two kinds of water-saturated sands with different grain sizes, except in the high-frequency range. Then, the velocity dispersion and attenuation in six kinds of water-saturated glass beads and four kinds of water-saturated silica sands with different grain sizes are measured in the frequency ranges of 80-140 and 300-700 kHz. The measured results are compared with those calculated using the BIMGS model plus some acoustic models. It is shown that the velocity dispersion and attenuation are well predicted by using the BIMGS model in the range of kd ≤ 0.5 (k: wavenumber in water, d: grain diameter) and by using the BIMGS model plus multiple scattering effects in the range of kd ≥ 0.5 in which negative velocity dispersion appears. [ABSTRACT FROM AUTHOR]

The ability of the grain shearing (GS) and viscous grain shearing (VGS) models to relate geophysical and acoustic properties is tested by a method based on the claimed tight coupling between compressional and shear wave speeds and attenuations, which allows the test result to be quantified in a single parameter. The VGS model is claimed to provide a better fit to the measured sound speed and attenuation in sandy sediments below 10 kHz. In situ measurements of wave speeds and attenuations from the Sediment Acoustics Experiment 1999 (SAX99) and published laboratory measurements by Prasad and Meissner [Geophysics 57, 710–719 (1992)] on a number of sand samples were used to test the models. By this metric, the SAX99 data show that the VGS model is no better than the original GS model because the improved agreement of compressional wave speeds at low frequencies is achieved at the expense of gross overestimation of the shear wave attenuation. When applied to the measurements by Prasad and Meissner, it is shown that the GS models are not applicable at any significant confining pressures, and at zero pressure they may only be applicable to a small subset of the sand samples. [ABSTRACT FROM AUTHOR]

Abstract: Characterization of natural gas hydrate bearing formations is important in the exploration and development of gas hydrate resources in subsea sediments. Solid hydrates can fill the voids of the matrix formed by sand grains and change their cementation condition, which may have a great impact on the electrical resistance and sound velocity of the sand matrix. In this study, experiments have been conducted to measure the ultrasonic velocity and electrical resistance in a large sandpack simulating the conditions of hydrate formation in subsea sediments. The effects of hydrate on the resistivity and ultrasonic velocity of hydrate bearing sand matrix have been revealed and modeled. The data can be used in well logging to determine hydrate saturation and other properties of hydrate bearing formations. [Copyright &y& Elsevier]

As with traditional sonar, synthetic aperture sonar (SAS) is susceptible to multipath contamination, reducing the quality and also modifying the statistics of the image. Such multipath contaminants may either be environmentally induced, as is often the case when attempting to image ranges greater than the water depth resulting in returns from the boundaries, or may be induced by the system's supporting structure itself. A clear understanding of such statistical impact is necessary to advance synthetic aperture formation algorithms and for predicting system performance. Broadband acoustic data suitable for SAS processing collected with a rail-mounted mobile-tower as part of the U.S. Office of Naval Research (ONR)-funded Sediment Acoustics eXperiment 2004 (SAX04) are analyzed in this paper. Analysis focused on both system structure and environmentally induced multipath using the K-distribution shape parameter as a metric. High-resolution sonar imagery often exhibited significantly non-Rayleigh, heavy-tailed envelope statistics, characterized by a low equivalent K-distribution shape parameter. Analysis showed a clear and significant increase in the estimated shape parameter in the presence of multipath, representing a trend toward a Rayleigh-distributed envelope. A model for reverberation is presented to provide bounds of the statistical impact using observable image intensity level increases in synthetic-aperture-formed images caused by multipath contamination. This model further shows potential for statistical impact when multipath arrivals are of similar level as the direct path even when not observable in the image (e.g., within 10 dB). [ABSTRACT FROM AUTHOR]

High-resolution seismic survey and sediment core sampling were conducted to investigate acoustic characteristics of gas-bearing sediments in Jinhae Bay, the southeast of Korea. The sediment in Jinhae Bay is mostly homogenous mud deposited after the Holocene transgression. Along with the 410 km of chirp seismic profiling, five piston core samples were collected on the track lines. Gassy sediments are common and occur widely in the bay. Core samples were analyzed for sediment texture, physical properties (porosity, water content, bulk density, and grain density), acoustic properties (compressional wave velocity and attenuation), and electrical resistivity. X-radiograph image analysis was also performed to observe the shape of degassing cracks. There is no significant downcore variation on physical and sediment textures regardless of existence of gas bubbles. However, compressional wave velocity dramatically decreases from average 1480 to 1380∼739 m/s for the cores that penetrate the gas-bearing zones. This is probably due to degassying cracks that developed by escaping gases and free gas bubbles that are still trapped in the cores. Electrical resistivity is the only geotechnical property that increases in the gas-bearing zone where compressional wave velocity abruptly decreases. This indicates the possibility of using both electrical resistivity as an index variable as well as to compressional wave velocity to identify gassy sediment microstructure because there are little changes in texture and composition of sediment. [ABSTRACT FROM AUTHOR]

This paper applies Bayesian inference, including model selection and posterior parameter inference, to inversion of seabed reflection data to resolve sediment structure at a spatial scale below the pulse length of the acoustic source. A practical approach to model selection is used, employing the Bayesian information criterion to decide on the number of sediment layers needed to sufficiently fit the data while satisfying parsimony to avoid overparametrization. Posterior parameter inference is carried out using an efficient Metropolis-Hastings algorithm for high-dimensional models, and results are presented as marginal-probability depth distributions for sound velocity, density, and attenuation. The approach is applied to plane-wave reflection-coefficient inversion of single-bounce data collected on the Malta Plateau, Mediterranean Sea, which indicate complex fine structure close to the water-sediment interface. This fine structure is resolved in the geoacoustic inversion results in terms of four layers within the upper meter of sediments. The inversion results are in good agreement with parameter estimates from a gravity core taken at the experiment site. [ABSTRACT FROM AUTHOR]

Acouso-physical properties of sea floor sediments in the southeast offshore sea area of Hainan Island on the northern continental shelf of the South China Sea are analyzed. In many cruises, conductivity-temperature-depth measurements of seawater, measurements of shallow stratum and side-scan sonar have been made. Acoustic parameters, basic sedimentary parameters, physical-mechanical parameters and 14C age, etc., have been measured. The sediment elastic parameters, including Young's modulus, bulk modulus, constrained modulus, rigidity modulus, Poisson's ratio, Lames constant, etc., have been calculated. Results show that the compression wave velocity of the seafloor sediment in the sea area ranges from 1474-1700 m/s, and there are high and low sound velocity sediment types in the different sea areas; the shear wave velocity is 150-600 m/s; at 100 kHz the sediment sound attenuation is 35-260 dB/m, the sediment density is 1.4-2.0 g/cm3; the sediment porosity is 42-88%. Sound field parameters and describing sound reciprocity between sea and seafloor are described. [ABSTRACT FROM AUTHOR]

Describes an acoustic tomographic system capable of performing in situ two-dimensional acoustic imaging of shallow water sediments. Description of the developed fine-scale acoustic tomography system; Performance of the system; Analysis of the laboratory and field experiment data.

Several persistent sonar clutter features were observed in 2002 during an experiment in the Straits of Sicily (Malta Plateau) in a region that is nominally flat and thickly sedimented. High frequency sidescan and seismic reflection data from the region indicated that mud volcanoes were present and were possibly expelling clouds of methane bubbles. In 2004 the region was revisited with oceanographic equipment including a multibeam sonar, an acoustic Doppler current profiler (ADCP), CTD’s, temperature loggers, a methane sensor, and a video camera. The visual observations at one site show carbonate mounds comprised of large heterogeneous blocks that suggest the presence of mud volcanism. A multibeam sonar survey of the area shows that the carbonate mounds are of order 5–10 m in height and 10–100 m in lateral dimension, and that they form in clusters. The multibeam backscatter data show high amplitudes in the region surrounding the carbonate mounds, possibly indicating the surficial extent of the carbonate material. High levels of methane were observed in the water column above the mounds, although conclusive evidence of the presence of methane bubble clouds was not found. However, high frequency backscatter recorded on the ADCP above multiple carbonate mounds showed plume-like features that may be bubble clouds, suspended fine-grained sediment, and/or schools of fish. [ABSTRACT FROM AUTHOR]

For sediment interfaces that are very rough on the scale of the acoustic wavelength (i.e., kh greater than 1 where k is 2*π/wavelength and h is the rms roughness of the water/sediment interface) it is possible to estimate what the reflection coefficient would be if the interface were flat. In order to do so, a large ensemble of forward scattering measurements are needed in order to reduce the statistical uncertainty of the estimated reflection coefficient. In addition to the statistical uncertainty there can be biases in the estimate (for some grazing angles) that must be taken into account. The above conclusions will be supported through the use of monte carlo simulations of scattering from a rippled interface. The simulations are carried out in the context of discriminating between alternative acoustic models of sand sediments. [Work supported by ONR.] [ABSTRACT FROM AUTHOR]

The article presents an erratum to the article “Low-frequency attenuation of acoustic waves in sandy/silty marine sediments,” by Allan D. Pierce and William M. Carey, which was published in the volume 124, 2008 issue.

The reflection coefficient from a sand/water interface is an important parameter in modeling the acoustics of littoral environments. Many models have been advanced to describe the influence of the sediment parameters and interface roughness parameters on the reflection coefficient. In this study, the magnitude and phase of the reflection coefficient from 30 to 160 kHz is measured in a bistatic experiment on a smoothed water/sand interface at grazing angles from 5 to 75 degrees. The measured complex reflection coefficient is compared with the fluid model, the elastic model and poro-elastic models. Effects of rough surface scattering are investigated using the Bottom Response from Inhomogeneities and Surface using Small Slope Approximation (BoRIS-SSA). Spherical wave effects are modeled using plane wave decomposition. Models are considered for their ability to predict the measured results using realistic parameters. [Work supported by ONR, Ocean Acoustics.] [ABSTRACT FROM AUTHOR]

This paper presents a nonlinear Bayesian inversion of high-resolution seabed reflection data to estimate visco-elastic parameters of the upper sediments. The inversion is applied to data from two sites in the Strait of Sicily. One site is characterized by low-velocity, silty-clay sediments, resulting in data with a well-defined angle of intromission. The second site is characterized by high-velocity clayey sand, resulting in a critical angle. The data were frequency averaged from 500–2000 Hz and inverted for visco-elastic parameters of a half-space seabed model. The likelihood function employed in the inversion is based on the assumption of independent, Gaussian-distributed data errors, with the standard deviation included as a nuisance parameter in the inversion. Statistical tests are applied to the data residuals a posteriori to validate these assumptions. Good results are obtained for sediment compressional-wave velocity, compressional attenuation, and density; shear parameters are less-well determined although low shear-wave velocities are indicated. The Bayesian analysis provides a quantitative comparison of inversion results for the two sites in terms of the resolution of specific geoacoustic parameters, and indicates that the geoacoustic information content is significantly higher for angle-of-intromission data. [ABSTRACT FROM AUTHOR]

This paper develops a non-linear Bayesian inversion for multi-frequency reflection-loss data with strongly correlated data errors to resolve density and sound-velocity gradients which are often observed in the uppermost sediment layer. Although data errors are usually assumed to be independent in geoacoustic inversion, in reality measured data often show strong error correlations. The inversion developed here is designed to take error correlations into account. A full data covariance matrix is estimated from initial residuals of non-uniformly sampled data. This covariance matrix is then used in the likelihood function of a fast Gibbs sampler to sample the posterior probability density and provide parameter estimates and credibility intervals. Rigorous statistical tests are applied to the resulting data residuals to illustrate the benefits of this error treatment. The approach is applied to reflectivity data collected at a site characterized by low-velocity, water-saturated sediments in the Strait of Sicily. Density and sound-speed gradients are clearly resolved by the reflectivity data and agree with core measurements from the experiment site within the credibility bounds. [ABSTRACT FROM AUTHOR]