Characterization of interface roughness of rippled sand off Fort Walton Beach, Florida

As part of the environmental characterization to model acoustic bottom scattering during the high-frequency sediment acoustics experiment (SAX99), fine-scale sediment roughness of a medium sand was successfully measured within a 600 /spl times/ 600-m area by two methods: stereo photography and a technique using a conductivity system. Areal coverage of the two methods, representing approximately 0.16 m/sup 2/ of the sea floor, was comparable, resulting in the depiction and quantification of half-meter wavelength sand ripples. Photogrammetric results were restricted to profiles digitized at 1-mm intervals; sediment conductivity results generated gridded micro-bathymetric measurements with 1- to 2-cm node spacing. Roughness power spectra give similar results in the low-spatial-frequency domains where the spectra estimated from both approaches overlap. However, spectra derived from higher resolution photogrammetric results appear to exhibit a multiple-power-law fit. Roughness measurements also indicate that spectrum changes as a function of time. Application of statistical confidence bounds on the power spectra indicates that roughness measurements separated by only 1-2 m may be spatially nonstationary.     

Relationships between the sound speed ratio and physical properties of surface sediments in the South Yellow Sea

Building empirical equations is an effective way to link the acoustic and physical properties of sediments. These equations play an important role in the prediction of sediments sound speeds required in underwater acoustics.Although many empirical equations coupling acoustic and physical properties have been developed over the past few decades, further confirmation of their applicability by obtaining large amounts of data, especially for equations based on in situ acoustic measurement techniques, is required. A sediment acoustic survey in the South Yellow Sea from 2009 to 2010 revealed statistical relationships between the in situ sound speed and sediment physical properties. To improve the comparability of these relationships with existing empirical equations, the present study calculated the ratio of the in situ sediment sound speed to the bottom seawater sound speed, and established the relationships between the sound speed ratio and the mean grain size, density and porosity of the sediment. The sound speed of seawater at in situ measurement stations was calculated using a perennially averaged seawater sound speed map by an interpolation method. Moreover, empirical relations between the index of impedance and the sound speed and the physical properties were established. The results confirmed that the existing empirical equations between the in situ sound speed ratio and the density and porosity have general suitability for application. This study also considered that a multiple-parameter equation coupling the sound speed ratio to both the porosity and the mean grain size may be more useful for predicting the sound speed than an equation coupling the sound speed ratio to the mean grain size.     

Acoustic backscattering experiments in a well characterized sand sediment: data/model comparisons using sediment fluid and Biot models

As part of the sediment acoustics experiment 1999 (SAX99), backscattering from a sand sediment was measured in the 20- to 300-kHz range for incident grazing angles from 10/spl deg/ to 40/spl deg/. Measured backscattering strengths are compared to three different scattering models: a fluid model that uses the mass density of the sediment in determining backscattering, a poroelastic model based on Biot theory and an "effective density" fluid model derived from Biot theory. These comparisons rely heavily on the extensive environmental characterization carried out during SAX99. This environmental characterization is most complete at spatial scales relevant to acoustic frequencies from 20 to 50 kHz. Model/data comparisons lead to the conclusions that rough surface scattering is the dominant scattering mechanism in the 20-50-kHz frequency range and that the Biot and effective density fluid models are more accurate than the fluid model in predicting the measured scattering strengths. For 50-150 kHz, rough surface scattering strengths predicted by the Biot and effective density fluid models agree well with the data for grazing angles below the critical angle of the sediment (about 30/spl deg/) but above the critical angle the trends of the models and the data differ. At 300 kHz, data/model comparisons indicate that the dominant scattering mechanism may no longer be rough surface scattering.     

Mid- to High-Frequency Acoustic Penetration and Propagation Measurements in a Sandy Sediment

During the recent 2004 Sediment Acoustics Experiment (SAX04), a buried hydrophone array was deployed in a sandy sediment near Fort Walton Beach, FL. The array was used to measure both the acoustic penetration into the sediment and sound speed and attenuation within the sediment while a smaller, diver-deployed array was also used to measure sound speed and attenuation. Both of these systems had been deployed previously during the 1999 Sediment Acoustics Experiment (SAX99). In that experiment, the buried array was used to make measurements in the 11–50-kHz range while the diver-deployed array made measurements in the 80–260-kHz range. For the SAX04 deployment, the frequency range for the measurements using the buried array was lowered to 2 kHz. The diver-deployed array was also modified to cover the 40–260-kHz range. Unlike the SAX99 deployment, there were no obvious sand ripples at the SAX04 buried array site at the time of the measurements. To examine the role of sand ripples in acoustic penetration over this new frequency range, artificial ripple fields were created. For the high frequencies, the penetration was consistent with the model predictions using small-roughness perturbation theory as in SAX99. As the frequency of the incident acoustic field decreased, the evanescent field became the dominant penetration mechanism. The sound speed measured using the buried array exhibits dispersion consistent with the Biot theory while the measured attenuation exceeds the theory predictions at frequencies above 200 kHz.      

Comparison of sound speed and attenuation measured in a sandy sediment to predictions based on the Biot theory of porous media

During the sediment acoustics experiment in 1999 (SAX99), several researchers measured sound speed and attenuation. Together, the measurements span the frequency range of about 125 Hz-400 kHz. The data are unique both for the frequency range spanned at a common location, and for the extensive environmental characterization that was carried out as part of SAX99. Environmental measurements were sufficient to determine or bound the values of almost all the sediment and pore water physical property input parameters of the Biot poroelastic model for sediment. However, the measurement uncertainties for some of the parameters result in significant uncertainties for Biot-model predictions. Here, measured sound-speed and attenuation results are compared to the frequency dependence predicted by Biot theory and a simpler "effective density" fluid model derived from Biot theory. Model/data comparisons are shown where the uncertainty in Biot predictions due to the measurement uncertainties for values of each input parameter are quantified. A final set of parameter values, for use in other modeling applications e.g., in modeling backscattering (Williams et al., 2002) are given, that optimize the fit of the Biot and effective density fluid models to the sound-speed dispersion and attenuation measured during SAX99. The results indicate that the variation of sound speed with frequency is fairly well modeled by Biot theory but the variation of attenuation with frequency deviates from Biot theory predictions for homogeneous sediment as frequency increases. This deviation may be due to scattering from volume heterogeneity. Another possibility for this deviation is shearing at grain contacts hypothesized by Buckingham; comparisons are also made with this model.     

Evaluation of Laser Scanning and Stereo Photography Roughness Measurement Systems Using a Realistic Model Seabed Surface

The topography of the seabed is influenced by sediment transport due to wave motion, current disturbance, and biological activities. The bottom roughness generated by these processes can substantially alter acoustic wave penetration into and scattering from the bottom, and therefore, it is essential to make accurate measurements of the bottom roughness for such acoustic applications. Methods to make direct measurements of bottom roughness include stereo photography, laser line scanning, and sediment conductivity. Roughness can also be measured indirectly using high-frequency sound backscatter. For optically-based methods, the accuracy of these measurements is typically evaluated using the elevations, lengths, or diameters of simple surface features of known dimensions. However, for acoustic applications, the statistical characteristics of the surface, e.g., the roughness spectrum, are more meaningful. In this paper, we present a fabricated rough surface milled into a 40 $,times ,$60 cm $^{2}$ plastic block for use as a benchmark in the assessment of two in situ roughness measurement systems: a laser scanning system and a digital stereo photography system. The surface has a realistic roughness power spectrum that is derived from the bottom roughness measured during the 1999 Sediment Acoustics Experiment (SAX99) and was fabricated by a computer numerical controlled milling machine. By comparing the fabricated surface spectrum to the measured spectrum, a determination of the accuracy of the roughness measurement is evaluated, which is of direct relevance to acoustic applications.      

Characterization of shallow ocean sediments using the airborne electromagnetic method

Experimental airborne electromagnetic (AEM) survey data collected in Cape Cod Bay are used to derive continuous profiles of water depth, electrical depth, water conductivity, and bottom sediment conductivity. Through a few well-known empirical relationships, the conductivities are used, in turn, to derive density, porosity, sound speed, and acoustic reflectivity of the ocean bottom. A commercially available Dighem III AEM system was used for the survey without any significant modification. The helicopter-borne system operated at 385 and 7200 Hz; both were in a horizontal coplanar configuration. The interpreted profiles show good agreement with available ground truth data. Where no such data are available, the results appear to be very reasonable. Compared with the shipborne electrode array method, the AEM method can determine the necessary parameters at a much higher speed with a better lateral resolution over a wide range of water depths from 0 to perhaps 100 m. The bottom sediment conductivity that can be measured by the AEM method is closely related to physical properties of sediments, such as porosity, density, sound speed, and, indirectly, sediment types that might carry broad implications for various offshore activities.     

High-frequency subcritical acoustic penetration into a sandy sediment

During the sediment acoustics experiment, SAX99, a hydrophone array was deployed in sandy sediment near Fort Walton Beach, Florida, in a water depth of 18 m. Acoustic methods were used to determine array element positions with an accuracy of about 0.5 cm, permitting coherent beamforming at frequencies in the range 11-50 kHz. Comparing data and simulations, it has been concluded that the primary cause of subcritical acoustic penetration was diffraction by sand ripples that were dominant at this site. These ripples had a wavelength of approximately 50 cm and RMS relief of about 1 cm. The level and angular dependence of the sound field in the sediment agree within experimental uncertainties with predictions made using small-roughness perturbation theory.     

Forward Scattering From a Rippled Sand/Water Interface: Modeling, Measurements, and Determination of the Plane Wave, Flat Surface Reflection Coefficient

In this paper, modeling results are presented demonstrating that, using an ensemble of forward-scattering measurements from a rippled sand/water interface, it is possible to accurately estimate the plane wave, flat surface reflection coefficient. The modeling effort was carried out in preparation for a sediment acoustics experiment in 2004 (SAX04). Guided by the modeling results, forward-scattering measurements were made during SAX04. The measurement instrumentation and procedure are presented. The plane wave reflection coefficients derived from these measurements are given and compared to reflection coefficients calculated using a fluid model and an approximation to the Biot porous medium model for the sand known as the effective density fluid model (EDFM). The model reflection coefficients were calculated using acoustic parameters determined from environmental measurements carried out by other researchers involved in SAX04. The reflection coefficient data/model comparison indicates that the sand at the SAX04 site is most accurately viewed as a porous medium for acoustic modeling purposes.      

南沙海域深水区表层沉积物声速与孔隙度相关关系

采用同轴差距测量法对南沙海域海底沉积物取样进行甲板现场声学测量,并计算了声速,结合沉积物取样分析获得的孔隙度,研究了海底表层100 cm沉积物的声速和孔隙度之间的相互关系,分析获得了孔隙度与海底沉积物声速关系的经验公式,结果表明此海域沉积物声速的临界孔隙度为67.80%。将此公式与国内外其他学者建立的经验公式进行了对比,分析发现各预测方程之间存在着差异,说明不同海域的沉积物声学特性具有差异性,经验公式具有区域性,同时分析了其他影响声速的因素。此项工作对于建立和研究南海声场环境具有重要意义。     

Variation of temperature-dependent sound velocity in unconsolidated marine sediments: Laboratory measurements

Laboratory measurements of sound velocity in unconsolidated marine sediment were performed to establish specific correction curves between temperature and sound velocity. Cores from the Hupo Basin and the southern sea of Geumo Island were cooled and sound velocity was measured while gradually increasing temperature (from 3 to 30°C). Sediment textural and physical properties (porosity, water content, and bulk density) were measured at the same depth. Sound velocity increases with temperature for clay, mud, silt, and sand sediment, resulting in values of approximately 2.65, 2.72, 2.78, and 3.10?m/s/°C, respectively. These results are similar to those of previous studies, and differences are likely due to density, porosity, and clay contents of the sediment. Using these results, we present correction curves for sound velocity temperature dependence for each sediment texture, which can be used to correct from laboratory to in situ values to develop accurate geoacoustic model.     

A method for estimating the physical and acoustic properties of the sea bed using chirp sonar data

This paper proposes a method, based on the Biot model, for estimating the physical and acoustic properties of surficial ocean sediments from normal incidence reflection data acquired by a chirp sonar. The inversion method estimates sediment porosity from reflection coefficient measurements and, using the estimated porosity and the measured change in fast wave attenuation with frequency, estimates the permeability of the top sediment layer. The spectral ratio of echoes from the interface at the base of the upper sediment layer and from the sediment-water interface provides a measure of the change in attenuation with frequency. Given the porosity and permeability estimates, the Kozeny-Carman equation provides the mean grain size and the inversion method yields the acoustic properties of top sediment layer. The inversion technique is tested using chirp sonar data collected at the 1999 Sediment Acoustics Experiment (SAX-99) site. Remote estimates of porosity, grain size, and permeability agree with direct measurements of those properties.     

Tests of models for high-frequency seafloor backscatter

The interaction of high-frequency sound with the seafloor is inherently a stochastic process. Inversion techniques must, therefore employ good stochastic models for bottom acoustic scattering. An assortment of physical models for bottom backscattering strength is tested by comparison with scattering strength data obtained at 40 kHz at three shallow water sites spanning a range of sediment types from fine silt to coarse sand. These acoustic data are accompanied by sediment physical property data obtained by core sample analysis and in situ probes. In addition, stereo photography was used to measure the power spectrum of bottom relief on centimeter scales. These physical data provided the inputs needed to test the backscatter models, which treat scattering from both the rough sediment-water interface and the sediment volume. For the three sites considered here, the perturbation model for scattering from a slightly rough fluid seafloor performs well. Volume scattering is predicted to be weak except at a site having a layer of methane bubbles     

Seafloor Roughness Measured by a Laser Line Scanner and a Conductivity Probe

To support modeling acoustic backscatter from the seafloor, a conductivity probe and a laser line scanner were deployed jointly to measure bottom roughness during an experiment off the New Jersey coast in summer 2006. The conductivity probe in situ measurement of porosity (IMP2) is impervious to water turbidity and yields a 1-D profile with 10-mm horizontal spacing and 1-mm resolution in the vertical direction. The laser line scanner is limited by water visibility but it provides 2-D grid points with resolutions 0.3 mm across track, 0.5 mm along track, and 0.3 mm in the vertical direction. Two sets of data, suitable to model mid- to high-frequency acoustic backscatter, were collected from two sites 900 m apart on August 14 and 17, 2006. The roughness spectra obtained from the laser scanning were compared to those measured by the IMP2. The spectra from the two methods are consistent over wave number range 0.0188–3 rad/cm, which are the wave number range common to both methods. The efficacy of the laser scanner is also confirmed by showing the spectral line created by the IMP2's periodic probing marks. The 2-D spectra generated from the laser scan data show that the bottom roughness at these sites is azimuthally isotropic, but significant spatial heterogeneity is observed.      

Acoustic density measurements of consolidating cohesive sediment beds by means of a non-intrusive “Micro-Chirp” acoustic system

A non-intrusive “Micro-Chirp” acoustic system and a signal-processing protocol have been developed to estimate the bulk density of consolidating cohesive sediment beds. Using high-frequency (300–700 kHz) Chirp acoustic waves, laboratory measurements were conducted with clay–water mixtures. Because acoustic echo strength is proportional to variations in acoustic impedance, and the speed of sound in the clay bed hardly changed during consolidation, the bulk density could be successfully estimated without disturbing the sediment bed. Based on acoustic signal analysis, this study demonstrates that the reflection coefficient and bulk density at the water–sediment interface increase with consolidation time, and that a single speed of sound value can be used for practical bulk density estimation in muddy environments.     

海底沉积物的基本地声结构与地声模型

为准确建立海底地声模型,本文探讨地声模型的基本组成和基本结构。通过样品实验室测量,分析南海海底表层沉积物的密度、孔隙度与声速随着埋深变化的关系,得出海底实际存在的低声速表面–声速缓慢变化类型、低声速表面–声速增大类型、高声速表面–声速缓慢变化类型和高声速表面–声速增大类型4种典型地声结构;对比钻探测量,分析黄海海底沉积物的密度、孔隙度与声速随埋深变化关系,得出海底地声模型分层特征与地声结构组合特征。研究表明,地声模型可以归结为4种基本地声结构的组合,通过与底层海水声速、同层内声速剖面以及与上层海底沉积物下表面声速的比较,可以建立各种海底地声模型;基于实验室测量法建立的地声模型可以作为参考地声模型,但需要考虑实际海底温度和压力梯度以及海底沉积物的频散特性等,借助于声速比校正法和频散性理论模型进行计算及修正。     

Sound velocity and related properties of seafloor sediments in the Bering Sea and Chukchi Sea

The Bering Sea shelf and Chukchi Sea shelf are believed to hold enormous oil and gas reserves which have attracted a lot of geophysical surveys. For the interpretation of acoustic geophysical survey results, sediment sound velocity is one of the main parameters. On seven sediment cores collected from the Bering Sea and Chukchi Sea during the 5th Chinese National Arctic Research Expedition, sound velocity measurements were made at 35, 50, 100, 135, 150, 174, 200, and 250 k Hz using eight separate pairs of ultrasonic transducers. The measured sound velocities range from 1 425.1 m/s to 1 606.4 m/s and are dispersive with the degrees of dispersion from 2.2% to 4.0% over a frequency range of 35–250 k Hz. After the sound velocity measurements, the measurements of selected geotechnical properties and the Scanning Electron Microscopic observation of microstructure were also made on the sediment cores. The results show that the seafloor sediments are composed of silty sand, sandy silt, coarse silt, clayey silt, sand-silt-clay and silty clay. Aggregate and diatom debris is found in the seafloor sediments. Through comparative analysis of microphotographs and geotechnical properties, it is assumed that the large pore spaces between aggregates and the intraparticulate porosity of diatom debris increase the porosity of the seafloor sediments, and affect other geotechnical properties. The correlation analysis of sound velocity and geotechnical properties shows that the correlation of sound velocity with porosity and wet bulk density is extreme significant, while the correlation of sound velocity with clay content, mean grain size and organic content is not significant. The regression equations between porosity, wet bulk density and sound velocity based on best-fit polynomial are given.     

声波在水-多孔介质海底界面上的反射与透射

声波在海底界面的反射和透射是海底散射、海底混响、海底目标探测的重要问题。利用Biot多孔介质声传播理论对声波在水-多孔海底界面上的反射和透射进行了分析,具体给出反射声波的反射系数,3种透射声波的透射系数以及声能透射系数随入射波入射角和频率(10~40 kHz)的变化关系,分析了各种透射波对透射声能的贡献。多孔海底介质参数分别采用Stoll和Chotiros给出的2组参数进行理论计算。     

Correlations between sound velocity and related properties of glacio-marine sediments: Barents sea

Statistical analysis of laboratory-measured compressional wave (sound) velocity, porosity, wet bulk density, and selected textural parameters of surface sediments from the Barents Sea reveals that clay content and mean grain size are the best indices to sound velocity. These parameters are followed closely by porosity and wet bulk density, while sand content provides the weakest index. Although Barents Sea surface sediments are characterized by fairly high variability, the results of the present study are in general agreement with studies of similar sediment types in other continental shelf environments.     

Porometric properties of siliciclastic marine sand: a comparison of traditional laboratory measurements with image analysis and effective medium modeling

During the 1999 sediment acoustics experiment (SAX99), porometric properties were measured and predicted for a well sorted, medium sand using standard laboratory geotechnical methods and image analysis of resin-impregnated sediments. Sediment porosity measured by laboratory water-weight-loss methods (0.372 /spl plusmn/ 0.0073 for mean /spl plusmn/1 standard deviation) is 0.026 lower than determined by microscopic image analysis of resin-impregnated sediments (0.398 /spl plusmn/ 0.029). Values of intrinsic permeability (m/sup 2/) determined from constant-head permeameter measurements (3.29 /spl times/ 10/sup -11/ /spl plusmn/ 0.60 /spl times/ 10/sup -11/) and by microscopic image analysis coupled with effective medium theory modeling (2.78 /spl times/ 10/sup -11/ /spl plusmn/ 1.01 /spl times/ 10/sup -11/) are nearly identical within measurement error. The mean value of tortuosity factor measured from images is 1.49 /spl plusmn/ 0.09, which is in agreement with tortuosity factor determined from electrical resistivity measurements. Slight heterogeneity and anisotropy are apparent in the top three centimeters of sediment as determined by image-based porometric property measurements. However, the overall similarity for both measured and predicted values of porosity and permeability among and within SAX99 sites indicates sediments are primarily homogeneous and isotropic and pore size distributions are fairly uniform. The results indicate that an effective medium theory technique and two-dimensional image analysis accurately predicts bulk permeability in resin-impregnated sands.