In situ permeabilities of selected coastal marine sediments

An in situ permeameter probe was deployed off Fort Walton Beach, Florida in shallow-water coastal sandy sediment. Stations were occupied in a 600 m /spl times/ 600 m area. Intrinsic permeability in sand at 17 stations varied from 0.3 to 6.1 /spl times/ 10/sup -11/ m/sup 2/ to subbottom depths of 50 cm. Permeability decreased with increasing subbottom depth and minimal compaction. The sediment is a well-sorted, medium quartz sand with a mean grain size of approximately 0.34-0.52 mm and with 5%-8% carbonate shells and shell fragments. The probe was tested in a slightly finer-grained sediment near the main study area and revealed permeabilities of 0.1-3.2 /spl times/ 10/sup -11/ m/sup 2/. The permeabilities measured are reasonable for the observed sandy and slightly finer-grained sediments.     

Dynamic measurement of sediment grain compressibility at atmospheric pressure: acoustic applications

Under certain conditions, Wood's equation can be used to predict sound speed in fluid/solid-grain suspensions if the bulk moduli and densities of the grains and fluid are known. In this paper, that relationship is used to estimate grain-bulk moduli in suspensions where sound speed, fluid density, fluid bulk modulus, grain density, and particle concentrations are known or accurately measured. Measured values of grain-bulk moduli for polystyrene beads suspended in water (mean = 4.15 /spl times/ 10/sup 9/ Pa) and soda-lime glass beads suspended in a "heavy liquid" (mean = 3.8 /spl times/ 10/sup 10/ Pa) are consistent with the values of bulk moduli for polystyrene beads and soda-lime glass beads found in the literature (3.6 to 4.2 /spl times/ 10/sup 9/ Pa and 3.4 to 4.0 /spl times/ 10/sup 10/ Pa, respectively). These measurements thus provide controls, which demonstrate the validity of the suspension technique to estimate values of particle bulk modulus. The values of bulk modulus, measured using the same suspension techniques, for Ottawa sand and quartz sand grains collected from the coastal sediments of the northeast Gulf of Mexico ranged between 3.8 and 4.7 /spl times/ 10/sup 10/ Pa, with 95% confidence limits between 3.0-5.7 /spl times/ 10/sup 10/ Pa. These measured values of bulk modulus are consistent with the range of handbook values for polycrystalline quartz (3.6 to 4.0 /spl times/ 10/sup 10/ Pa). The use of the lower bulk modulus (i.e., 7.0 /spl times/ 10/sup 9/ Pa) recently suggested by Chotiros is therefore inappropriate and traditional handbook values of sediment grain-bulk moduli should be used as inputs for sediment acoustic modeling.     

Absolute positioning of an autonomous underwater vehicle using GPS and acoustic measurements

Kinematic global positioning system (GPS) positioning and underwater acoustic ranging can combine to locate an autonomous underwater vehicle (AUV) with an accuracy of /spl plusmn/30cm (2-/spl sigma/) in the global International Terrestrial Reference Frame 2000 (ITRF2000). An array of three precision transponders, separated by approximately 700 m, was established on the seafloor in 300-m-deep waters off San Diego. Each transponder's horizontal position was determined with an accuracy of /spl plusmn/8 cm (2-/spl sigma/) by measuring two-way travel times with microsecond resolution between transponders and a shipboard transducer, positioned to /spl plusmn/10 cm (2-/spl sigma/) in ITRF2000 coordinates with GPS, as the ship circled each seafloor unit. Travel times measured from AUV to ship and from AUV to transponders to ship were differenced and combined with AUV depth from a pressure gauge to estimate ITRF2000 positions of the AUV to /spl plusmn/1 m (2-/spl sigma/). Simulations show that /spl plusmn/30 cm (2-/spl sigma/) absolute positioning of the AUV can be realized by replacing the time-difference approach with directly measured two-way travel times between AUV and seafloor transponders. Submeter absolute positioning of underwater vehicles in water depths up to several thousand meters is practical. The limiting factor is knowledge of near-surface sound speed which degrades the precision to which transponders can be located in the ITRF2000 frame.     

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.     

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.     

Characterization of North Atlantic right-whale (Eubalaena glacialis) sounds in the Bay of Fundy

Hydrophone recordings, made in the presence of North Atlantic right whales (Eubalaena glacialis) in the Bay of Fundy during the daytime in July 1999, are used to determine the characteristics of the recorded sounds. A spectrogram-based method was implemented to discriminate whale sounds from background noise and the time-frequency envelope of the primary harmonic in the spectrogram was used as the basis for sound characterization. Sounds were typically (82%, n=45) in the 300- to 600-Hz range with up- and downsweeping modulations. Lower (<200 Hz) and higher (>900 Hz) frequency sounds were relatively rare. Most sounds were frequency modulated, with 95% of the observed instantaneous relative frequency variation being within /spl plusmn/4.5% of the mean peak frequency. Harmonics were observed in 18% of the sounds. The average sound duration was 0.42 s/spl plusmn/0.26 SD. The sounds occurred at a rate of between 0.3 and 0.7 min/sup -1/. The time intervals between adjacent sounds (2-700 s) were not randomly distributed. The number of sounds occurring among different waiting times did not reflect a Poisson distribution and a clustering of sounds (2 to 5 cluster/sup -1/) was observed. The sound characteristics are compared to those documented elsewhere and as reported for the southern right whale.     

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.      

Fine-scale volume heterogeneity measurements in sand

As part of the effort to characterize the acoustic environment during the high frequency sediment acoustics experiment (SAX99), fine-scale variability of sediment density was measured by an in situ technique and by core analysis. The in situ measurement was accomplished by a newly developed instrument that measures sediment conductivity. The conductivity measurements were conducted on a three-dimensional (3-D) grid, hence providing a set of data suited for assessing sediment spatial variability. A 3-D sediment porosity matrix is obtained from the conductivity data through an empirical relationship (Archie's Law). From the porosity matrix, sediment bulk density is estimated from known average grain density. A number of cores were taken at the SAX99 site, and density variations were measured using laboratory techniques. The power spectra were estimated from both techniques and were found to be appropriately fit by a power-law. The exponents of the horizontal one-dimensional (1-D) power-law spectra have a depth-dependence and range from 1.72 to 2.41. The vertical 1-D spectra have the same form, but with an exponent of 2.2. It was found that most of the density variability is within the top 5 mm of the sediment, which suggests that sediment volume variability will not have major impact on acoustic scattering when the sound frequency is below 100 kHz. At higher frequencies, however, sediment volume variability is likely to play an important role in sound scattering.     

Geotechnical Properties of Surface Sediments in the INDEX Area

As a part of the environmental impact assessment studies, geotechnical properties of sediments were determined in the Central Indian Basin. The undrained shear strength and index properties of the siliceous sediments were determined on 20 box cores of uniform dimension collected from various locations in five preselected sites. The maximum core length encountered was 41 cm and most of the sediments were siliceous oozes consisting of radiolarian or diatomaceous tests. The shear strength measurements revealed that surface sediments deposited in recent times (0-10 cm) have a shear strength of 0-1 kPa; this value increases with depth, reaching 10 kPa at 40 cm deep. Older sediments have greater strength because of compaction. Water content varies in the wide range of 312-577% and decreases with depth. The clay minerals such as smectite and illite are dominant and show some control over water content. Wet density, specific gravity, and porosity do not indicate any notable variation with depth, thereby indicating a uniform, slow rate of sedimentation. The average porosity of sediments is 90.2%, specific gravity 2.18, and wet bulk density 1.12 g/cm 3 . Sediments exhibit medium to high plasticity characteristics, with the average plasticity index varying between 105% and 136%. Preliminary studies on postdisturbance samples showed an increase in natural water content and a decrease in undrained shear strength of sediments in the top 10- to 15-cm layer.     

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.      

In situ measurements of near-surface porosity in shallow-water marine sands

An in situ resistivity profiler was developed to measure with minimal disruption, the near-surface porosity of shallow-water marine sands. Results from a siliciclastic site off NW Florida and two Bahamian carbonate sites (an ooid shoal and coral reef sand flat) suggest the following general features. First, there is a 5- to 15-mm thick zone of elevated porosity adjacent to the sediment-water interface. Porosity in this layer was from 0.05 to 0.25 (decimal porosity) greater than the subjacent values, and would be difficult to resolve using traditional measurement techniques. Second, average porosity at >10-mm depth was 0.38 /spl plusmn/ 0.01 at the siliciclastic site, 0.39 /spl plusmn/ 0.01 at the ooid shoal site, and 0.49 /spl plusmn/ 0.02 at the coral reef sand flat site; consistent with literature values. Third, individual profiles exhibited 0.05-0.15 fluctuations about the mean, with vertical length scales of 5-15 mm. These fluctuations may be the result of grain packing heterogeneities caused by hydrodynamic sorting during deposition and subsequent physical and biological mixing or could be artifacts caused by disruption of the grain framework. Fourth, ripple troughs at the siliciclastic sand site had a significantly higher near-surface porosity compared to ripple crests, due most likely to the presence of detrital material in the troughs.     

Geoacoustic inversion by matched-field processing combined with vertical reflection coefficients and vertical correlation

The wide-band source (WBS) signals measured in the Asian Seas International Acoustics Experiment (ASIAEX) in the East China Sea (ECS) were used to invert for geoacoustic parameters. Sound speed and density were inverted using the matched-field processing method combined with the vertical reflection coefficients and sea-bed attenuation coefficients were inverted from the vertical correlation data. For a half infinite liquid sea-bottom model, the inverted equivalent bottom sound speed is 1610/spl plusmn/12 m/s and the bottom density is 1.86 g/cm/sup 3/. The inverted attenuation coefficients are well described by a nonlinear relationship of the form /spl alpha//sub b/=0.28f/sup 1.58/ dB/m (f is in units of kilohertz) in the frequency range of 100-600 Hz.     

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.     

Geotechnical Properties of Surface Sediments in the INDEX Area

Abstract

As a part of the environmental impact assessment studies, geotechnical properties of sediments were determined in the Central Indian Basin. The undrained shear strength and index properties of the siliceous sediments were determined on 20 box cores of uniform dimension collected from various locations in five preselected sites. The maximum core length encountered was 41 cm and most of the sediments were siliceous oozes consisting of radiolarian or diatomaceous tests. The shear strength measurements revealed that surface sediments deposited in recent times (0–10 cm) have a shear strength of 0–1 kPa; this value increases with depth, reaching 10 kPa at 40 cm deep. Older sediments have greater strength because of compaction. Water content varies in the wide range of 312–577% and decreases with depth. The clay minerals such as smectite and illite are dominant and show some control over water content. Wet density, specific gravity, and porosity do not indicate any notable variation with depth, thereby indicating a uniform, slow rate of sedimentation. The average porosity of sediments is 90.2%, specific gravity 2.18, and wet bulk density 1.12 g/cm³. Sediments exhibit medium to high plasticity characteristics, with the average plasticity index varying between 105% and 136%. Preliminary studies on postdisturbance samples showed an increase in natural water content and a decrease in undrained shear strength of sediments in the top 10- to 15-cm layer.     

Correlation of shear strength,hydraulic conductivity,and thermal gradients with sediment disturbance: South Pass region,Mississippi Delta

The degree of sediment disturbance in the South Pass area is correlated to the average hydraulic conductivity, shear strength, and thermal gradient. Hydraulic conductivity averages 18, 6, and 4 × 10^–7 cm/s in the undisturbed, moderately disturbed, and most disturbed sediments, respectively. Shear strength also decreases with increasing disturbance, from 7.6 to 4.4 to 3.5 kPa. Excluding the four stations dominated by annual temperature variations, the remaining 19 thermal gradients correlate well with sediment disturbance. The average gradient is positive in all of the disturbed sediments (0.12 ± 0.07° C/m) and 0 in the undisturbed sediments (0.02 ± 0.05° C/m).     

Non-destructive porosity determinations of antarctic marine sediments derived from resistivity measurements with an inductive method

The fractional porosity of marine sediments is one of the fundamental index properties of rocks. For the determination of porosity a nondestructive, inductive method was used on unsplit sediment cores. The results were compared with galvanically measured resistivities using a miniature Wenner array on split cores. The measurements agree well except for cores with high clay content where measurement frequency related effects cause a resistivity difference of about 10%. Porosities were derived from resistivities using a published resistivity-porosity relationship by Boyce (1968) and compared with sample porosities. A comparison of both data sets shows good agreement.     

A technique for measuring in-situ compressional wave speed dispersion in marine sediments

Compressional speed dispersion exists in all marine sediments. If the dispersion is great enough it may play a significant role in acoustic interaction with the seabed. On the other hand if dispersion is weak, seabed models and databases can be substantially simpler. The ocean acoustics community is divided on this issue, in part because of the lack of observations. One of the experimental challenges has been to measure speed over several decades of frequency using a single technique so that observed speed changes cannot be due to different biases in the techniques. A simple in-situ experimental approach was developed that measures the critical angle as a function of frequency and thus infers the speed dispersion. Measurements on the mid to outer continental shelf (Malta Plateau and the New Jersey Shelf STRATAFORM area) show a weak dispersion over the band from /spl sim/10/sup 2/-10/sup 4/ Hz. This implies (via the Kramers-Kronig relations) that the compressional wave attenuation for these sediments is small and/or approximately linear over this band.     

受控三轴应力-应变下沉积物声速与物理力学性质的关系

对南海某海域深度100～400 m的海底浅层（约2 m埋深范围）沉积物柱状样在接近海底水压力下进行三轴应变-声学同步测量,结果表明沉积物纵波声速有两个特征:（1）从应变过程开始到结束,沉积物纵波声速不断变化;（2）平均声速随着平均静弹性模量的增加,由大变小又由小变大,存在声速最小值。这些结果与海底浅表层沉积物的物理力学性质、围压、颗粒的结合状态改变有关。此外,沉积物动弹性模量和孔隙度呈良好的负相关性,这与孔隙度增大含水量增大有关;动弹性模量是静弹性模量的10～100倍,这主要与三轴应变试验的应变数量级与声波振动产生的应变数量级的差异大有关。采用本论文实验测量的数据分别建立了双复合参数-声速和孔隙度-声速经验公式,分析结果表明双复合参数-声速公式声速预报误差约是孔隙度-声速公式的1/4,表明双复合参数-声速公式更加有效。     

南海海底沉积物声学物理参数测定的温度和时间优化研究

海洋沉积物含水率、密度和孔隙度等物理参数是沉积物声学特性研究中的重要指标。由于南海沉积物类型多样、成分复杂, 特别是深海沉积物样品珍贵, 需精确测定沉积物声学及物理参数并无损害地保持沉积物化学性质。文章以黏土、粉砂和砂三种典型海底沉积物为研究对象, 使用环刀法和烘干法, 在不同温度条件下(60℃、80℃、100℃和120℃)测定和分析了这三种沉积物的含水率、密度、孔隙度随烘干时间的变化趋势及特征, 并进行了回归分析和综合研究。结果表明: 1) 对同类型沉积物, 温度越高, 完全失去孔隙水的时间越短, 且失水过程具有阶段性;2) 同一温度下, 三种典型沉积物完全失去孔隙水的时间为t_砂<t_粉砂<t_黏土, 且不同时间段, 失去孔隙水的速率差异较大, 这主要与沉积物的颗粒大小、颗粒间的间隙大小以及烘干后期沉积物中所含的水分均已大部分流失有关;3) 建议声学沉积物样品的烘干温度以80℃左右为宜, 并给出三种沉积物完全烘干的参考时长和临界时间;4) 在温度为80℃时, 将临界时间处的物理参数带入经验方程进行声速预报是可行的。     

Biot反演在夏威夷钙质沉积物原位测量声速和衰减中的应用

对夏威夷檀香山岛的两个站(H3和H4)钙质沉积物进行了20～100kHz的原位纵波声速和声衰减测量.它们均有轻微的频散.随频率的增加H3站位声速从1691m/s增加到1708m/s,H4站位的声速从1579m/s增加到1585m/s.随频率的增加H3站位的有效衰减从15dB/m增加到75dB/m,H4站位的有效衰减从22dB/m增加到62dB/m.运用Biot-Stoll模型对所测得的纵波速度和声衰减数据进行了Biot模型未知参数反演,发现粒径较大的H3站的沉积物(孔隙率为45%)比粒径较小的H4站的沉积物(孔隙率为56%)具有曲率小和渗透率及孔隙半径都大的性质.