Dajun Tang

An overview of SAX99: acoustic measurements

A high-frequency acoustic experiment was performed at a site 2 km from shore on the Florida Panhandle near Fort Walton Beach in water of 18-19 m depth. The goal of the experiment was, for high-frequency acoustic fields (mostly In the 10-300-kHz range), to quantify backscattering from the seafloor sediment, penetration into the sediment, and propagation within the sediment. In addition, spheres and other objects were used to gather data on acoustic detection of buried objects. The high-frequency acoustic interaction with the medium sand sediment was investigated at grazing angles both above and below the critical angle of about 30/spl deg/. Detailed characterizations of the upper seafloor physical properties were made to aid in quantifying the acoustic interaction with the seafloor. Biological processes within the seabed and the water column were also investigated with the goal of understanding their impact on acoustic properties. This paper summarizes the topics that motivated the experiment, outlines the scope of the measurements done, and presents preliminary acoustics results.

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.

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 (Archies 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.

The acoustic signature of marine seep bubbles

Observations of seabed bubbles (62m depth) at a natural marine hydrocarbon seep by passive acoustic and optical approaches are compared. The acoustic and optical methods observed a bimodal distribution with peaks at 1500 and 1750Hz, and 2200 and 2800μm radius, respectively. Radii were ~20% lower than predicted by the Minnaert formula. Frequency shifts were observed for bubbles emitted within a few milliseconds and were attributed to coupling between nearby bubbles. Surfactants also may have played a role.

Acoustic Backscattering From a Sand and a Sand/Mud Environment: Experiments and Data/Model Comparisons

The results from two bottom backscattering experiments are described in this paper. These experiments occurred within about 1 km of each other but were separated by approximately five years (1999 and 2004). The experimental methods used in the second experiment were changed based on lessons learned in the first experiment. These changes and the motivation for them are discussed. The sediment at each experiment site would generally be classified as the same (as a well-sorted medium sand sediment) before the weather events (Hurricane Ivan and Tropical Storm Matthew) that occurred in late September and early October 2004. As a result of these weather events, the sediment present during the October 18, 2004 experiments was much more complicated than that in 1999 and in many places had a mud/sand surface layer. The environmental measurements in both experiments were sufficient to separate physical mechanisms responsible for scattering. For shallow grazing angles (less than 45deg), backscattering at frequencies between 20 and 150 kHz was attributable to sediment interface roughness in 1999, whereas volume scattering dominated in 2004. Furthermore, in 2004, volume heterogeneity within the mud/sand surface layer is a probable mechanism for the scattering feature seen in the data in the 20deg-30deg region. Above 200 kHz, the frequency dependence of both the 1999 data and the 2004 data indicates that a new scattering mechanism is coming into play. Other results within this issue [Ivakin, IEEE J. Ocean. Eng., vol. 34, no. 4, Oct. 2009] indicate that scattering from shells is a viable candidate for explaining the data above 200 kHz.

Sediments in the East China Sea

This paper describes measurements of sediments during the 2000-2001 Asian Seas International Acoustic Experiment in the East China Sea. A number of techniques were used to infer properties of these sediments, including gravity and piston cores, subbottom profiling using a water gun, long-range sediment tomography, and in situ measurement of conductivity. Historical data from echosounder records and cores showed two regions of surficial sediments in the experimental area: a silty area to the west and a sandy area to the east. The tomography, cores, and water-gun measurements confirm the two surficial sediment regions seen in the historical data and also indicate that the subbottom structure at the experimental site consists of a thin (0-3 m thick) layer of sandy sediment directly beneath the sea floor. Below this layer, there is an extensive package of sediment with relatively uniform acoustic attributes. Core analysis shows that the surface sediment layer varies in compressional wave speed from a low near 1600 m/s in the west side of the experiment area to 1660 m/s in the east side of the experiment area. Long-range sediment tomography inversions show a similar spatial variation in the surface layer properties. In addition, the layer thickness as determined from tomography is consistent with the estimates from subbottom profiling.

Analyses of high‐frequency bottom and subbottom backscattering for two distinct shallow water environments

High‐frequency (40‐kHz) bottom backscattering data obtained by the Benthic Acoustic Measurement System from two different shallow water locations were analyzed. One of the sites has a hard sandy bottom, and the other a soft silty bottom. Since the data were recorded on two slightly separated arrays, the differential phase technique was used to locate the scatterers. It is found that scattering at the sandy site originated from the seafloor interface, while the scattering at the silty bottom was caused by a scattering layer about 1 m beneath the seafloor (believed to be methane gas voids). The backscattering strengths at the silty bottom were about 3 dB higher than those at the sandy bottom. A simple sub‐bottom scattering model for the silty bottom is assumed and compared with the data. When the sound refraction and attenuation are taken into account, the model‐data fit yields reasonable results.

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 IMP2s 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.

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

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

Plane-wave reflection from a random fluid half-space

The reflection of a monochromatic plane wave impinging from a homogeneous fluid half‐space onto a random fluid half‐space is studied using an integrodifferential equation method. The coherent reflection coefficient is presented as a function of frequency and statistical parameters characterizing the random medium.