Mean Sound Speed Research Articles

Correcting bathymetry measurements for water sound speed effects using inversion theory

We present a method to accurately estimate the bathymetry and water sound speed in shallow waters using overlapping swaths obtained from a Multi-Beam Echo Sounder (MBES). The method is designed to correct the errors in the bathymetry manifested mainly in the outer beams of the MBES. The errors are caused by deviations in sound speed, which occur, for example, in estuaries where fresh river water mixes with seawater. Simulations show that we are able to simultaneously estimate the mean sound speed and the bathymetry. This is accomplished by minimizing the differences between the MBES measurements in the overlap region using a Levenberg-Marquardt optimization routine. We also present an application of the method to real data obtained at the fairway to the harbour of Rotterdam, The Netherlands. Analysis shows that the inverted sound speed agrees well with the measured sound speed at the transducer. However, the uncertainties are larger when the track-to-track distance is ̃2 times the depth. Therefore, we also discuss the optimal distance between sailed tracks. The method appears to be a promising tool for the accurate mapping of sea floors in shallow-water areas with a complex sound-speed profile.

TH‐D‐M100J‐06: Evaluation of Breast Density Using Sound Speed Measurements

Purpose: Women with high mammographic breast density are at a 4‐ to 6‐fold increased risk of developing breast cancer compared to women with fatty breasts. The purpose of this investigation was to confirm preliminary acoustic velocity measurements for breast density assessment, and to evaluate them with the current standard of care. Method and Materials: A sample of ∼100 patients was imaged with our computed ultrasound tomography clinical prototype with each dataset comprised of 45–75 tomograms ranging from near the chest wall through the nipple region. Whole breast acoustic velocity was determined by creating image stacks and evaluating the sound speed frequency distribution. The acoustic measures of breast density were evaluated by comparing these results to two mammographic density measures: (1) qualitative, as determined by a certified radiologist using the BI‐RADS Categorical Assessment based on a 1 (fatty) to 4 (dense) scale, and (2) quantitative, via digitization and computerized analysis of archival mammograms. The former involved a radiologist's visual assessment of each mammogram, while the latter required digitizing cranio‐caudal (CC) and mediolateral oblique (MLO) mammograms and implementing semi‐automatic segmentation routines. Results: Approximately 140 m/s difference in acoustic velocity was found between the fatty and dense BI‐RADS Categories, with statistically significant differences in mean sound speed demonstrated between each category (ANOVA, α= 0.05). In addition, an increased sound speed was exhibited with increased BI‐RADS Category. Furthermore, moderately strong positive associations were demonstrated between mean acoustic velocity and quantitative measures of mammographic percent breast density for both MLO and CC views (r2 = 0.65–0.76), respectively. Conclusion: These results confirm the hypothesis that utilizing acoustical velocity can be used as an indicator of breast tissue density. Having an operator independent and non‐ionizing means of whole‐breast density evaluation has important clinical implications which include tracking temporal changes, identifying high‐risk populations, and evaluating treatment response.

Optimal localization of a seafloor transponder in shallow water using acoustic ranging and GPS observations

This study utilized circular and straight-line survey patterns for acoustic ranging to determine the position of a seafloor transponder and mean sound speed of the water column. To reduce the considerable computational burden and eliminate the risk of arriving at a local minimum on least-squares inversion, the position of a seafloor transponder was estimated by utilizing optimization approaches. Based on the implicit function theorem, the Jacobian for this inverse problem was derived to investigate the constraints of employing circular and straight-line survey patterns to estimate the position of a transponder. Both cases, with and without knowledge of the vertical sound speed profile, were considered. A transponder positioning experiment was conducted at sea to collect acoustic and GPS observations. With significant uncertainties inherent in GPS measurements and the use of a commercial acoustic transponder not designed for precise ranging, experimental results indicate that the transponder position can be estimated accurately on the order of decimeters. Moreover, the mean sound speed of the water column estimated by the proposed optimization scheme is in agreement with that derived from conductivity, temperature, and density (CTD) measurements.

Effects of offshore mesoscale eddies and fronts on inshore shallow water acoustic propagation

Shallow water shelf areas inside of western boundary currents have two distinctly different ocean acoustic environments determined by the type of front that separates deep offshore from shallow inshore. Prograde fronts allow for the ducting of offshore internal waves up onto the shelf as the main source the sound speed fluctuations over the internal wave band. Retrograde fronts block the propagation of offshore internal waves setting up stability conditions that allow for propagation of locally generated, large amplitude non‐linear solitary waves as the major source of sound speed variability. Here, acoustic propagation is examined for both environments with data from two similar fixed system propagation experiments, one for the prograde environment off the coast of south Florida near the site off the Acoustic Observatory, and the second for the retrograde front environment of the Mid‐Atlantic Bight the SWO6 site. Offshore mesoscale features of fronts and eddies are shown to determine mean sound speed profiles and the energy of the internal wave fields. In turn, intensity fluctuations and temporal coherencies of broadband acoustic signals over several octaves are observed to vary with variations of the sound speed. For some locations, observations of mesoscale features alone can predict sonar performance.

Does decalcification alter the tissue sound speed of rabbit supraspinatus tendon insertion? In vitro measurement using scanning acoustic microscopy

Failure of the tendon or ligament insertions is one of the most common injuries in the Orthopaedic field. To elucidate the pathogenesis of those injuries, the authors had attempted to measure the tissue sound speed that could be correlated to its elasticity using scanning acoustic microscopy (SAM). For the application of SAM to tendon or ligament insertions, it was necessary to determine the role of decalcification in SAM measurements since mineralized tissues including bone or mineralized fibrocartilage were present at the insertion site. To assess whether decalcification alters the tissue sound speed or not, supraspinatus tendon insertion of six Japanese white rabbits were measured with SAM operating in the frequency range of 50–150 MHz. Right supraspinatus tendons attached to the humeral head were cut into two pieces at the center of the tendon. Then, they were fixed with 10% neutralized formalin for 12 h. In each specimen, medial half was not decalcified, while lateral half was decalcified with ethylene-diamine-tetra-acetic acid (EDTA). After embedding in paraffin, 5 μm thick specimens were prepared for the measurement using SAM. The mean sound speed in each histologic zone was evaluated, and subsequently compared to that measured in the undecalcified and the decalcified specimens. Mean sound speed of non-mineralized fibrocartilage was 1544 m/s in the undecalcified specimens, while the value of 1541 m/s was determined in the decalcified ones. On the other hand, it decreased 2–3% after decalcification in the mineralized tissue including mineralized fibrocartilage and bone (mineralized fibrocartilage: undecalcified = 1648 m/s, decalcified = 1604 m/s; bone: undecalcified = 1716 m/s, decalcified = 1677 m/s). However, no significant differences were found between the undecalcified and the decalcified specimens (non-mineralized fibrocartilage: p = 0.84, mineralized fibrocartilage: p = 0.35, bone: p = 0.28). These results indicate that SAM could be applied to determine the properties of the tendon or the ligament insertions after decalcification with EDTA. Although SAM is applicable only for in vitro experimental study, it is expected that these data will contribute to better understanding concerning the biomechanics of tendon or ligament insertions as well as the pathogenesis of their failure at a microscopic level.

A Hot Helium Plasma in the Galactic Center Region

Recent X-ray observations by the space mission Chandra confirmed the astonishing evidence of a diffuse, hot, thermal plasma at a temperature of ~9 × 107 K (~8 keV) that was found by previous surveys to extend over a few hundred parsecs in the Galactic center region. This plasma coexists with the usual components of the interstellar medium, such as cold molecular clouds and a soft (0.8 keV) component produced by supernova remnants, and its origin remains uncertain. First, simple calculations using a mean sound speed for a hydrogen-dominated plasma have suggested that it should not be gravitationally bound, and thus it requires a huge energy source to heat it in less than the escape time. Second, an astrophysical mechanism must be found to generate such a high temperature. No known source has been identified to fulfill both requirements. Here we address the energetics problem and show that the hot component could actually be a gravitationally confined helium plasma. We illustrate the new prospects that this opens up by discussing the origin of this gas and by suggesting possible heating mechanisms.

Cuvier's beaked whale (Ziphius cavirostris) head tissues:physical properties and CT imaging

Tissue physical properties from a Cuvier's beaked whale (Ziphius cavirostris) neonate head are reported and compared with computed tomography (CT) X-ray imaging. Physical properties measured include longitudinal sound velocity, density, elastic modulus and hysteresis. Tissues were classified by type as follows: mandibular acoustic fat, mandibular blubber, forehead acoustic fat (melon), forehead blubber, muscle and connective tissue. Results show that each class of tissues has unique, co-varying physical properties. The mandibular acoustic fats had minimal values for sound speed (1350+/-10.6 m s(-1)) and mass density (890+/-23 kg m(-3)). These values increased through mandibular blubber (1376+/-13 m s(-1), 919+/-13 kg m(-3)), melon (1382+/-23 m s(-1), 937+/-17 kg m(-3)), forehead blubber (1401+/-7.8 m s(-1), 935+/-25 kg m(-3)) and muscle (1517+/-46.8 m s(-1), 993+/-58 kg m(-3)). Connective tissue had the greatest mean sound speed and density (1628+/-48.7 m s(-1), 1087+/-41 kg m(-3)). The melon formed a low-density, low-sound-speed core, supporting its function as a sound focusing organ. Hounsfield unit (HU) values from CT X-ray imaging are correlated with density and sound speed values, allowing HU values to be used to predict these physical properties. Blubber and connective tissues have a higher elastic modulus than acoustic fats and melon, suggesting more collagen structure in blubber and connective tissues. Blubber tissue elastic modulus is nonlinear with varying stress, becoming more incompressible as stress is increased. These data provide important physical properties required to construct models of the sound generation and reception mechanisms in Ziphius cavirostris heads, as well as models of their interaction with anthropogenic sound.

TH-C-I-609-09: Quantifying Breast Density for Improved Evaluation of Breast Cancer Risk

Purpose: To quantify a preliminary relationship between breast density and acoustic velocity using ultrasoundcomputed tomography. The existence of such a relationship may provide quantitative prognostic information that can be used to determine breast cancer risk. Method and Materials: A sample of female patients was imaged using the CURE, or Computerized Ultrasound Risk Evaluation, device. The mammographic densities of each patient were assigned by a certified radiologist on a 1 (fatty) to 4 (dense) scale. Regions of interest (160 mm2) were selected contralateral to present anomalies in multiple adjacent slices centrally located on the breast, away from the chest wall and nipple regions. The minimum, maximum, mean, and standard deviation of the acoustic velocity were recorded for each slice. Results: This statistical information was then correlated against breast density to investigate the presence of meaningful relationships. Initial assessment of this sample suggests that the maximum sound speed values increase with increasing mammographic density, while the minimum values appear to decrease with increasing density. The mean sound speed shows a weak but steady trend with increasing density. The behavior of these correlations suggests that the intrinsic scatter of sound speeds within the breast tissue is a strong function of the mammographic breast density. However, these relationships are statistically weak due to a small sample size, and further investigation will be required to strengthen this association. Conclusion: This preliminary examination suggests that a quantifiable relationship between breast density and sound speed may exist. If these results are further validated in larger studies, then it may be possible to utilize CURE measurements to assess breast cancer risk with a quantitative score. Results will be available for forty additional patients in July, and a more definitive conclusion regarding these findings can then be provided.

Direct numerical simulation of passive scalar in decaying compressible turbulence

The passive scalars in the decaying compressible turbulence with the initial Reynolds number (defined by Taylor scale and RMS velocity) Re=72, the initial turbulent Mach numbers (defined by RMS velocity and mean sound speed) Mt=0.2-0.9, and the Schmidt numbers of passive scalar Sc=2-10 are numerically simulated by using a 7th order upwind difference scheme and 8th order group velocity control scheme. The computed results are validated with different numerical methods and different mesh sizes. The Batchelor scaling with k(-1) range is found in scalar spectra. The passive scalar spectra decay faster with the increasing turbulent Mach number. The extended self-similarity (ESS) is found in the passive scalar of compressible turbulence.

Acoustic propagation in the coastal environment of the Florida Straits—recent experimental results

An autonomous source was moored at ranges of 10 and then 20 km from a vertical receiver array with 32 elements in a depth of 145 m of water. M sequences were transmitted for 28 days at six center frequencies from 100 to 3200 in one octave increments. Arrivals and paths are identified with models and then fluctuation statistics, coherence, and predictability are examined in a parameter space of frequency, range, and receiver depth. A group of refracted-bottom-reflected (RBR) modes/rays has nearly equal group velocities and tends to focus in time and depth forming intense arrivals especially at the depth of the transmitted. A second group of surface reflected bottom reflected (SRBR) modes produce arrivals that fan out in time. Coastal areas inside western boundary currents have exceptionally variable sound speed fields owing to dynamical effects such as meanders, shelf waves, eddies, coastal upwelling and energetic internal waves and tides. Sound speed fluctuations are observed to be an order greater than the deep ocean. Very large changes in mean sound speed profiles and extreme gradients occur at subinertial periods. Also, potential energy of the internal wave field varies with the same longer periods as do statistical properties of observed acoustic signals.

Regional variability of the phenomenon of sound penetration into shadow zones in the ocean with fine-structure stratification

The regional variability of the phenomenon of the shadow zone insonification in the ocean is manifested in the variability of the main parameters of the sound signals that penetrate into these zones because of the scattering by the fine-structure inhomogeneities of the refractive index. The intensity of the phenomenon is governed by a combination of the vertical distribution of intensity of the fine-structure inhomogeneities and the caustics that exist in the insonified domains, along with the caustic intensity and position, both of which depend on the mean sound speed profile and on the geometry of the experiment. For the chosen typical regions of the ocean, the characteristics of the fine structure are systematized, and the phenomenon under study is analyzed. The results obtained offer a justified approach to solving inverse problems and a way to perform practical-purpose studies aimed at improving the ultimate performance of underwater observation and monitoring systems.

1302 直接数値シミュレーションによる圧縮性溝乱流場の構造解析(GS-5 数値流体力学と乱流(1))

To investigate the effect of comprssibelity on near-wall turblent structures , direct numerical simulation (DNS) is conducted for a supersonic wall-bounded channel flow. Mach number defined as bulk mean velocity and sound speed at the wall, is 1.5. In the nearwall region, typical turbulence structure, such as streaky structure and coherent vortices are observed similar to the incompressible turbulent flow. However, quantitatively the number or length scale of vortices is distinct from incompressible case. A conditional sampling technique is applied to study the flow field surrounding the strong down-wash events. The diltation dissipation denoting compressiblity effect is fairly small around vortices, the solenoidal dissipation due to the vortex itself is crucial even in this compressible flow.

Fine-scale acoustic tomographic imaging of shallow water sediments

In acoustic tomographic system capable of performing in situ two-dimensional (2D) acoustic imaging of shallow water sediments is described. This system is capable of resolving inhomogeneities greater than 10 cm and differentiating sound-speed variations greater than 2%, A tomographic inversion is performed in a 2D vertical slice of about 1 m/sup 2/ (1 m/spl times/1 m) using three identical probes, with each consisting of 70 evenly distributed transducers. In normal deployments, two of the probes are oriented vertically and are separated by about 1 meter, and the third is positioned horizontally right above the two vertical probes. The additional horizontal probe greatly improves the horizontal resolution of the system compared to conventional crosshole tomographic setups. Numerical simulations are performed to evaluate the influences of arrival time detection error and transducer position error on the performance of the tomography system. For an arrival time of 500 ns (standard deviation) and a position error of 4 mm (standard deviation), sound-speed anomalies of greater than 0.8% can be correctly predicted near the upper portion (close to the horizontal probe) and are resolvable near the lower portion. A controlled laboratory experiment was conducted to evaluate the performance of the system. The location of a polyurethane block (Conap EN22) used as a known target is correctly predicted while the inverted sound speed is about 9% lower than that from its actual value. Field data taken from a saturated muddy site are presented and analyzed. The inverted mean sound speed and attenuation are about 1480 ms/sup -1/ and 20 dBm/sup -1/, respectively.

Very-high-frequency normal mode modeling

The relevant quantity that determines the complexity of modeling via the normal mode method is the number of modes required in the problem. That is only partly determined by the frequency. Other factors that enter the problem are the water column depth and the mean sound speed in the water column and the bottom. Crudely the number of modes is proportional to the product of the water column depth and the frequency times what we call the hardness parameter and as well as the inverse of the mean speed of sound in the water column. The hardness parameter is equal to the square root of the factor of 1 minus the square of the ratio of the mean sound speed in the water column to that in the bottom. We determine the actual equation for the number of modes in a refractive wave guide with a layered bottom. We illustrate the modal spectrum for the general problem. Based on that we show some tricks useful to model high-frequency problems on the order of several thousands of modes.

Heat capacity and vibrational spectra of monolayer films adsorbed in nanotubes

Carbon or boron nitride nanotubes can adsorb condensed-phase monolayer films on their inner surfaces. Examples include inert gas or hydrogen films, which form at low temperatures in nanotubes with radius $g~5 \mathrm{\AA{}}.$ We study the phonon modes and thermodynamic properties of such films. Azimuthal quantization yields quasi-one-dimensional behavior at low temperature, manifested by a heat capacity linear in T, with quasi-two-dimensional behavior at higher temperature; the crossover between these regimes has a universal form, depending only on the ratio of the film radius to the thermal phonon wavelength. The film radius and mean sound speed can be extracted from the temperature dependence of the heat capacity.

The inner and outer flow fields due to a pair of spheres moving unsteadily through a compressible fluid: rectilinear motion

Two rigid spheres with equal radii a 0 move with variable speed along their line of centres, through an inviscid compressible fluid. The acoustic velocity potential is determined asymptotically when a typical speed u 0 of the spheres is small compared with the mean sound speed c 0 and the separation of the two spheres varies on a time scale t 0 = a 0 /u 0 . Inner and outer asymptotic approximations are matched and the distant sound field is expressed in terms of point sources of monopole, dipole and quadrupole type.

THE SOUND FIELD GENERATED BY A PAIR OF CIRCULAR CYLINDERS MOVING IN UNSTEADY RECTILINEAR MOTION

Two parallel circular cylinders with equal radii a 0 move with variable speed through an inviscid compressible fluid. Their centres are at (±h,0), where h varies with time, so that they move along a common straight line with equal and opposite speeds. The acoustic velocity potential is determined asymptotically when the typical speed u 0 of the cylinders is small compared with the mean sound speed c 0 and the separation function h varies on a time scale t 0 = a 0 /u 0 . Inner and outer asymptotic approximations are matched and the distant sound field is expressed in terms of line sources of monopole and quadrupole type.

Void-fraction measurements in bubble plumes generated by 2-D and 3-D breaking waves.

Recent experiments have confirmed that low-frequency sound (10 to 300 Hz) is generated under breaking waves. It has been suggested that volume oscillations of the bubble plume generated by breaking is the mechanism responsible for the generation of this sound. Confirmation of this process requires independent measurement of the void fraction, and therefore sound speed, in the bubbly mixture. Detailed measurements of the evolution of the void-fraction field in bubble plumes generated by 2-D and large scale 3-D laboratory breaking waves are presented and compared. Various moments of the bubble plume void-fraction field give information on the (a) volume of air entrained, (b) cross-sectional area of the plume, (c) mean void fraction and sound speed, (d) bubble plume resonant frequencies, (e) potential energy of the bubble plume, (f) horizontal and vertical displacement and velocity of the bubble plume. These measurements along with independent measurements of the sound radiated (Loewen and Melville, this meeting) support the hypothesis that low-frequency sound is generated by the volume oscillation of the bubble plume. The radial dependence of the bubble plume’s void-fraction (and sound speed) field and its effect on low-frequency sound generation is investigated and the influence of the void-fraction instrumentation detection threshold in estimating the above moments is also examined. [Work supported by ONR (Ocean Acoustics) and NSF (Physical Oceanography).]

The initial and ultimate motions of an impulsively started cylinder in a compressible fluid

A cylinder with circular cross section of radius a 0 , in an inviscid, compressible fluid, is subjected to an impulse so that it moves in a direction perpendicular to its length. In addition, a force, which is taken to be constant, acts in the direction of motion. The cylinder’s velocity is found to decrease rapidly during the initial stages of the motion on the timescale T 0 = a 0 / c 0 , where c 0 is the mean sound speed in the fluid. After a time of order T 0 it reaches a stage when the velocity varies more slowly. If u 0 is a typical value of the velocity and is small compared with c 0 , then a matching argument in time leads to the ultimate, slowly varying velocity defined on the slow timescale t 0 = a 0 / u 0 . The acoustic field induced by the cylinder’s motion is determined and, for the later stages of the motion, a matching argument in space leads to an expression for the field at distances much greater than a 0 from the cylinder in terms of moving line source and dipole potentials.

The sound field due to an elliptic cylinder in rectilinear motion

The sound field induced by a cylinder moving perpendicular to its length in an inviscid, compressible fluid is determined. The cylinder has elliptic cross section with semimajor axis a´ and its velocity, characterized by U 0 , varies on the timescale t 0 = a´ / U 0 and is small compared with c 0 , the mean sound speed in the fluid. It is also assumed that a´ is very much less than c 0 t 0 , a characteristic wavelength, so that the cylinder is compact. By using a modified matching argument, an asymptotic solution for the velocity potential is obtained. In particular, the acoustic field at distances much greater than á is described in terms of moving line source and multipole potentials whose singularities lie at the centre of the cylinder.