Sound Speed Variations Research Articles

Evaluation of aging, diabetes mellitus, and skin wounds by scanning acoustic microscopy with protease digestion.

ABSTRACTScanning acoustic microscopy (SAM) can assess tissue stiffness by calculating the speed of sound (SOS) through tissues. SOS increases as tissue stiffness increases. Sensitivity to protease digestion depends on protein type, concentration, and modification. We analyzed the SOS images of formalin-fixed paraffin-embedded skin sections from elderly, young, diabetic, and nondiabetic subjects, as well as chronic and acute wounds. SAM provided high-resolution histology similar to LM and revealed characteristic SOS alteration following pepsin treatment. SOS values of dermis samples from elderly subjects (especially females) were lower than those of younger adults, which was indicative of age-related dermal softening and loosening. SOS values of elderly females were lower than those of younger females and elderly males. Dermal SOS showed a positive correlation with epidermal thickness. SOS values of epidermis of elderly subjects were higher than those of younger adults and showed a rapid decline 0.5h after protease digestion. Reticular dermis of diabetic patients exhibited greater pepsin resistance than that of nondiabetic patients. Chronic wounds exhibited greater SOS values and pepsin resistance than acute wounds. SOS variation with aging, diabetes mellitus, and wound fibrosis reflected histological and mechanical changes associated with senescence and disease duration. Epidermal thickness reflects age-related changes in dermal stiffness.

Target Localization for a Distributed SIMO Sonar With an Isogradient Sound Speed Profile

A distributed single-input multiple-output (SIMO) sonar system makes use of a sound source and a distributed star receiver network to localize targets. Compared with the terrestrial target localization, there are more challenges for underwater target localization. The underwater sound speed varies with depth, temperature, and salinity. As a result, the propagation path of the acoustic signal is not a straight line. It makes existing straight-line based distance measurements less accurate. Furthermore, time synchronization, which is closely related to time-based localization, has to be carried out because of the absence of GPS and clock differences of underwater devices. In this paper, we take the sound speed variation and time synchronization into account and propose two underwater target localization algorithms, namely, the space-alternating generalized expectation maximization-based underwater target localization (SAGE-UTL) and the nonlinear weighted least squares based underwater target localization (NWLS-UTL), for the distributed SIMO sonar system. The SAGE-UTL algorithm is designed for the situation, where the central receiver of the star receiver network needs to remain silent for its own safety. On the other hand, the NWLS-UTL algorithm is designed for the other situation, where the silence of the central receiver is not required. We assume that all the receivers are not synchronized, and the speed variation can be approximately modeled by a depth-dependent sound speed profile. Our proposed algorithms can jointly achieve target localization and time synchronization. We evaluate the SAGE-UTL and the NWLS-UTL algorithms with several numerical simulations. The results show good performance of our proposed algorithms compared with the Cramer–Rao bound and the tailored benchmark algorithms, such as the approximate nonlinear least squares algorithm and the approximate space-alternating generalized expectation maximization based underwater target localization algorithm.

Mobile Node Localization in Underwater Wireless Networks

Localizing mobile nodes in underwater networks is a highly challenging endeavor due to range errors caused by the mobility and uncertainty of sound speed. We propose a novel localization approach, which incorporates time alignment and range bending compensation to meet this challenge. Given the lengthy and varied propagation delays to different anchor nodes, we use a Kalman filter to align different time instants and locations as a mobile node receives timestamps from different anchors during a localization period. Based on Snell’s law, the ray tracing theory is applied to compensate for sound speed variations. These two steps minimize the errors caused by mobility and sound speed uncertainty. A penalty convex–concave procedure approach is also applied to accurately solve a nonconvex optimization problem to minimize localization errors. Deep sea trial results show that the final localization error for the mobile node is only 1.44 m (with the differential GPS as the true-value reference), marking a substantial improvement over existing state-of-the-art solutions.

Two-layer heterogeneous breast phantom for photoacoustic imaging.

Photoacoustic tomography (PAT) is emerging as a potentially important aid for breast cancer detection. Well-validated tissue-simulating phantoms are needed for objective, quantitative, and physically realistic testing for system development. Prior reported PAT phantoms with homogenous structures do not incorporate the irregular layered structure of breast tissue. To assess the impact of this simplification, we design and construct two-layer breast phantoms incorporating vessel-simulating inclusions and realistic undulations at the fat/fibroglandular tissue interface. The phantoms are composed of custom poly(vinyl chloride) plastisol formulations mimicking the acoustic properties of two breast tissue types and tissue-relevant similar optical properties. Resulting PAT images demonstrate that in tissue with acoustic heterogeneity, lateral size of imaging targets is sensitive to the choice of sound speed in image reconstruction. The undulating boundary can further degrade a target's lateral size due to sound speed variation in tissue and refraction of sound waves at the interface. The extent of this degradation is also influenced by the geometric relationship between an absorber and the boundary. Results indicate that homogeneous phantom matrixes may underestimate the degradation of PAT image quality in breast tissue, whereas heterogeneous phantoms can provide more realistic testing through improved reproduction of spatial variations in physical properties.

Thermoacoustic tomography for an integro-differential wave equation modeling attenuation

In this article we study the inverse problem of thermoacoustic tomography (TAT) on a medium with attenuation represented by a time-convolution (or memory) term, and whose consideration is motivated by the modeling of ultrasound waves in heterogeneous tissue via fractional derivatives with spatially dependent parameters. Under the assumption of being able to measure data on the whole boundary, we prove uniqueness and stability, and propose a convergent reconstruction method for a class of smooth variable sound speeds. By a suitable modification of the time reversal technique, we obtain a Neumann series reconstruction formula.

TARS

Efficient medium access control (MAC) is desirable for underwater sensor networks (UWSNs). However, designing an efficient underwater MAC protocol is challenging due to the long propagation delay of the underwater acoustic channel and spatial-temporal uncertainty. In this article, we propose a novel Traffic-Adaptive Receiver-Synchronized underwater MAC protocol, TARS, for throughput maximization. We divide time into equal-sized slots, each the size of one packet transmission time plus a guard time to cope with network dynamics. We adjust the packet transmission phase in a slot, determined by the sender-receiver distance, to align packet receptions for collision reduction. Both the sound propagation speed variation and the node mobility are considered in setting the transmission phase and slot size. We employ a queue-aware utility-optimization framework to determine the optimal transmission strategies dynamically, taking into account both the interference and data queue status. Extensive simulation results show that compared to the existing representative protocols, TARS achieves better performance with higher network throughput and lower packet delay (e.g., about 13%−146% higher in throughput and 13%−21% lower in delay than others in a mobile ad hoc network), as well as robustness under network mobility. Thus, TARS is highly suitable for mobile and traffic-varying UWSNs.

Accounting for speed of sound variations in volumetric hand-held optoacoustic imaging

Hand-held implementations of recently introduced real-time volumetric tomography approaches represent a promising path toward clinical translation of the optoacoustic technology. To this end, rapid acquisition of optoacoustic image data with spherical matrix arrays has attained exquisite visualizations of three-dimensional vascular morphology and function deep in human tissues. Nevertheless, significant reconstruction inaccuracies may arise from speed of sound (SoS) mismatches between the imaged tissue and the coupling medium used to propagate the generated optoacoustic responses toward the ultrasound sensing elements. Herein, we analyze the effects of SoS variations in three-dimensional hand-held tomographic acquisition geometries. An efficient graphics processing unit (GPU)-based reconstruction framework is further proposed to mitigate the SoS-related image quality degradation without compromising the high-frame-rate volumetric imaging performance of the method, essential for real-time visualization during hand-held scans.

Assessment of thermal stratification, stability and characteristics of deep water zone of the southern Caspian Sea

Variations of seawater properties and stratification of water column in the southern deep water zone (470 m depth) of the Caspian Sea were evaluated based on field measured data. Variations of physical characteristics (temperature, salinity, density and sound speed) of seawater mainly were limited to the upper 100 m layers. Below this layer, seasonal changes of seawater parameters were minor. Vertical structure of temperature in the southern Caspian Sea waters was characterized by a significant seasonal thermocline approximately between 20 and 50 m depths in summer with a vertical gradient of around 16 °C. Destruction of the thermocline occurred with the general cooling of the sea surface water and deepening of the mixed layer during late of autumn and winter. Frequency of stability in water column often across the upper layers (surface mixed and thermocline) was more than deeper levels. Variations of salinity were around 12.4 PSU in the time of measurements. Structures of thermocline and pycnocline were characterized by a significant homogeneity especially in offshore stations during the year. According to high agreement between density and temperature in the Caspian Seawater, a strong seasonal pycnocline was observed in the position of the thermocline. Furthermore, structure of sound speed variations followed vertical variations of seawater temperature. Variations of chemical characteristics (dissolved oxygen and pH) of seawater mainly were continued from sea surface to near the sea bed in deep water zone.

Full Modeling of High-Intensity Focused Ultrasound and Thermal Heating in the Kidney Using Realistic Patient Models.

High-intensity focused ultrasound (HIFU) therapy can be used for noninvasive treatment of kidney (renal) cancer, but the clinical outcomes have been variable. In this study, the efficacy of renal HIFU therapy was studied using nonlinear acoustic and thermal simulations in three patients. The acoustic simulations were conducted with and without refraction in order to investigate its effect on the shape, size, and pressure distribution at the focus. The values for the attenuation, sound speed, perfusion, and thermal conductivity of the kidney were varied over the reported ranges to determine the effect of variability on heating. Furthermore, the phase aberration was studied in order to quantify the underlying phase shifts using a second-order polynomial function. The ultrasound field intensity was found to drop on average 11.1dB with refraction and 6.4dB without refraction. Reflection at tissue interfaces was found to result in a loss less than 0.1dB. Focal point splitting due to refraction significantly reduced the heating efficacy. Of all the tissue parameters, perfusion was found to affect the heating the most. Small changes in temperature were seen with varying attenuation and thermal conductivity, but no visible changes were present with sound speed variations. The aberration study revealed an underlying trend in the spatial distribution of the phase shifts. The results show that the efficacy of HIFU therapy in the kidney could be improved with aberration correction. A method is proposed by which patient specific pretreatment calculations could be used to overcome the aberration and therefore make ultrasound treatment possible.

Acoustic Waveform Inversion for Ocean Turbulence

AbstractThe seismic oceanography method is based on extracting and stacking the low-frequency acoustic energy scattered by the ocean heterogeneity. However, a good understanding on how this acoustic wavefield is affected by physical processes in the ocean is still lacking. In this work an acoustic waveform modeling and inversion method is developed and applied to both synthetic and real data. In the synthetic example, the temperature field is simulated as a homogeneous Gaussian isotropic random field with the Kolmogorov–Obukhov spectrum superimposed on a background stratified ocean structure. The presented full waveform inversion method is based on the ray-Born approximation. The synthetic seismograms computed using the ray-Born scattering method closely match the seismograms produced with a more computationally expensive finite-difference method. The efficient solution to the inverse problem is provided by the multiscale nonlinear inversion approach that is specifically stable with respect to noise. Full waveform inversion tests are performed using both the stationary and time-dependent sound speed models. These tests show that the method provides a reliable reconstruction of both the spatial sound speed variation and the theoretical spectrum due to fully developed turbulence. Finally, the inversion approach is applied to real seismic reflection data to determine the heterogeneous sound speed structure at the west Barents Sea continental margin in the northeast Atlantic. The obtained model illustrates in more detail the processes of diapycnal mixing near the continental slope.

Dynamic measurement of speed of sound in n-Heptane by ultrasonics during fuel injections

The paper presents a technique to measure the speed of sound in fuels based on pulse-echo ultrasound. The method is applied inside the test chamber of a Zeuch-type instrument used for indirect measurement of the injection rate (Mexus). The paper outlines the pulse-echo method, considering probe installation, ultrasound beam propagation inside the test chamber, typical signals obtained, as well as different processing algorithms. The method is validated in static conditions by comparing the experimental results to the NIST database both for water and n-Heptane. The ultrasonic system is synchronized to the injector so that time resolved samples of speed of sound can be successfully acquired during a series of injections. Results at different operating conditions in n-Heptane are shown. An uncertainty analysis supports the analysis of results and allows to validate the method. Experimental results show that the speed of sound variation during an injection event is less than 1%, so the Mexus model assumption to consider it constant during the injection is valid.

Direct numerical simulations of supersonic turbulent channel flows of dense gases

The influence of dense-gas effects on compressible wall-bounded turbulence is investigated by means of direct numerical simulations of supersonic turbulent channel flows. Results are obtained for PP11, a heavy fluorocarbon representative of dense gases, the thermophysics properties of which are described by using a fifth-order virial equation of state and advanced models for the transport properties. In the dense-gas regime, the speed of sound varies non-monotonically in small perturbations and the dependency of the transport properties on the fluid density (in addition to the temperature) is no longer negligible. A parametric study is carried out by varying the bulk Mach and Reynolds numbers, and results are compared to those obtained for a perfect gas, namely air. Dense-gas flow exhibits almost negligible friction heating effects, since the high specific heat of the fluids leads to a loose coupling between thermal and kinetic fields, even at high Mach numbers. Despite negligible temperature variations across the channel, the mean viscosity tends to decrease from the channel walls to the centreline (liquid-like behaviour), due to its complex dependency on fluid density. On the other hand, strong density fluctuations are present, but due to the non-standard sound speed variation (opposite to the mean density evolution across the channel), the amplitude is maximal close to the channel wall, i.e. in the viscous sublayer instead of the buffer layer like in perfect gases. As a consequence, these fluctuations do not alter the turbulence structure significantly, and Morkovin’s hypothesis is well respected at any Mach number considered in the study. The preceding features make high Mach wall-bounded flows of dense gases similar to incompressible flows with variable properties, despite the significant fluctuations of density and speed of sound. Indeed, the semi-local scaling of Patelet al.(Phys. Fluids, vol. 27 (9), 2015, 095101) or Trettel & Larsson (Phys. Fluids, vol. 28 (2), 2016, 026102) is shown to be well adapted to compare results from existing surveys and with the well-documented incompressible limit. Additionally, for a dense gas the isothermal channel flow is also almost adiabatic, and the Van Driest transformation also performs reasonably well. The present observations open the way to the development of suitable models for dense-gas turbulent flows.

Extension of the array invariant to deep-water environments

The array invariant utilizes multiple arrivals separated in beam angle and travel time for broadband signals, achieving robust source-range estimation without detailed knowledge of the environments in shallow water. This approach includes the waveguide invariant parameter β, which is close to one for surface/bottom-interacting shallow-water environments. In deep water, however, β significantly varies from large negative to positive values depending on the group of modes, while being sensitive to the sound speed variation within the water column. In this paper, two different approaches are investigated: 1) an extension of the array invariant by integrating multiple arrivals with varying β and 2) the use of dispersion relationship between several pairs of arrivals. These two approaches will be applied to a deep-water environment (4-km deep) using a 13-m aperture vertical array deployed to the sound channel axis (e.g., 330-m) from the Medium Frequency Noise experiment (MFN), where a high-frequency source (above 1 kHz) also was positioned around the sound channel axis.

Acoustic wave propagation in horizontally variable shallow water

A unique normal mode solution which approximates sound speed variation with depth by a sequence of isovelocity layers is verified for a sample of range independent environments. Within each layer, two closed-form fundamental solutions which satisfy the separated depth-dependent wave equation are formulated with complex analogues to be used in evanescent regions. Piecewise-continuous potential mode solutions are constructed by satisfying an upper boundary condition and extending through isovelocity layers to the bottom. The value of the Wronskian for the Green’s function thus obtained is used to locate the eigenvalues of the normal modes comprising the propagating field. A closed form solution in a modified two-layer Pekeris waveguide with step-wise changes in the upper layer sound speed with range is used to study horizontal mode coupling through a series of such environments and to predict propagation loss as a function of range.

Broadband acoustic wave propagation in three-dimensional shallow waveguide with variable sound speed profile and boundary roughness

Propagation of broadband acoustic signals in shallow water environment is a complex four-dimensional problem that needs to be addressed with input from the spatial and temporal physical parameters of waveguides with rough boundaries. To construct a numerical model of this four-dimensional problem, methods such as the Parabolic Equation (PE), Horizontal Rays and Vertical Modes, 3D Ray Method, and Nx2D PE have been utilized in recent years. However, data/model comparison still remains a challenge and accurate comparison between measured and modeled acoustic fields in the waveguide is badly needed. Lack of environmental input to use for modeling is one reason, but with proper sampling of environment it may be overcome by novel experimental design based on the knowledge of waveguide physics. Acoustic frequency can also be utilized as one of the key parameters to simplify the problem and adopt strategies in conducting calibrated experiments. In this paper, we provide a broad view of recent advancements in three-dimensional acoustic wave propagation in shallow water waveguides in the presence of variable volumetric and boundary conditions. The effect of broadband acoustic wave center frequency and bandwidth with respect to the scale of environmental variability is also discussed. [Work supported by ONR 322 OA.]

Acoustic mode based internal tide tomography from the PhilSea 2009 experiment

The Philippine Sea 2009 experiment transmitted low frequency broadband sound pulses to a mode resolving array located at 185 km. The entire experiment was conducted over a period of one month, during which, most of the transmissions took place every 5 min, and the rest every 3 h. This talk estimates the travel times of the mode pulses and inverts them for the internal tide induced sound speed variability across the propagation path. A first order perturbation theory approach models mode travel times as range-averaged function of the sound speeds. Depending on the propagation distance, the contributions from some internal tide wavelengths are enhanced while others suppressed in the mode travel times. The talk shows the month-long time-series of the mode peak arrival times and estimates a spectrum for the observations. The presentation then derives an expression to relate the spectral components of the mode arrival times in terms of the internal tide spatial wavenumbers. The magnitude of the arrival time spectral components at the tide frequencies are used to invert for the respective internal tides across the propagation path. The tomographic inverse for the internal tides are compared with CTD measurements at the receiver and the source arrays.

Analysis of Iterative Methods in Photoacoustic Tomography with Variable Sound Speed

In this article, we revisit iterative methods for solving the inverse source problem of photoacoustic tomography in free space. Recently, there have been interesting developments on explicit formulations of the adjoint operator, demonstrating that iterative methods are an attractive choice for photoacoustic image reconstruction. In this work, we propose several modifications of current formulations of the adjoint operator which help speed up the convergence and yield improved error estimates. We establish a stability analysis and show that, with our choices of the adjoint operator, the iterative methods can achieve a linear rate of convergence, in the $L^2$-norm as well as $H^1$-norm. In addition, we analyze the normal operator from the microlocal analysis point of view. This gives insight into the convergence speed of the iterative methods and choosing proper weights for the mapping spaces. Finally, we present numerical results using various iterative reconstruction methods for full as well as limited vi...

The depth dependence of ambient noise in deep ocean trenches

A series of experiments aimed at measuring the power spectrum and vertical and horizontal noise coherence (directionality) in the deep ocean were carried out between 2009and 2015 using a family of autonomous instrument platforms named “Deep Sound.” Deep Sound is a free-falling acoustic recorder designed to descend from the ocean’s surface to a pre-assigned depth where it drops an iron weight and returns to the surface under its own buoyancy, while recording pressure time series on four hydrophones with vertical and horizontal spacing over the bandwidth 5 Hz—30 kHz. The complete vertical noise profile was recorded from the surface to 6 km in the Philippine Sea, 8.5 km in the Tonga Trench, and 9 km in the Serena and Challenger Deeps in the Mariana Trench. Generally, the vertical noise coherence is well described by the Cron & Sherman surface noise model with some depth-dependence explained by seawater attenuation and local sound speed variations. Below the reciprocal or critical depth (depth at which the so...

Ocean remote sensing with acoustic daylight: Lessons from experiments in the Florida Straits

Ambient and shipping noise in the ocean provides acoustic illumination, which can be used, similarly to daylight in the atmosphere, to characterize the environment. Phase information, which is particularly sensitive to sound speed variations and current velocity, can be retrieved from noise observations by the process known as noise interferometry. Approximations to acoustic Green's functions, which describe sound propagation between two locations, are estimated by cross-correlating time series of diffuse noise measured at those locations. Noise-interferometry-based approximations to Green's functions can be used as the basis for a variety of inversion algorithms, thereby providing a purely passive alternative to active-source ocean acoustic remote sensing. This paper gives an overview of results from noise interferometry experiments conducted in the Florida Straits at 100 m depth in December 2012, and at 600 m depth in September/October 2013. Under good conditions for noise interferometry, estimates of cross-correlation functions are shown to allow one to perform advanced phase-coherent signal processing techniques to: perform waveform inversions; estimate currents by exploiting nonreciprocity; perform time-reversal/back-propagation calculations; and investigate modal dispersion using time-warping techniques. Conditions which are favorable for noise interferometry are identified and discussed. [Work supported by NSF and ONR.]

Broadband Processing in a Noisy Shallow Ocean Environment: A Particle Filtering Approach

When a broadband source propagates sound in a shallow ocean the received data can become quite complicated due to temperature-related sound-speed variations and therefore a highly dispersive environment. Noise and uncertainties disrupt this already chaotic environment even further because disturbances propagate through the same inherent acoustic channel. The broadband (signal) estimation/detection problem can be decomposed into a set of narrowband solutions that are processed separately and then combined to achieve more enhancement of signal levels than that available from a single frequency, thereby allowing more information to be extracted leading to a more reliable source detection. A Bayesian solution to the broadband modal function tracking, pressure-field enhancement, and source detection problem is developed that leads to nonparametric estimates of desired posterior distributions enabling the estimation of useful statistics and an improved processor/detector. To investigate the processor capabilities, we synthesize an ensemble of noisy, broadband, shallow-ocean measurements to evaluate its overall performance using an information theoretical metric for the preprocessor and the receiver operating characteristic curve for the detector.