Piecewise Acoustic Source Imaging with Unknown Speed of Sound Using a Level-Set Method

We investigate the following inverse problem:starting from the acoustic wave equation,reconstruct a piecewise constant passive acoustic source from a single boundary temporal measurement without knowing the speed of sound.When the amplitudes of the source are known a priori,we prove a unique determination result of the shape and propose a level set algorithm to reconstruct the singularities.When the singularities of the source are known a priori,we show unique determination of the source amplitudes and propose a least-squares fitting algorithm to recover the source amplitudes.The analysis bridges the low-frequency source inversion problem and the inverse problem of gravimetry.The proposed algorithms are validated and quantitatively evaluated with numerical experiments in 2D and 3D.

An improved algorithm based on equivalent sound velocity profile method at large incident angle

With the development of ultra-wide coverage technology,multibeam echo-sounder(MBES)system has put forward higher requirements for localization accuracy and computational efficiency of ray tracing method.The classical equivalent sound speed profile(ESSP)method replaces the measured sound velocity profile(SVP)with a simple constant gradient SVP,reducing the computational workload of beam positioning.However,in deep-sea environment,the depth measurement error of this method rapidly increases from the central beam to the edge beam.By analyzing the positioning error of the ESSP method at edge beam,it is discovered that the positioning error increases monotonically with the incident angle,and the relationship between them could be expressed by polynomial function.Therefore,an error correction algorithm based on polynomial fitting is obtained.The simulation experiment conducted on an inclined seafloor shows that the proposed algorithm exhibits comparable efficiency to the original ESSP method,while significantly improving bathymetry accuracy by nearly eight times in the edge beam.

Precise Positioning Method for Seafloor Geodetic Stations Based on the Temporal Variation of Sound Speed Structure

At present,GNSS-Acoustic(GNSS-A)combined technology is widely used in positioning for seafloor geodetic stations.Based on Sound Velocity Profiles(SVPs)data,the equal gradient acoustic ray-tracing method is applied in high-precision position inversion.However,because of the discreteness of the SVPs used in the forementioned method,it ignores the continuous variation of sound velocity structure in time domain,which worsens the positioning accuracy.In this study,the time-domain variation of Sound Speed Structure(SSS)has been considered,and the cubic B-spline function is applied to characterize the perturbed sound velocity.Based on the ray-tracing theory,an inversion model of“stepwise iteration&progressive corrections”for both positioning and sound speed information is proposed,which conducts the gradual correction of seafloor geodetic station coordinates and disturbed sound velocity.The practical data was used to test the effectiveness of our method.The results show that the Root Mean Square(RMS)errors of the residual values of the traditional methods without sound velocity correction,based on quadratic polynomial correction and based on cubic B-spline function correction are 1.43 ms,0.44 ms and 0.21 ms,respectively.The inversion model with sound velocity correction can effectively eliminate the systematic error caused by the change of SSS,and significantly improve the positioning accuracy of the seafloor geodetic stations.

The profile of sound speed and dissolved oxygen in the polymetallic nodules depositional area in the Western Pacific

With the consumption of terrestrial metal resources,the exploitation of deep-sea polymetallic nodule minerals has been widely concerned around the world.Therefore,the environmental impact of deep-sea polymetallic nodule mining cannot be ignored.However,duo to the lacks in stable and safe deep-sea(the depth>1000 m)vertical profile observation systems and consequently in long-term in-situ observation data,the sound speed and dissolved oxygen and the other water environment factors in the deposition areas of polymetallic nodules remains poorly understood.In this study,a deep-sea in-situ observation system was designed and deployed,and the water environment data of the polymetallic nodule deposition area were collected and analyzed.Result shows that the dissolved oxygen in the depth of 0–600 m was mainly affected by biological factors,while that in the area deeper than 600 m was affected by physical factors.The sound speed in the water body was mainly affected by temperature and pressure.At depths below 840 m,the sound speed is mainly controlled by temperature,and at depths between 840 m and 5700 m,the sound speed is mainly controlled by pressure.The correlations of sound speed vs.pressure and vs.temperature were regressed into equation.The resuspension of sediments rich in various metals may result in the reduction of dissolved oxygen and the improvement of redox potential.This environmental impact caused by a single sediment resuspension could last for 24 h or more.These findings enrich the understanding of the background value of the water environment in the polymetallic nodule deposition area.

Reconstructions of time-evolving sound-speed fields perturbed by deformed and dispersive internal solitary waves in shallow water

The high-fidelity reconstruction of sound speeds is crucial for predicting acoustic propagation in shallow water where internal solitary waves(ISWs)are prevalent.Mapping temperatures from time series to spatial fields is an approach widely used to reproduce the sound speed perturbed by deformed internal waves.However,wave-shape distortions are inherent in the modeling results.This paper analyzes the formation mechanism and dynamic behavior of the distorted waveform that is shown to arise from the mismatch between the modeled and real propagation speeds of individual solitons within an ISW packet.To mitigate distortions,a reconstruction method incorporating the dispersion property of an ISW train is proposed here.The principle is to assign each soliton a real speed observed in the experiment.Then,the modeled solitons propagate at their intrinsic speeds,and the packet disperses naturally with time.The method is applied to reconstruct the sound speed perturbed by ISWs in the South China Sea.The mean and median of the root-mean-square error between the reconstructed and measured sound speeds are below 2 m/s.The modeled shape deformations and packet dispersion agree well with observations,and the waveform distortion is reduced compared with the original method.This work ensures the high fidelity of waveguide-environment reconstructions and facilitates the investigation of sound propagation in the future.

Sound speed profiles in high spatiotemporal resolution using multigrid three-dimensional variational method:a coastal experiment off northern Shandong Peninsula

It is essential to ac quire sound speed profiles(SSPs)in high-precision spatiotemporal resolution for undersea acoustic activities.However,conventional observation methods cannot obtain high-resolution SSPs.Besides,S SPs are complex and changeable in time and space,especially in coastal areas.We proposed a new space-time multigrid three-dimensional variational method with weak constraint term(referred to as STC-MG3DVar)to construct high-precision spatiotemporal resolution SSPs in coastal areas,in which sound velocity is defined as the analytical variable,and the Chen-Millero sound velocity empirical formula is introduced as a weak constraint term into the cost function of the STC-MG3DVar.The spatiotemporal correlation of sound velocity observations is taken into account in the STC-MG3DVar method,and the multi-scale information of sound velocity observations from long waves to short waves can be successively extracted.The weak constraint term can optimize sound velocity by the physical relationship between sound velocity and temperature-salinity to obtain more reasonable and accurate SSPs.To verify the accuracy of the STC-MG3DVar,SSPs observations and CTD observations(temperature observations,salinity observations)are obtained from field experiments in the northern coastal area of the Shandong Peninsula.The average root mean square error(RMSE)of the STC-MG3DVar-constructed SSPs is 0.132 m/s,and the STC-MG3DVar method can improve the SSPs construction accuracy over the space-time multigrid 3DVar without weak constraint term(ST-MG3DVar)by 10.14%and over the spatial multigrid 3DVar with weak constraint term(SC-MG3DVar)by 44.19%.With the advantage of the constraint term and the spatiotemporal correlation information,the proposed STC-MG3DVar method works better than the ST-MG3DVar and the SCMG3DVar in constructing high-precision spatiotemporal re solution SSPs.

Image-Based Ultrasound Speed Estimation: Phantom and Human Liver Studies

Purpose: A novel image-based method for speed of sound (SoS) estimation is proposed and experimentally validated on a tissue-mimicking ultrasound phantom and normal human liver in vivo using linear and curved array transducers. Methods: When the beamforming SoS settings are adjusted to match the real tissue’s SoS, the ultrasound image at regions of interest will be in focus and the image quality will be optimal. Based on this principle, both a tissue-mimicking ultrasound phantom and normal human liver in vivo were used in this study. Ultrasound image was acquired using different SoS settings in beamforming channels ranging from 1420 m/sec to 1600 m/sec. Two regions of interest (ROIs) were selected. One was in a fully developed speckle region, while the other contained specular reflectors. We evaluated the image quality of these two ROIs in images acquired at different SoS settings in beamforming channels by using the normalized autocorrelation function (ACF) of the image data. The values of the normalized ACF at a specific lag as a function of the SoS setting were computed. Subsequently, the soft tissue’s SoS was determined from the SoS setting at the minimum value of the normalized ACF. Results: The value of the ACF as a function of the SoS setting can be computed for phantom and human liver images. SoS in soft tissue can be determined from the SoS setting at the minimum value of the normalized ACF. The estimation results show that the SoS of the tissue-mimicking phantom is 1460 m/sec, which is consistent with the phantom manufacturer’s specification, and the SoS of the normal human liver is 1540 m/sec, which is within the range of the SoS in a healthy human liver in vivo. Conclusion: Soft tissue’s SoS can be determined by analyzing the normalized ACF of ultrasound images. The method is based on searching for a minimum of the normalized ACF of ultrasound image data with a specific lag among different SoS settings in beamforming channels.

Acoustic sound speed profile inversion based on orthogonal matching pursuit

The estimation of ocean sound speed profiles(SSPs)requires the inversion of an acoustic field using limited observations.Such inverse problems are underdetermined,and require regularization to ensure physically realistic solutions.The empirical orthonormal function(EOF)is capable of a very large compression of the data set.In this paper,the non-linear response of the sound pressure to SSP is linearized using a first order Taylor expansion,and the pressure is expanded in a sparse domain using EOFs.Since the parameters of the inverse model are sparse,compressive sensing(CS)can help solve such underdetermined problems accurately,efficiently,and with enhanced resolution.Here,the orthogonal matching pursuit(OMP)is used to estimate range-independent acoustic SSPs using the simulated acoustic field.The superior resolution of OMP is demonstrated with the SSP data from the South China Sea experiment.By shortening the duration of the training set,the temporal correlation between EOF and test sets is enhanced,and the accuracy of sound velocity inversion is improved.The SSP estimation error versus depth is calculated,and the 99%confidence interval of error is within±0.6 m/s.The 82%of mean absolute error(MAE)is less than 1 m/s.It is shown that SSPs can be well estimated using OMP.

Matched-field inversion of sound speed profile in shallow water using a parallel genetic algorithm

A sound speed profile plays an important role in shallow water sound propagation.Concurrent with in-situ measurements,many inversion methods,such as matched-field inversion,have been put forward to invert the sound speed profile from acoustic signals.However,the time cost of matched-field inversion may be very high in replica field calculations.We studied the feasibility and robustness of an acoustic tomography scheme with matched-field processing in shallow water,and described the sound speed profile by empirical orthogonal functions.We analyzed the acoustic signals from a vertical line array in ASIAEX2001 in the East China Sea to invert sound speed profiles with estimated empirical orthogonal functions and a parallel genetic algorithm to speed up the inversion.The results show that the inverted sound speed profiles are in good agreement with conductivity-temperature-depth measurements.Moreover,a posteriori probability analysis is carried out to verify the inversion results.

Speeds of Sound and Excess Isentropic Compressibilities of Butyl Acetate＋Aromatic Hydrocarbons

Speed of sound data for butyl acetate＋benzene, or toluene, or o-xylene, or m-xylene, or p-xylene binary mixtures have been measured over the entire range of mole fraction at 308.15 K. The excess isentropic compressibilities （ Ks^E ） were computed from speed of sound and density data, derived from molar excess volume data. The Ks^E values were analyzed by using graph theoretical approach. The Ks^E values evaluated by graph theory compared reasonably well with their corresponding experimental values. The Ks^E data were also expressed in terms of Redlich-Kister polynomial equation to derive the coefficients and the standard deviation.

Enhancement of photoacoustic tomography in the tissue with speed-of-sound variance using ultrasound computed tomography

The speed-of-sound variance will decrease the imaging quality of photoacoustic tomography in acoustically inhomo- geneous tissue. In this study, ultrasound computed tomography is combined with photoacoustic tomography to enhance the photoacoustic tomography in this situation. The speed-of-sound information is recovered by ultrasound computed to- mography. Then, an improved delay-and-sum method is used to reconstruct the image from the photoacoustic signals. The simulation results validate that the proposed method can obtain a better photoacoustic tomography than the conventional method when the speed-of-sound variance is increased. In addition, the influences of the speed-of-sound variance and the fan-angle on the image quality are quantitatively explored to optimize the image scheme. The proposed method has a good performance even when the speed-of-sound variance reaches 14.2%. Furthermore, an optimized fan angle is revealed, which can keep the good image quality with a low cost of hardware. This study has a potential value in extending the biomedical application of photoacoustic tomography.

Sound Source Localisation for a High-Speed Train and Its Transfer Path to Interior Noise

Noise is one of the key issues in the operation of high-speed railways, with sound source localisation and its transfer path as the two major aspects. This study investigates both the exterior and interior sound source distribution of a high-speed train and presents a method for performing the contribution analysis of airborne sound with regard to the interior noise. First, both exterior and interior sound source locations of the high-speed train are identified through in-situ measurements. Second, the sound source contribution for di erent regions of the train and the relationships between the exterior and interior noises are analysed. Third, a method for conducting the contribution analysis of airborne sound with regard to the interior noise of the high-speed train is described. Lastly, a case study on the sidewall area is carried out, and the contribution of airborne sound to the interior noise of this area is obtained. The results show that, when the high-speed train runs at 310 km/h, dominant exterior sound sources are located in the bogie and pantograph regions, while main interior sound sources are located at the sidewall and roof. The interior noise, the bogie area noise and the sound source at the middle of the coach exhibit very similar rates of increase with increasing train speed. For the selected sidewall area, structure-borne sound dominates in most of the 1/3 octave bands.

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.

Study on Sound-Speed Dispersion in A Sandy Sediment at Frequency Ranges of 0.5–3 kHz and 90–170 kHz

In order to study the properties of sound-speed dispersion in a sandy sediment, the sound speed was measured both at high frequency （90-170 kHz） and low frequency （0.5-3 kHz） in laboratory environments. At high frequency, a sampling measurement was conducted with boiled and uncooked sand samples collected from the bottom of a large water tank. The sound speed was directly obtained through transmission measurement using single source and single hydrophone. At low frequency, an in situ measurement was conducted in the water tank, where the sandy sediment had been homogeneously paved at the bottom for a long time. The sound speed was indirectly inverted according to the traveling time of signals received by three buried hydrophones in the sandy sediment and the geometry in experiment. The results show that the mean sound speed is approximate 1710-1713 m/s with a weak positive gradient in the sand sample after being boiled （as a method to eliminate bubbles as much as possible） at high frequency, which agrees well with the predictions of Biot theory, the effective density fluid model （EDFM） and Buckingham＇s theory. However, the sound speed in the uncooked sandy sediment obviously decreases （about 80%） both at high frequency and low frequency due to plenty of bubbles in existence. And the sound-speed dispersion performs a weak negative gradient at high frequency. Finally, a water-unsaturated Biot model is presented for trying to explain the decrease of sound speed in the sandy sediment with plenty of bubbles.

Reconstructing Sound Speed Profiles with Sea Surface Data

Ocean sound speed profile(SSP) is the key factor affecting acoustic propagation. The acquisition of SSPsin real time with high precision is meaningful for underwater activities. By means of the remote sensing method, thesea surface data could be obtained in near-real time. Typically, the subsurface fields are correlated with the sea surfaceparameters. Thus, the SSPs could be obtained by means of satellite remote sensing. In this paper, the history as wellas the current research over the reconstruction of subsurface fields by means of sea surface data is introduced. Thentwo methods to reconstruct the SSPs with sea surface data, including the linear regression method using the empiricalorthogonal function, and the self-organizing method based on the big data theory, are described in detail in the paper.

Sound propagation in inhomogeneous waveguides with sound-speed profiles using the multimodal admittance method

The multimodal admittance method and its improvement are presented to deal with various aspects in underwater acoustics,mostly for the sound propagation in inhomogeneous waveguides with sound-speed profiles,arbitrary-shaped liquid-like scatterers,and range-dependent environments.In all cases,the propagation problem governed by the Helmholtz equation is transformed into initial value problems of two coupled first-order evolution equations with respect to the modal components of field quantities(sound pressure and its derivative),by projecting the Helmholtz equation on a constructed orthogonal and complete local basis.The admittance matrix,which is the modal representation of Direchlet-to-Neumann operator,is introduced to compute the first-order evolution equations with no numerical instability caused by evanescent modes.The fourth-order Magnus scheme is used for the numerical integration of differential equations in the numerical implementation.The numerical experiments of sound field in underwater inhomogeneous waveguides generated by point sources are performed.Besides,the numerical results computed by simulation software COMSOL Multiphysics are given to validate the correction of the multimodal admittance method.It is shown that the multimodal admittance method is an efficient and stable numerical method to solve the wave propagation problem in inhomogeneous underwater waveguides with sound-speed profiles,liquid-like scatterers,and range-dependent environments.The extension of the method to more complicated waveguides such as horizontally stratified waveguides is available.

Effect of risedronate on speed of sound in postmenopausal women with osteoporosis

AIM: To examine the effects of treatment with risedronate for 1 year on speed of sound(SOS) of the calcaneus and bone turnover markers in postmenopausal women with osteoporosis. METHODS: Thirty-eight postmenopausal women with osteoporosis who had been treated with risedronate for > 1 year were enrolled in the study. The SOS and bone turnover markers were monitored during treatment with risedronate for 1 year. RESULTS: The urinary levels of cross-linked N-terminal telopeptides of type Ⅰ collagen and serum levels of alkaline phosphatase were significantly decreased at 3 mo(-34.7%) and 12 mo(-21.2%), respectively, compared with the baseline values. The SOS increased modestly, but significantly by 0.65% at 12 mo compared with the baseline value. Treatment with risedronate elicited an increase in the SOS of the calcaneus exceeding the coefficient of variation in vivo(0.27%). CONCLUSION: The present study confirmed that risedronate suppressed bone turnover and elicited a clinically significant increase in the SOS of the calcaneus in postmenopausal women with osteoporosis.

Comprehensive Study of Inversion Methods for Sound Speed Profiles in the South China Sea

Traditional acquisition method of sound speed profiles using hydro-acoustic instruments is accurate but time-consuming and costly.To overcome this problem,some inversion methods have been developed over the last few decades.In this study,a comprehensive comparison of two inversion methods–the acoustic inversion method(AIM)and the satellite observation reconstruction method(SOR)–is presented.For AIM,the sound speed profile is first parameterized by the empirical orthogonal function(EOF)and the optimal parameters are searched by simulated annealing algorithm with respect to the cross-correlation function of the receiving signal and the simulation signal.For SOR,remotely sensed data are used to construct sound speed profiles.An experiment was conducted in the northeast of the South China Sea to verify the two methods.Both methods can obtain sound speed profiles quickly and cheaply.Compared with the sound speed profiles obtained by a conductivity-temperature-depth(CTD)instrument,the root-meansquare-error(RMSE)of AIM is 0.55 m s^(−1) and that of SOR is 1.71 m s^(−1).It is clear that AIM provides better inversion performance than SOR.Another primary benefit of AIM is that this method has no limitation to the inversion depth.The simulation results of sound propagation in regard to the inversed sound speed profiles show that the transmission losses of AIM and CTD are consistent and that of SOR is adversely affected by the inversion error of the sound speed and the inversion depth.But SOR has particular advantages in the inversion coverage.Together,all of these advantages make the AIM particularly valuable in practice.

Speed of Sound in Atmosphere of the Earth

It is demonstrated that contemporary conception on adiabaticity of sound in the Earth atmosphere is fair in sufficient approximation only for altitudes z ≤ 103 m. At higher altitudes adiabaticity of sound is violated and essential dependence of its speed on altitude is revealed which is related to heterogeneity of the atmosphere in gravitation field of the Earth. It became possible to reveal the factor of gravity field due to the fact that in the equation of the state of atmosphere considered to be ideal gas, the entropy s is taken into consideration and is written down as ρ = (p, s) instead of generally accepted ρ = ρ(p) which is fair only for isentropic media and is not applicable to the Earth. Such approach enabled to determine that apart from adiabatic mechanism of generation of sound wave there exists isobaric one and exactly this mechanism leads to dependence of sound speed on altitude which is the same as dependence on density.

Valid Regions of Formulas of Sound Speed in Bubbly Liquids

There are numerous formulae relating to the predictions of sound wave in the cavitating and bubbly flows. However, tile valid regions of those formulae are rather unclear from the view point of physics. In this work, the validity of the existing formulae is discussed in terms of three regions by employing the analysis of three typical lengths involved （viscous length, thermal diffusion length and bubble radius）. In our discussions, viscosity and thermal diffusion are both considered together with the effects of relative motion between bubbles and liquids. The importance of relative motion and thermal diffusion are quantitatively discussed in a wide range of parameter zones （including bubble radius and acoustic frequency）, The results show that for large bubbles, the effects of relative motion will be prominent in a wide region.