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.

Geoacoustic Inversion Based on Dispersion Characteristic of Normal Modes in Shallow Water

A geoacoustic inversion method based on dispersion characteristic of normal modes is presented. An adaptive time-frequency analysis technique with a high resolution in both time and frequency domains is applied to derive the dispersion characteristic of normal modes from the broadband propagation signal. The bottom acoustic parameters are inverted by matching the calculated group delays of normal modes with the experimental data. Finally, some experimental results which could validate the inversion method are given.

Space and Time Coherence of Acoustic Field in Shallow Water

New experimental measurements of signal coherence in shallow water are presented. For signals with Iow fre quencies of about 500 Hz in iso-velocity shallow water with a silt-sand bottom and a water depth of about 45 tn, the vertical coherence has no distinct depth dependence at ranges of 18.5, 55.5 and 92.5 kin, but it has obvious range dependence. The horizontal coherence lengths are all greater than 40 wavelengths, and the time coherence lengths are all greater than 510s at these ranges. These experimental results show that a low-frequency acoustic field has strong spatial coherence and temporal stability in iso-velocity shallow water.

Frequency Dependence of Longitudinal Correlation Length in the Yellow Sea

Spatial correlation coefficient is one of the most important parameters for the description of sound propagation in shallow water. Frequency dependence of the longitudinal correlation length is still an open topic. We observe in a shallow water experiment that the longitudinal correlation length in units of wavelength increases with the increase of frequency. This phenomenon has not been seen in the published papers. The theoretical analysis and numerical simulations indicate that the non-linear frequency relationship of the bottom attenuation is the main cause of this phenomenon.

Coherence-Time of Matched-Field Processing in Shallow Water in the Presence of Linear Internal Waves

Relationships among the signal coherence-time of matched-field processing （MFP）, the acoustic frequency, the source-receiver range, and the sound speed standard deviation （STD） caused by internal waves in shallow water, are numerically investigated based on oceanographic data from two shallow water experiments. It is found that the coherence-time can be fitted with an inverse square-root power of range, a near inverse 1 power of frequency, and inverse 1.3 power of sound speed STD.

Improvement of Low-Frequency Acoustic Spatial Correlation by Frequency-Shift Compensation

Interference of normal modes can lead to longitudinal decorrelation of an ocean acoustical field, so It can aecrease the gain of a wide-aperture array. Based on waveguide invariance, we propose a frequency-shift compensation method that can be used to improve the signal correlation received at different locations with certain longitudinal separations. The method is verified by experimental data.

A Novel Nonlinear Optimization Method of Inversion for Sea-Bottom Properties

A nonlinear optimization method is proposed to inverse the sea-bottom properties.The method is called seed division algorithm(SDA)and can be used to solve global optimization problem.A numerical test is carried out to inverse the sea-bottom properties.The test is based on SDA and beam displacement ray mode theory.

Horizontal Correlation of Ambient Noise near a Sea Route

Ambient noise data measured in an experiment conducted near the sea route are analysed. It is found that at low frequency, the measured horizontal correlation coefficients at different separations oscillate much larger than that predicted by the classical ambient noise model. The theoretical analyses show that the observed phenomenon is mainly caused by windy noise together with the discrete shipping noise nearby. An ambient noise model is proposed to include the effects caused by both the noise sources and can be used to forecast the ambient noise field near a sea route.

Frequency Dependence of Transverse Correlation Coefficient in the Yellow Sea

Horizontal correlation is one of the most important characteristic parameters in ocean acoustics. In a shallow water acoustic experiment, we observed that the transverse correlation coemcient is an oscillation function of frequency. A model based on adiabatic normal mode theory in a random inhomogeneous waveguide which is statistically isotropic in a horizontal plane is developed. The theory indicates that the oscillation of the transverse correlation coefficient is mainly due to the normal mode interference.

Geoacoustic Inversion Based on a Vector Hydrophone Array

We propose a geoacoustic inversion scheme employing a vector hydrophone array based on the fact that vector hydrophone can provide more acoustic field information than traditional pressure hydrophones. Firstly, the transmission loss of particle velocities is discussed. Secondly, the sediment sound speed is acquired by a matchedfield processing （MFP） procedure, which is the optimization in combination of the pressure field and vertical particle velocity field. Finally, the bottom attenuation is estimated from the transmission loss difference between the vertical particle velocity and the pressure. The inversion method based on the vector hydrophone array mainly has two advantages： One is that the MFP method based on vector field can decrease the uncertain estimation of the sediment sound speed. The other is that the objective function based on the transmission loss difference has good sensitivity to the sediment attenuation and the inverted sediment attenuation is independent of source level. The validity of the inverted parameters is examined by comparison of the numerical results with the experimental data.

Time-Domain Geoacoustic Inversion Based on Normal Incidence Reflection from Layered Sediment

We present a new geoacoustic inversion approach based on the stable inversion ot sediment acoustic impeaance from normal incident reflection data. The Hamilton empirical formulae are utilized to separate the impedance into velocity and density. The inverted results are evaluated by the measurement on the core samples. The predicted transmission losses （TLs） using the inverted results are in good agreement with the measured TLs from the explosive sources.

Anomalous sound propagation due to the horizontal variation of seabed acoustic properties

The sound propagation in shallow water is greatly influenced by the acoustic properties of seabed. An anomalous transmission loss was observed in an experiment, and a range dependent bottom model with horizontal variation of seabed acoustic property is proposed and could be well used to explain the anomalous phenomena. It is shown that the horizontal variation of bottom properties has a great effect on underwater sound propagation, and it should be given much attention in sound propagation and geoacoustic inversion problems.

An oscillation phenomenon of low frequency reverberation in the shallow water and its physical explanation

An oscillation phenomenon of the low frequency reverberation intensity was observed in several shallow water reverberation experiments. This phenomenon cannot be explained by the widely used incoherent reverberation theory. In this paper, to explain the observed oscillation phenomenon, a normal mode based coherent reverberation theory is presented. The theoretical analysis and numerical results show that modal interference can cause the regular oscillation phenomenon of the low frequency reverberation intensity, and the oscillation frequency is determined by the normal mode eigen-values. A new method to estimate the bottom sound speed based on the oscillation frequency of reverberation intensity was presented in this paper. The experimental results at three different sites indicate that the bottom sound speed estimated from the oscillation frequency of reverberation intensity agrees with that inverted from Matched Field Processing (MFP) well.

Interference structure of shallow water reverberation in time-frequency distribution

The striations of the reverberation spectrum in the time-frequency distribution were observed in a shallow water acoustic experiment in 2002. A model following the coherent reverberation model developed in 2002 is presented to explain the observed striations. To examine the consistency between the measured data and numerical predictions, we have used a method based on Radon transform for determining the slope of the striations to the measured reverberation data and numerical predictions. The results indicate that the previously developed coherent reverberation model can predict the interference structure of the reverberation intensity in the time-frequency distribution.

The effects of the internal waves on temporal correlation of matched-field processing in shallow water

Effects of linear and solitary internal waves on the temporal correlation of matched- field processing （MFP） in shallow water are numerically investigated for acoustic sources with different frequencies and depths based on oceanographic data from an experiment. It is shown that the temporal correlation of MFP decreases as the amplitude of solitary internal waves or the average energy density of linear internal waves increases. For acoustic source with lower frequency or located below the thermocline, the temporal correlation of MFP is less affected by internal waves, and the temporal correlation length of MFP is longer. Moreover, the effects of the range from solitary internal waves to acoustic source on the temporal correlation of MFP are relatively small.

Two inverse methods for characterization of an inaccessible thin film under a plate by low-frequency ultrasound

Two ultrasonic methods were studied for determining the parameters of a composite consisting of a soft thin film attached beneath a hard plate substrate from resonant frequencies and amplitude spectra in low frequency region,respectively.Analyses of the sensitivity function, the objective function and the error transfer function were presented for discussing the effects of the two methods for both single parameter and two-parameter inversion.Two techniques were employed in experiment to characterize a composite consisting of a film attached beneath an aluminum plate substrate with a 7 MHz center frequency broadband transducer.It was observed that the results were similar for single parameter inversion and the method based on resonant frequencies got better result for two-parameter inversion under the same condition. The relative errors of the method based on resonant frequencies for inversing acoustic impedance and time-of-flight were 3.4%and 4.7%,respectively,and the method based on amplitude spectra got no results.

The effect of spatial coherence of sound field on subspace-based high-resolution spatial spectrum estimation

When the signals received by any two array elements are not fully correlated, the performance of DOA （Direction Of Arrival） estimation methods might degrade if the planewave model assumption is still used. For narrowband uniform linear array, by utilizing the property of Toeplitz matrix, it is proved that the received signals can be expressed as the sum of several plane-wave signals, which can be viewed as the ＂splitting＂ parts of the true sources. The ＂splitting＂ of sources makes it difficult to estimate the true number of sources. As a result, spurious peaks might occur in the spatial spectrum when subspace-based high-resolution spatial spectrum estimation technique is used.

Adaptive nonlinear prediction of ocean reverberation

An adaptive nonlinear prediction algorithm is proposed to predict ocean reverberation based on the phase space reconstruction of nonlinear dynamic system. The prediction algorithm is tested by experimental reverberation data measured in two areas, and the one-step forward prediction results are in good agreement with the experimental data. If the errors between the predicted and experimental data are chosen as the variable to detect the target in the reverberation series, the reverberation is suppressed and the signal-to-reverberation ratio is improved.

Sound scattering from underwater elastic spherical shell covered with multilayered medium approximated acoustic cloak

The anechoic performance and mechanism of underwater elastic spherical shell covered with coating are studied at low frequencies.The acoustic cloak is anisotropic material,which can be designed with homogeneous isotropic materials on the basis of effective medium approximation theory.The analytic expression of scattering acoustic field from the shell covered with multilayered medium is formulated and the scattering form function,resonance mode,acoustic field distribution are computed,the scattering characteristics and mechanism of transmission are analyzed.The results show that the direction of sound transmission inside the multilayered medium is changed,the acoustic field is deflected gradually,and the acoustic energy flux is guided around the target,which reduces the scattering intensity at low frequencies,the acoustic intensity of target＇s surface is very weak.Excepting the first resonance peak in spectrum produced by the zero order partial wave,the other resonance modes of elastic spherical shell are not excitated and the multilayered medium can suppress the resonance of the spherical shell effectively.

A hybrid scheme for geoacoustic inversion

A hybrid inversion scheme of geoacoustic parameters, which combines dispersion characteristics of normal modes with transmission loss, has been proposed based on the facts that different parameters have different effects on sound field. First, considering the dispersion characteristics of normal modes were sensitive to the density and the sound speed of seabottom, and not sensitive to the seabottom attenuation coefficient, the group time delays of different modes and different frequencies were acquired through adaptive optimal kernel time-frequency representation, then the group time delays were taken as the cost function, and using a global optimal algorithm, the seabottom density and the profile of seabottom sound speed were deduced. Successively, the validity of inversion results was evaluated by the a posteriori probability. Last, the attenuation coefficient of sediment is inverted using the transmission loss data recorded from the ship radiation noise. The validity of the total inversion results has been evaluated by making use of the matched field processing source localization.