Temporal Arrival Structure Analysis and Source Localization in Bottom Bounce Model from a Vertical Array Data

Low-frequency transmission loss in the Arctic can be effectively described by the reflection/scattering loss at the ice-water interface due to the upward refractive sound-speed profile; the sound field that interacts with the bottom is quickly attenuated as to be unimportant at long ranges. Various models for the reflection/scattering loss have been proposed in the literature and can be shown to agree with single hydrophone data by adjusting the ice roughness or other parameters in the model. However, a critical test of the model predictions has to be conducted using vertical array data, since the depth dependence of the sound field provides a more meaningful measurement of the angular dependence of the reflection/scattering loss. In this paper, the angular and frequency dependence of the reflection/scattering coefficients was investigated, based on the mode attenuation coefficients previously deduced from vertical array data. It was found that the measured reflection/scattering loss was larger than most model predictions using realistic ice roughness parameters. A transmission loss model, based on a simple parametric fit to the reflection/scattering data, was shown to agree well with previously published data.

The minimum variance (MV) adaptive beamformer has been widely proposed for matched‐field processing because it provides a means of suppressing ambiguous beampattern ‘‘sidelobes.’’ In this paper, the performance of three robust MV methods is evaluated using vertical array data from the Mediterranean Sea collected by the NATO SACLANT Centre north of Elba [D. F. Gingras and P. Gerstoft, 3234 (1994)]. The three MV methods considered are (1) the reduced MV method (RMV) [C. L. Byrne et al., 2493–2502 (1990)], (2) the MV method with neighborhood location constraints (MV‐NLC) [H. Schmidt et al., 1851–62 (1990)], and (3) the MV method with environmental perturbation constraints (MV‐EPC) [J. Krolik, 1408–19 (1992)]. Real data results indicate that each approach has different merits. Projecting the data onto the reduced space defined by the modal eigenfunctions as in the RMV is necessary to avoid instabilities caused by highly correlated noise. Using neighboring location constraints as in the MV‐NLC reduces the need to finely sample the ambiguity surface. And using constraints derived from the second‐order statistics of the signal wavefront averaged over an ensemble of perturbed environmental parameters as in the MV‐EPC increases robustness to environmental mismatch. [Work supported by ONR.]

In shallow water, modal wavenumbers are useful for many applications. This paper presents an approach for wavenumber estimation of range-independent waveguide based on pressure and its finite difference (FD) data matrices built on the vertical linear array (VLA) data. The core of this method is that wavenumber information is expressed in the singular value decomposition (SVD) of the FD data matrix. Therefore, wavenumbers can be estimated directly by combining the SVDs of the pressure data matrix and FD data matrix. The method is validated on simulated data. The dependence of the proposed method on relevant parameters is discussed.

The Belleplaine test site, located in the island of Guadeloupe (French Lesser Antilles) includes a three-accelerometer vertical array, designed for liquefac- tion studies. The seismic response of the soil column at the test site is computed using three methods: the spectral ratio method using the vertical array data, a numerical method using the geotechnical properties of the soil column, and an operative fre- quency domain decomposition (FDD) modal analysis method. The Belleplaine test site is characterized by a mangrove layer overlaid by a stiff sandy deposit. This con- figuration is widely found at the border coast of the Caribbean region, which is exposed to high seismic hazard. We show that the buried mangrove layer plays the role of an isolation system equivalent to those usually employed in earthquake engineering aimed at reducing the seismic shear forces by reducing the internal stress within the structure. In our case, the flexibility of the mangrove layer reduces the distortion and the stress in the sandy upper layer, and consequently reduces the potential of liquefaction of the site.

In this article, we compare field estimates of near-surface attenuation, as captured by site-specific o-values (i.e., o0) with laboratory-based estimates of minimum shear-strain damping (Ÿmin). We propose models for Ÿmin based on o0 measured at selected stations of the KiK-net database, which are found to be generally larger than lowstrain damping values obtained from laboratory testing. The latter can only quantify intrinsic material damping, whereas other attenuation mechanisms such as scattering of the wavefield contribute to field-based estimates. In addition, we evaluate the difference in damping at the surface and at borehole stations to determine the contribution of shallow layers to attenuation as captured by o0-values at the surface. Thus, values of o0 are computed at the surface and at the downhole instrument depth. The difference between both values, correlates well with the averaged shear-wave velocity over the top 30 m of the profile, VS30, and with the depth to bedrock. Estimates of o0 for hard-rock and stiff sites in Japan are also examined and compared with other regional o0-values proposed for high VS30 materials in New Zealand, Greece, and Switzerland. Two values of o0, which are lower than the corresponding estimates for the aforementioned regions, are deemed potential descriptors of hard-rock conditions in Japan. The ability of the proposed o0-consistent damping models to predict ground motions using the vertical array data from the KiK-net sites has yet to be tested.

In this paper source localization results obtained by matched-field (MF) processing of vertical line array (VLA) data from the PACIFIC ECHO III sea trial are presented. The experimental site was located in a region of significant bathymetric variation. An harmonic source was towed on a series of arcs, providing data for a sequence of different ranges and azimuths. In spite of the significant bathymetric variation, good MF localization results were obtained for a large fraction of the trial time. In the first part of this analysis it is assumed that the azimuth of the source is known. Substantial improvements in MF localization were obtained by using an adiabatic normal mode model to calculate replica acoustic fields for the range-dependent bathymetry. The improvement over the results obtained with flat-bottom waveguide replicas is demonstrated for data from the longer range arcs. In the second part of the analysis, the concept of environmental signal processing, an enhancement to MF processing that is obtained by making use of the complexity of the oceanic waveguide environment, is investigated. The azimuthal variation of bathymetry for the source tracks along the arcs was used to obtain estimates of the source bearing from the VLA data. In many instances, the bearing estimates were in reasonable agreement with the known source bearings.

Detecting and imaging dense non-aqueous phase liquids (DNAPLs) in the subsurface is a challenging problem that is of interest to the Department of Energy, Department of Defense, and several local and state agencies. Current DNAPL detection techniques, such as Direct Push Probe Technologies (DPT) and In-Situ Tracers (IST) have risks and limitations. Cross-well radar (CWR) is a radar-based geophysical technique with low invasiveness for real time monitoring of DNAPLs. This technique uses electromagnetic waves transmitted and received through antennas in the subsurface. The computerized tomography is applied to a pilot-scale experimental facility constructed by the authors (referred as SoilBED) in this research. Cross-tomography data at multiple depths and locations were collected to simulate a 1/100 scaled contaminated soil problem. The experimental results are converted to the time domain signals. The resulting signal travel times are compared with the simulated results by FDTD. The transformed signals are used to prepare signal intensity and travel time tomograms of the SoilBED cross-sectional slices, for background and scattered field to study the signature of the scatterers. The results are able to visualize the dielectric objects in the saturated soil. Introduction Cross-well radar is a potential technique for DNAPL detection. Most of the research and field works done in Cross-well radar and other radar based geophysical techniques deal with a single frequency. Selecting one single frequency may be effective for one non-dispersive medium, but perhaps not for other non-dispersive or most dispersive media. This research covers a broadband of frequency from 400 MHz to 2.2 GHz. Having broadband frequency data helps to either use the entire frequency range and work in frequency domain or select a single frequency or narrower frequency range that seems to have enough detection potential out of the entire observed frequency response. It is important to consider cost and practicality of the system. Establishing practical and cost effective systems require strong theoretical and laboratory background conditions. Therefore, both theoretical and experimental aspects of the research are studied in parallel, to evaluate the feasibility of DNAPL detection using CWR and to study wave propagation behavior and forward problem. Collecting repeatable and reproducible frequency response data is a very challenging process. This difficultly achieved data can be manipulated first or directly used towards image processing. As any research subject, conducting both experimental and theoretical approaches is the most desirable way to advance fundamental knowledge in the field of interest. Frequency response and arrival time can be converted to each other. Thus, frequency domain results can be transformed to time domain and used towards techniques such as travel time tomography and vice versa. The experimental frequency domain results can be converted to CTF (Channel Transfer Functions) and compared to the simulated frequency domain results by the first order Born approximation (Farid 2004, Farid et al. 2005, and Zhan 2005). The FDTD simulation code (Farid 2002 and 2003) is available as the theoretical tool to perform the comparison in time domain and study travel time. If the experimental results are converted to time domain and transformed to travel time, they can be compared to the FDTD simulated results. Travel Time Computed by FDTD To establish a better knowledge of wave propagation in soil, both dry and saturated backgrounds are studied. The sandy soil in the SoilBED facility with 3.88% moisture content is referred to as dry soil.

A grid-free compressive sensing (CS) based method for extracting the normal modes of acoustic propagation in the ocean waveguide from vertical line array (VLA) data is presented. Extracting the normal modes involves the estimation of mode horizontal wavenumbers and the corresponding mode shapes. Sparse representation of the waveguide propagation using modes at discrete horizontal wavenumbers enables CS to be applied. Grid-free CS, based on group total-variation norm minimization, is adopted to mitigate the issues of the wavenumber search grid discretization in the conventional CS. In addition, the suggested method can process multiple sensor data jointly, which improves performance in estimation over single sensor data processing. The method here uses data on a VLA from a source at several ranges, and processes the multiple sensor data at different depths jointly. The grid-free CS extracts the mode wavenumbers and shapes even with no a priori environmental knowledge, a partial water column spanning array data, and without the mode orthogonality condition. The approach is illustrated by numerical simulations and experimental SWellEx-96 (shallow water evaluation cell experiment 1996) data.

Various line array configurations are evaluated for the source localization performance based on the analysis of mode decomposition matrix in this paper. The guideline of array shape design focuses on improving the localization performance of matched filed processing, meanwhile reducing the difficulty of deploying equipment in practical experiments. In the shallow water environment, when the environment is well known, the source localization result can be obtained by matched field processing algorithms effectively, but the source localization performance is affected by the array parameters, such as array length, the number of sensors, and the configurations of various horizontal and vertical line arrays. The modal decomposition method provides a useful insight into the questions of how many modes are needed and how to design the array to resolve the modes. Therefore, the method of utilizing a normal mode acoustic propagation model to decompose mode is proposed by vertical line array, horizontal line array and combined array respectively. Then we can evaluate the source localization performance of various line array configurations by studying the characteristic of normal mode decomposition matrix, thus establishing a qualitative or even quantitative relationship between each other. The more the normal mode decomposition matrix tends to be diagonalized, the better performance of line array localization will be obtained. Simulation results show that the localization performance of matched field processing with the combined arrays will be severely degraded when the mode amplitudes cannot be accurately deduced by one of the sub-arrays. Considering the requirements for the practical experiments and various environments, the source localization performance of short vertical line array and combined array are mainly discussed in this paper. The combined array can increase the azimuth and depth information of the source and realize three-dimensional target detection while the vertical array provides range-depth information and the horizontal array provides bearing information. Simulation result indicates that the design guidelines based on the normal mode decomposition are appropriate for arrays employed for matched filed processing. Meanwhile, the combined arrays perform better than the short vertical array, which is benefited by the horizontal array's suppressing the side lobes, which leads the ratio of peak to sidelobe to increase, and thus improving the location accuracy. The values of localization accuracy of combined arrays are all above 90% according to the simulation experiment. Take the practical application into account, the combined array is undoubtedly a compromise choice for the localization performance and the test complexity.

Exploring and understanding the temporal structure of dynamic networks attract extensive attention over the past few years. Most of these current research focuses on temporal community detection, evolution analysis or link prediction from a mission-oriented perspective. In fact, these three tasks should be not isolated but mutually reinforcing. Transforming these three tasks into a unified framework, it is crucial to extract the evolution pattern, which helps to understand the time-varying characteristics of temporal structure in essence. In addition, to the best of our knowledge, there is no work focusing on modeling and uncovering the local and global evolution pattern hidden in temporal community structure, simultaneously. In this paper, we propose a novel framework based on Orthogonal Nonnegative Matrix Factorization to Explore the Evolution Pattern (ONMF-EEP) for analyzing and predicting the time-varying structures in dynamic networks from local and global perspectives. The nature of this framework assumes that community structures are subject to a local evolution pattern (LEP) at each snapshot, and these LEPs are from a common global evolution pattern (GEP). The framework can synchronously detect temporal community structure, extract evolution pattern, and predict structure including communities and future snapshot links. The extensive experiments on real-world networks and artificial networks demonstrate that our proposed framework is highly effective on the tasks of dynamic network analysis.

Bottom loss inference from ambient noise inversion has been shown to yield accurate and robust predictions of undersea sound propagation and sonar detection performance using vertical line arrays and a conventional beamformer [Arvelo, J. Acoust. Soc. Am., 123, 679, 2008]. However, any array with a vertical aperture should be good candidates for ambient noise inversion. The main problem with vertical line arrays is that they are not well suited to rapidly survey a large area. Platform translation with a vertical array is expected to cause extreme array shape distortions that could seriously limit the accuracy of the results. To mitigate array shape distortion, a smaller array with vertical aperture could be mounted on the survey platform, which would allow it to survey the area at greater speeds. The platform's diameter certainly limits the vertical aperture of the array. A limited vertical aperture limits the frequency range, resolution, and accuracy of the estimated bottom loss. Since the array is rigidly fixed to the platform's body, an aggressive adaptive beamformer may be adopted to increase the resolution and accuracy of the inverted bottom loss to very low frequencies. The performance is demonstrated via comparisons against conventional beamformers with numerical simulations and measurements.

The spatial and energy resources available on small underwater platforms are limited, imposing strict restrictions on the structure, weight, and channel count of the sonar array they carry. To enhance the direction of arrival (DOA) estimation capabilities of small platform sonars under these constraints, this paper examines a combined vertical array (CVA), composed of a single vector hydrophone and pressure hydrophones. The vertical line array is insensitive to azimuthal variations, but the CVA is capable of horizontal direction estimation with better gain compared to a single vector hydrophone. A novel DOA estimation algorithm, based on the joint processing of sound pressure and particle velocity signals from a CVA, has been proposed: By jointly processing the sound pressure signals from the vertical array and the particle velocity signals from a single vector hydrophone, the method achieves gain transformation of the CVA in three-dimensional space, effectively enhancing the horizontal bearing estimation capability. Expressions for the horizontal gain of the CVA were derived, and the impact of array and signal parameters on the horizontal direction estimation accuracy of the CVA was simulated using the Monte Carlo method. Numerical simulations and experimental results indicate that the horizontal direction estimation accuracy of the CVA surpasses that of traditional vector hydrophones, with high-frequency gain stability unaffected by grating lobes in the elevation direction. This allows for high-precision, unambiguous estimation of target directions.

In the 1992 Barents Sea Polar Front Experiment, bandpass acoustic tomography signals centered at 224 Hz were received with very high SNR on a 16‐element vertical hydrophone array at a range of 35 km from the source. The depth of the ocean varied from 120 m at the source to 280 m at the vertical array. A critical part of travel time tomography is the measurement of the arrival time of individual modes and rays. Identifying and tracking these mode and rays can be extremely difficult using a single hydrophone in coastal waters due to signal interference. A vertical array has the potential to solve these problems. An efficient signal processing scheme is described, written in MATLAB, that simultaneously (1) beamforms for the bandpass modes or rays, (2) compensates for clock drift, sampling skew, and array tilt, and (3) compresses the maximal‐length, phase‐encoded signal. Data are presented that show that the beamforming along with acoustic modeling provides identification of the arrivals with a high degree of confidence. Errors in measuring the arrival times are estimated to range from 2 ms for low modes to 10 ms for high angle rays making the arrivals useful for inversions. [Work supported by ONR, Code 1125AR.]

In deep water, multipath time delays or frequency-domain interference periods of the acoustic intensity combined with multipath arrival angles are typically used for source localization. However, depth estimate is hard to achieve for a narrowband source at a remote part of the direct arrival zone as the required bandwidth increases with the source range. In this paper, a passive source localization method with a vertical line array, suitable for both broadband and narrowband sources, is proposed. Based on the variation trends of multipath angles with source range and depth, source localization is achieved by only matching the measured angles of the direct path and surface-reflected path with model-based values of a predefined grid of potential source locations. Considering the angle resolution limited by the array aperture and the presence of coherent multipath, sparse Bayesian learning is used and compared with the conventional beamforming and the minimum-variance distortionless-response beamforming to resolve and estimate the multipath angles. Simulations and experimental data of explosive sources collected by a vertical line array in the South China Sea are carried out to illustrate the method and demonstrate the performance.

Horizontal and vertical beamformer (waveguide) invariants are used to robustly localize broadband noise sources (ships) using signals in the 50–200-Hz band. The localization is realized by using signatures recorded on one horizontal and three vertical arrays. In brief, waveguide invariant theory applied to beamforming by two vertical arrays provides a range ratio of the source to the receivers. Beamforming by a horizontal array provides time-evolving spectrum for a particular look direction (LOFARgram). As a result the trajectories of the striations observed in LOFARgrams can be used to estimate the speed, range and direction of a broadband source. In December 2003 the Naval Research Laboratory moored three vertical arrays and one horizontal array at ranges of 10, 20 and 30 km from two fixed acoustic sources on the New Jersey (USA) shelf. Acoustic signatures from passing merchant and research vessels were recorded on the arrays and used to demonstrate source localization. [Work supported by ONR.]