A time series analysis of sound propagation in a strongly multipathshallow water environment with an adiabatic normal mode approach

Measured time series were generated by small omnidirectional explosive sources in a shallow water area. A bottom-mounted hydrophone recorded sound signals that propagated over a sloping bottom. The time series in the 250-500 Hz band were analyzed with a broad-band adiabatic normal mode approach. The measured waveforms contain numerous bottom interacting multipaths that are complicated by the subbottom structure that contains high-velocity layers near the water-sediment interface. Several of the details of the geoacoustic structure and the depth of the water column at the receiver are inferred from comparisons of the measured data to simulated time series. The sensitivity of broad-band matched-field ambiguity surfaces in the range-depth plane for a single receiver to selected waveguide parameters is examined. A consistent analysis is made where the simulated time series are compared to the measured time series along with the single-receiver matched-field localization solutions for ranges out to 5 km. In this range interval, it was found that the peak cross-correlation between the measured and simulated time series varied between 0.84 and 0.69. The difference between the GPS range and the range obtained from the matched-field solution varied from 0 to 63 m. The geoacoustic structure obtained in the analysis consists of an 8-m low-velocity sediment layer over an 8-m high-velocity layer followed by a higher velocity, infinite half-space     

Estimation of Geoacoustic Properties of Marine Sediment Using a Hybrid Differential Evolution Inversion Method

This paper reports the inversion of midfrequency (1500–4500 Hz) chirps from a short-range transmission experiment conducted on the New Jersey Continental Shelf during the 2006 Shallow Water Experiment (SW06). The source was held at different depths and the sound signals were recorded at a vertical line array to investigate the interactions with the sea bottom at various grazing angles. Strong reflections from the sediment layer were seen in the data for all of the sources. Due to the presence of complex microstructures in the thermocline of the oceanic sound-speed profile, fluctuations both in amplitude and arrival time of the direct path arrivals were observed. Time variation of the water-column environment was also evident during the source transmissions. To mitigate the effects of the ocean environment on the seabed property estimation, a multistage optimization inversion was employed. The sound speed and the experimental geometry were inverted first using only the travel times of the water-column arrivals. The bottom sound speed and the sediment layer thickness were then inverted by matching the travel times of the bottom and sub-bottom reflections. The average of the estimated values for the sediment sound speed is 1598 m/s, consistent with in situ measurements from other experiments in the vicinity.      

Modal Inversion Analysis for Geoacoustic Properties of the New Jersey Continental Shelf in the SWAT Experiments

In this paper, inversion for bottom sediment properties at a site on the New Jersey continental shelf is studied as part of the Shallow Water Acoustic Technology (SWAT) project. A source towed at a constant water depth over a range of some tens of kilometers transmitted low-frequency continuous wave (cw) signals, which were measured on a bottom-moored vertical line array of receivers. For the along-shelf geometry, the zeroth-order asymptotic Hankel transform is then applied to the acoustic field at 50 Hz measured on the resulting synthetic aperture horizontal array created at each receiver depth. The resulting horizontal wave number spectra, which have peaks corresponding to the mode eigenvalues, are observed to have slightly different values at different receiver depths, and therefore, stochastic mode inversion is exploited to utilize all of the observed peak position information. The estimated sound-speed profile (SSP) for the upper 10 m of sediment is then compared with an inversion result obtained using midfrequency (2–16 kHz) chirp sonar pulses reflected at normal incidence from the sediment. Although obtained using totally different inversion techniques, both estimated profiles are shown to be in good agreement in the top 10 m of sediment. The acoustic field simulated using the inverted SSP also agrees well with the measured acoustic field at each receiver depth. Furthermore, simulated sound fields which use this profile as input data are shown to be effective in predicting the measurements obtained at a different frequency (125 Hz) and for a different (cross-shelf) geometry.      

Real- and synthetic-array signal processing of buried targets

Results from two field experiments aimed at investigating the detection and classification of buried targets are presented. In both experiments a 2-16-kHz parametric source was used. In the first experiment, the source was used in combination with a 12-m horizontal line array and in the second with a 1.4-m vertical line array which was displaced horizontally along an underwater rail to form a 10 m /spl times/ 1.4 m two-dimensional synthetic aperture sonar (SAS). To increase the SAS integration time, the parametric source was electronically scanned in azimuth during the displacement along the rail, as in spotlight mode. It is shown that both arrays allow important signal-to-reverberation gains, enhancing the detection of sub-bottom echoes. A new, environmentally adaptive, matched filter which further improves the signal to reverberation ratio while allowing discrimination between proud and buried targets is presented and validated experimentally. The use of resonant scattering for target classification of buried objects is discussed, in the particular case of spherical shells.     

Measurement of sound transmission and signal gain in the complex Strait of Korea

The experiment, The Acoustic Characterization Test III, was conducted in the oceanographically complex Strait of Korea to accurately measure the sound transmission under known environmental conditions. Geoacoustic profiles derived from geophysical measurements, measured bathymetry, and sound-speed profiles were the basis for range dependent parabolic equation (PE) calculations. Agreement between measured and calculated transmission loss was obtained with an attenuation profile in the near water-sediment interface layer with a dependence on frequency to the 1.8 power consistent with measurements in other sand-silt areas. Since the environment was oceanographically complex and the shipping noise levels were high, the coherency of the sound transmission was estimated using relative signal gain (RSG). RSG was taken as the difference between the gain calculated with PE and measured with the array and at longer ranges and higher frequencies was found to be approximately -2 dB with a signal gain coefficient of variation of 5%. This RSG degradation, attributed to the random signal phase fluctuations resulting from scattering from the surfaces and volume of the waveguide, yielded using a Gaussian coherence function a spatial coherence length of 30/spl lambda/ @ 400 Hz-40 km. In addition, high resolution imaging of five targets with two bottom mounted arrays illustrate the achievable performance of low-to-mid frequency active sonar in this environment.     

An environmental assessment in the Strait of Sicily: measurementand analysis techniques for determining bottom and oceanographicproperties

In October 1997, the EnVerse 97 shallow-water acoustic experiments were jointly conducted by SACLANT Centre, TNO-FEL, and DERA off the coast of Sicily, Italy. The primary goal of the experiments was to determine the sea-bed properties through inversion of acoustic data. Using a towed source, the inversion method is tested at different source/receiver separations in an area with a range-dependent bottom. The sources transmitted over a broadband of frequencies (90-600 Hz) and the signals were measured on a vertical array of hydrophones. The acoustic data were continuously collected as the range between the source and receiving array varied from 0.5 to 6 km. An extensive seismic survey was conducted along the track providing supporting information about the layered structure of the bottom as well as layer compressional sound speeds. The oceanic conditions were assessed using current meters, satellite remote sensing, wave height measurements, and casts for determining conductivity and temperature as a function of water depth. Geoacoustic inversion results taken at different source/receiver ranges show sea-bed properties consistent with the range-dependent features observed in the seismic survey data. These results indicate that shallow-water bottom properties may be estimated over large areas using a towed source fixed receiver configuration     

Broad-band geoacoustic inversion in shallow water from waveguideimpulse response measurements on a single hydrophone: theory andexperimental results

The paper discusses an inversion method that allows the rapid determination of in situ geoacoustic properties of the ocean bottom without resorting to large acoustic receiving apertures, synthetic or real. The method is based on broad-band waterborne measurements and modeling of the waveguide impulse response between a controlled source and a single hydrophone. Results from Yellow Shark '94 experiments in Mediterranean shallow waters using single elements of a vertical array are reviewed, inversion of the bottom parameters is performed with an objective function that includes the processing gain of a model-based matched filter (MBMF) receiver relative to the conventional matched filter. The MBMF reference signals incorporate waveguide Green's functions for known geometry and water column acoustic model and hypothesized bottom geoacoustic models. The experimental inversion results demonstrated that, even for complex environmental conditions, a single transmission of a broad-band (200-800 Hz) coded signal received at a single depth and a few hundred forward modeling runs were sufficient to correctly resolve the bottom features. These included the sound speed profile, attenuation, density, and thickness of the top clay sediment layer, and sound speed and attenuation of the silty clay bottom. Exhaustive parameter search proved unequivocally the low-ambiguity and high-resolution properties of the MBMF-derived objective. The single-hydrophone results compare well with those obtained under identical conditions from matched-field processing of multitone pressure fields sampled on the vertical array. Both of these results agree with expectations from geophysical ground truth. The MBMF has been applied successfully to a field of advanced drifting acoustic buoys on the Western Sicilian shelf, demonstrating the general applicability of the inversion method presented     

孤立内波对过渡海域声场干涉结构的影响分析

南海北部陆坡海域是孤立内波的活跃区,孤立内波在该海域能够引起水体环境较强烈的水平不均匀性,从而影响声场干涉结构。将描述宽带声信号强度干涉条纹斜率的波导不变量视为一种分布,能更准确地分析声场的距离*频率干涉结构。本文研究了孤立内波环境下过渡海域声场的距离*频率干涉结构,依据实测孤立内波海洋环境,得到孤立内波环境下随距离变化的声速剖面,利用抛物方程方法仿真过渡海域声场干涉结构。在此基础上,利用拉东变换和傅里叶变换结合的谱值分离方法在低信噪比环境下提取波导不变量分布。分析表明孤立内波环境下过渡海域的声场类影区、类会聚区的波导不变量取值更丰富。     

Sensor position estimation and source ranging in a shallow water environment

Underwater acoustic transient signals are generated mechanically at known positions along a wharf. These signals are received by a wide aperture planar array of four underwater acoustic sensors, whose positions relative to the wharf are unknown. A method is described that enables the positions of the sensors to be estimated from accurate differential time-of-arrival measurements (with 0.1 /spl mu/s precision) as the signal wavefronts traverse the array. A comparison of the estimated positions with the nominal positions of the first three sensors, which form a 20-m-wide aperture horizontal line array, reveals a 2-cm displacement of the middle sensor from the line array axis. This slight bowing of the line array results in overranging (bias error of 3%) when the wavefront curvature method is used with the nominal collinear sensor positions to locate a static source of active sonar transmissions at a range of 59.2 m. The use of the spherical intersection method coupled with the estimated sensor positions of the line array provides an order of magnitude improvement in the range estimate (within 0.3% of the actual value). However, systematic ranging errors are observed when the sound propagation medium becomes nonstationary. Next, the differences in the arrival times of the direct path and boundary-reflected path signals at the middle sensor of the wide aperture line array are estimated using the differential phase residue of the analytic signal at the sensor output. These multipath delays are used to estimate the range and depth of the source. Although the average value of the multipath range estimates is within 0.5% of the actual value, the variance of the range estimates is 50 times larger when compared with the results of the spherical intersection and wavefront curvature methods. The multipath delay data are also processed to provide a reliable estimate of the temporal variation in the water depth enabling the tidal variation to be observed.     

Modal evolution and inversion for seabed geoacoustic properties inweakly range-dependent shallow-water waveguides

In a shallow-water ocean environment, the range dependent variation of the geoacoustic properties of the seabed is one of the crucial factors affecting sound propagation. Since the local modes of propagation depend on the spatial changes in the bottom sediments, the local eigenvalues of these modes are useful as tools for examining the range dependence of the sediment properties. In order to extract the local eigenvalues from measurements of the pressure field in a laterally inhomogeneous waveguide, the zeroth-order asymptotic Hankel transform with a short sliding window is utilized. The local peak positions in the output spectra differ from the local eigenvalues due to both the range variation of the local modes and the interference of adjacent modes. The departure due to the former factor is evaluated analytically by using the stationary phase method. In order to reduce the error induced by the latter factor, mode filtering is utilized by incorporating data from a fixed vertical array of receivers. The methods developed are applied to simulated pressure field data as well as experimental field data, and it is shown that the range evolution of the local modes can be successfully estimated. In addition, field measurements are used to demonstrate that the modal trajectories in range can be used to infer the range-dependent geoacoustic properties of the seabed     

The effects of array shape perturbation on beamforming and passive ranging

The problem of beam formation from a towed line array whose shape has been distorted is considered. Emphasis is placed on the beam broadening and range estimation effects of array shape perturbations and how the resulting losses can be regained if the actual element positions are known. Specific illustrations are provided for various levels of shape distortion. For example, a 15-m bow in a 232.5-m-long array broadens the beamwidth by a factor of 3 at 50 Hz. As another example, a 6-m bow in an 800-m-long array leads to a 20-pereent range underestimation at 10 km for a 100-500-Hz broadband source.     

Low-frequency bottom loss

The shallow refracted path through sea floor sediments plays a significant role in the transmission of acoustic energy at low frequencies. For bottom grazing angles of 90/spl deg/ to 25/spl deg/, low-frequency acoustic energy was observed to come from reflected paths. For bottom grazing angles of 25/spl deg/ to 10/spl deg/ the dominant source of low-frequency acoustic energy is from shallow refracted paths through the sediments. At angles less than 10/spl deg/, low-frequency acoustic energy is received from both the refracted and the reflected paths. The refracted path is possible because of the positive gradient within the sediment. The sudden emergence of the refracted arrival is related to the overall sound path length in the sediment and sediment absorption of sound. Since sediment absorption is directly proportional to frequency, only low-frequency energy is transmitted via this path. The refracted path may well exist where unconsolidated sediments of at least a few hundred feet are present.     

Specific Features of the Influence of the Lower Boundary of a Waveguide on the Angular Structure of the Sound Field in the Shelf Zones in the Northwest Part of the Black Sea

We analyze the possibility of existence of critical angles of incidence of sound waves along two paths of propagation of sound typical of the northwest shelf of the Black Sea from the viewpoint of the characteristics of the lower boundary of a waveguide and the space and time structure of the field of sound velocity. The lower boundary of the waveguide may possess the property of acoustic transparency both in the case of a subsurface sound channel and under the conditions of negative refraction if the bottom is formed by fine-aleurite silts responsible for significant losses in the process of propagation of sound. The angles of total internal reflection exist for bottoms formed by shell rocks under all hydrological conditions typical of this region. At the same time, for bottoms formed by fine-aleurite silts, these angles exist only for a certain vertical structure of the field of sound velocity, which enables one to use the range of angles in which the losses caused by the reflections of sound waves from the bottom can be neglected.     

Geoacoustic inversion by matched-field processing combined with vertical reflection coefficients and vertical correlation

The wide-band source (WBS) signals measured in the Asian Seas International Acoustics Experiment (ASIAEX) in the East China Sea (ECS) were used to invert for geoacoustic parameters. Sound speed and density were inverted using the matched-field processing method combined with the vertical reflection coefficients and sea-bed attenuation coefficients were inverted from the vertical correlation data. For a half infinite liquid sea-bottom model, the inverted equivalent bottom sound speed is 1610/spl plusmn/12 m/s and the bottom density is 1.86 g/cm/sup 3/. The inverted attenuation coefficients are well described by a nonlinear relationship of the form /spl alpha//sub b/=0.28f/sup 1.58/ dB/m (f is in units of kilohertz) in the frequency range of 100-600 Hz.     

Adaptive beamforming of a towed array during a turn

During maneuvering, towed array beamforming degrades if a straight array is assumed. This is especially true for high-resolution adaptive beamforming. It is experimentally demonstrated that adaptive beamforming is feasible on a turning array, provided that array shape is estimated. The array shape can be inferred solely from the coordinates of the tow vessel's Global Positioning System (GPS) without any instrumentation in the array. Based on estimated array shape from the GPS, both the conventional beamformer and the white noise constrained (WNC) adaptive beamformer are shown to track the source well during a turn. When calculating the weight vector in the WNC approach, a matrix inversion of the cross-spectral density matrix is involved. This matrix inversion can be stabilized by averaging the cross-spectral density matrix over neighboring frequencies. The proposed algorithms have been tested on real data with the tow-vessel making 45/spl deg/ turns with a 500-m curvature radius. While turning, the improvement in performance over the assumption of a straight array geometry was up to 5 dB for the conventional beamformer and considerably larger for the WNC adaptive beamformer.     

Acoustic model-based matched filter processing for fadingtime-dispersive ocean channels: theory and experiment

It is shown that the performance of a conventional matched filter can be improved if the reference (replica) channel compensates for the distortion by the ocean medium. A model-based matched filter is generated by correlating the received signal with a reference channel that consists of the transmitted signal convolved with the impulse response of the medium. The channel impulse responses are predicted with a broadband propagation model using in situ sound speed measured data and archival bottom loss data. The relative performance of conventional and model-based matched filter processing is compared for large time-bandwidth-product linear-frequency-modulated signals propagating in a dispersive waveguide. From ducted propagation measurements conducted in an area west of Sardinia, the model-based matched filter localizes the depths of both the source and receiving array and the range between them. The peak signal-to-noise ratio for the model-based matched filter is always larger than that of the conventional filter     

Absolute positioning of an autonomous underwater vehicle using GPS and acoustic measurements

Kinematic global positioning system (GPS) positioning and underwater acoustic ranging can combine to locate an autonomous underwater vehicle (AUV) with an accuracy of /spl plusmn/30cm (2-/spl sigma/) in the global International Terrestrial Reference Frame 2000 (ITRF2000). An array of three precision transponders, separated by approximately 700 m, was established on the seafloor in 300-m-deep waters off San Diego. Each transponder's horizontal position was determined with an accuracy of /spl plusmn/8 cm (2-/spl sigma/) by measuring two-way travel times with microsecond resolution between transponders and a shipboard transducer, positioned to /spl plusmn/10 cm (2-/spl sigma/) in ITRF2000 coordinates with GPS, as the ship circled each seafloor unit. Travel times measured from AUV to ship and from AUV to transponders to ship were differenced and combined with AUV depth from a pressure gauge to estimate ITRF2000 positions of the AUV to /spl plusmn/1 m (2-/spl sigma/). Simulations show that /spl plusmn/30 cm (2-/spl sigma/) absolute positioning of the AUV can be realized by replacing the time-difference approach with directly measured two-way travel times between AUV and seafloor transponders. Submeter absolute positioning of underwater vehicles in water depths up to several thousand meters is practical. The limiting factor is knowledge of near-surface sound speed which degrades the precision to which transponders can be located in the ITRF2000 frame.     

Simultaneous reception of long-range low-frequency backscattered sound by a vertical and a horizontal array

Simultaneous measurements of low-frequency sound generated by an explosive source and backscattered from the seafloor in the eastern Mediterranean were made with two receiver configurations: a towed horizontal array and a vertical array. Images of the scattering features on the beam-time data of the horizontal array were useful in the interpretation of the scattering process and in estimating areas of scatterers received by the vertical array, and permitted scattering strengths to be estimated for both configurations. Images of the vertical array data provided information about the vertical arrival angles at the array from specific scatterers. At long range, the sound from the scattering features was received at grazing angles less than13deg. The scattering strengths for three features varied from - 47 to - 25 dB. The mean frequency dependence over the band 125 to 700 Hz varied from 0 to 2.5 dB/octave with greater variations occurring within smaller bands.