Broadband source localization with a single hydrophone

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.

The arriving-angle structure for the forward scattered wave on a vertical line array is obtained upon a modified scattering model in the Pekeris waveguide. The structure is investigated and interpreted by the array invariant theory combined with target induced modal coupling effect. Compared with that of the direct blast, the arriving-angle structure of the forward scattering wave owns multi-striations as well as an increased vertical array invariant. The forward scattered angle structure is dependent on the target position on the source-receiver line. Simulations indicate a potential separation for the forward scattered wave overwhelmed by the direct blast.

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.

The inversion of broad-band low-frequency acoustic signals received on sparse arrays can lead to robust and efficient estimations of sea-bed properties. This paper describes a shallow-water geoacoustic inversion scheme based on the use of a model-based matched-impulse response on a single hydrophone. Results from the INTIMATE'96 experiment on the Portuguese shelf break are reviewed. In order to minimize the effects of strong time variability due to internal tides, only the time-stable waterborne bottom-surface reflected arrivals are exploited. A quasi-linear inversion algorithm is first applied to refine the geometry of the experiment. Then, inversion of bottom parameters is performed with an objective function that only makes use of the bottom-surface reflected arrivals' amplitudes. The experimental results show that broad-band transmissions (300-800 Hz) received on a single hydrophone, combined with the use of a simple eigenray code, are sufficient to correctly resolve geometrical parameters and bottom features. The analysis of the reflection coefficients both on simulated and real data helps to understand the validity of the inverted parameters and to derive the basis of an equivalent medium concept for geoacoustic inversion based on a "through-the-sensor" approach.

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.]

A method of range and depth estimation was studied using a single hydrophone based on the dispersive characteristic and time-frequency analysis for low frequency underwater acoustic pulse signals in shallow water environment. First, the signal received on a single hydrophone can be decomposed into a series of modes within the frame work of normal mode theory, and then the dispersive characteristic of the propagating modes can be analyzed using the time-frequency analysis. In order to improve the time-frequency resolution, the use of the time-frequency distribution with adaptive radial-Gaussian kernel extracts the arrival time difference of propagating modes in dispersion curve, which can be used to estimate source range. Mode energy can be extracted using binary time-frequency mask filtering based on multi-mode joint matching processing; and the source depth can be estimated by comparing the differences of the mode energy of the real data and simulated replica data, yielding a contrast function. Simulation results from a shallow-water Pekeris waveguide show that the time-frequency distribution with adaptive radial-Gaussian kernel represents well the dispersion characteristics of the underwater acoustic pulse signals, provides higher time-frequency resolution and overcomes the problem of the inherent limit for the time resolution and frequency resolution in the traditional short-time Fourier transform, so that the modes can be separated and identified more easily in the time-frequency plane. From the result of the range estimation, the different mode combinations have different results of the range estimation. The range estimation result can be obtained accurately by using the mode with high energy in the time-frequency plane. The relative error in range estimation is less than 2% by using the mode with high energy. In terms of the depth estimation, the more the number of joint matching mode, the more sharp peak and low fake peaks the contrast function has, so that the depth estimation is further improved by incorporating more modes. This research has great significance for studying the extraction and separation of low frequency underwater acoustic pulse signals.

During the LFASE experiment conducted in the summer of 1989, a vertical hydrophone array (VHA) was deployed at the site of DSDP Hole 534b in the Blake-Bahama basin. The VHA consisted of 16 hydrophones spaced 30 m apart rising vertically above the seafloor with the bottom of the array anchored to a digital recording package located on the seafloor. The main purpose of the experiment was to record seismic signals and noise above, on, and beneath the seafloor at the same site; the VHA recorded data from above the seafloor, an ocean-bottom seismograph (OBS) was on the seafloor, and a borehole geophone array (BHA) was in a borehole beneath the seafloor. Signal-to-noise measurements of P waves made for airgun shots recorded simultaneously by the VHA and the OBS show that the OBS has a signal-to-noise ratio (SNR) 5 to 10 dB greater than that of a single hydrophone, but P-wave stacks of the VHA data have SNRs for body waves 10 to 15 dB greater than the OBS. This implies that a vertical array is capable of recording distinct body waves out to significantly greater ranges than is a single receiver and so might be a useful tool in seismic refraction experiments. Other stacks of the VHA data revealed S waves not visible on the data from a single hydrophone or geophone. The VHA was also used to construct images of the wavefronts arriving from a distant source and to determine the vertical direction of the source. This array processing capability shows the potential of VHAs for use in 3D seismic reflection surveys, particularly in cases where it is not convenient to use a multi-channel streamer.

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.]

This paper considers concurrent matched-field processing of data from multiple, spatially separated acoustic arrays with application to towed-source data received on two bottom-moored horizontal line arrays from the SWellEx-96 shallow water experiment. Matched-field processors are derived for multiple arrays and multiple-snapshot data using maximum-likelihood estimates for unknown complex-valued source strengths and unknown error variances. Starting from a coherent processor where phase and amplitude is known between all arrays, likelihood expressions are derived for various assumptions on relative source spectral information (amplitude and phase at different frequencies) between arrays and from snapshot to snapshot. Processing the two arrays with a coherent-array processor (with inter-array amplitude and phase known) or with an incoherent-array processor (no inter-array spectral information) both yield improvements in localization over processing the arrays individually. The best results with this data set were obtained with a processor that exploits relative amplitude information but not relative phase between arrays. The localization performance improvement is retained when the multiple-array processors are applied to short arrays that individually yield poor performance.

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.

This paper describes an analytical study of range-depth ambiguity surfaces obtained on a vertical array in shallow water through the spatial correlation of the incident acoustic field with a replica of that field. The results indicate that a high-quality ambiguity surface (narrow main lobe-low sidelobe levels) can be obtained provided that the incident acoustic field is formed from a significant number of “effective” modes and that the array spans a large fraction of the water column. Changes in the acoustic environment that reduce the effective number of modes diminish the quality of the ambiguity surface. Thus the quality is reduced by either a decrease in the water depth or the source frequency or by an increase in either the source range or the bottom loss. Furthermore, although the array length is a key parameter, hydrophone spacings over twice that required to satisfy a Nyquist criteria (water depth/number modes) can be used without substantial degradation. Finally, errors in the bottom parameters, which produce errors in the replica field, shift the main-lobe peak, whereas errors in the sound-speed profile can degrade the ambiguity surface to the point where the main-lobe peak is indistinguishable from the sidelobe peaks.

Summary Microseismic data acquired during hydraulic stimulations are a key tool in determining well spacing, stimulation parameters, frac barriers, and natural and hydraulic fracturing relationships. In addition to hypocenter locations of the fracturing mechanism, attributes associated with the microseismic event including magnitude and P-S amplitude ratios provide valuable insight into the stimulation. These attributes can be used as weights in the calculation of microseismic derived stimulated rock volumes to determine effective draining areas. High velocity layers associated with carbonate debris flows are known to distort the seismic wave field and may have an effect on associated microseismic derived attributes. In this study we use a 3D elastic finite difference algorithm to model double couple and open tensile source mechanisms at different levels in the Wolfcamp shale of West Texas to determine the sensitivity of microseismic attributes and location to the geology and array configuration. Magnitude, P-S ratio, and event location from a horizontal and vertical array are examined. 3D snapshot movies of the different mechanisms show the complexity of the developing wave field and the location of nodal planes which could interfere with the position or observation of microseismic events. The modeling showed that attributes derived from a vertical array are more robust than attributes from a horizontal array. Attributes from events that occurred near high velocity layers such as carbonate debris flows may be less accurate than events that occurred in more consistent lower velocity layers. Event location and microseismic event attributes recorded from a vertical array are more accurate than events recorded from a horizontal array.

Acoustic propagation experiments have been carried out in the Florida Straits with a multi-frequency broadband source that transmitted m-sequence pulses over a range of 10 km to a sparse-filled vertical line array. This paper presents results of matched field inversions of the acoustic field data at low frequencies to estimate geoacoustic model parameters for the experimental site. Two approaches were taken for the inversions. The first was a conventional matched field inversion using multi-frequency data centered at 200 and at 400 Hz from the vertical array. The second approach was designed to model the low frequency waveform at a single hydrophone. For the very long range experimental geometry, the waveform was modeled in terms of modes. Each inversion was cast as an optimization problem using the adaptive simplex simulated annealing algorithm. The inversions provide a comparison between approaches that take advantage of the spatial coherence in one case, and the time coherence in the received signal in the other case. Both inversions give similar results for the parameters of a simple geoacoustic bottom model, and the sensitivities and relative uncertainties of the model parameters are consistent for the two approaches. Notably, the inversions are sensitive to compressional wave attenuation.

This paper applies geoacoustic inversion to low-frequency narrow-band acoustic data from a quiet surface ship recorded on a bottom-moored horizontal line array in shallow water. A Bayesian matched-field inversion method is employed which quantifies geoacoustic uncertainties and allows for meaningful comparison of inversion results from different data sets. Geoacoustic inversion results for ship-noise data are compared with inversion results for multitone data from a towed controlled source collected in the same experiment, and with independent geophysical measurements. To increase the information content of low-level ship-noise data, the effect of including multiple, independent data segments in the inversion is investigated and shown to significantly reduce geoacoustic parameter uncertainties. Geoacoustic uncertainties are also shown to depend on ship range and orientation, with increased uncertainties for long ranges and for the ship stern oriented away from the array.

Shallow water bottom properties can be very difficult to estimate and are usually done so by means of vertical line arrays over a range of frequencies. This work will examine the behavior of the minimization broadband processor at low frequencies (25–100 Hz) for a variety of horizontal line arrays at endfire in simulated Shallow Water 2006 environments. Do we ever have enough sensitivity and resolution to estimate any of the bottom properties when using a horizontal array?