Acoustic Arrivals Research Articles

Effects of range-dependent sound speed on acoustic seaglider arrivals in the Canada Basin

Two seagliders equipped with temperature, conductivity and pressure sensors, acoustic recorders, and acoustic Doppler current profilers were deployed in the Canada Basin as part of a large scale acoustic tomography experiment in the summer of 2016 and 2017. The seagliders received transmissions from moored acoustic sources transmitting linear frequency modulated sweeps with 100 Hz bandwidth and center frequencies on the order of 250 Hz. Sources were moored at approximately 175 m depth within an acoustic duct (sometimes referred to at the Beaufort Duct), which is present in the Canada Basin due to Pacific Winter Water, enabling acoustic transmission to long ranges. Acoustic Seagliders recorded transmissions at ranges up to 480 km. The variability and stability of the acoustic duct with range was measured by temperature and salinity profiles recorded by the gliders as they transited between moored source locations. The range dependence of this duct will be explored through oceanographic measurements made in the summer of 2016 and 2017, with particular attention paid to glider transects which roughly overlapped between the two years. The effect of the range dependent sound-speed environment on the acoustic arrivals received on the gliders will be explored through parabolic equation models and received acoustic data.

Resolution, identification, and stability of broadband acoustic arrivals in Fram Strait.

An ocean acoustic tomography system consisting of three moorings with low frequency, broadband transceivers and a moored receiver located approximately in the center of the triangle formed by the transceivers was installed in the central, deep-water part of Fram Strait during 2010-2012. Comparisons of the acoustic receptions with predictions based on hydrographic sections show that the oceanographic conditions in Fram Strait result in complex arrival patterns in which it is difficult to resolve and identify individual arrivals. In addition, the early arrivals are unstable, with the arrival structures changing significantly over time. The stability parameter α suggests that the instability is likely not due to small-scale variability, but rather points toward strong mesoscale variability in the presence of a relatively weak sound channel as being largely responsible. The estimator-correlator [Dzieciuch, J. Acoust. Soc. Am. 136, 2512-2522 (2014)] is shown to provide an objective formalism for generating travel-time series given the complex propagation conditions. Because travel times obtained from the estimator-correlator are not associated with resolved, identified ray arrivals, inverse methods are needed that do not use sampling kernels constructed from geometric ray paths. One possible approach would be to use travel-time sensitivity kernels constructed for the estimator-correlator outputs.

Određivanje pozicije akustičnih izvora korišćenjem metode diskretne raspodele verovatnoća podržane združenom detekcijom i procenom vremena nailaska signala

The localization of various acoustic sources in a battlefield (such as weapon rounds, mortars, rockets, mines, improvised explosive devices, vehicle-borne improvised explosive devices and airborne vehicles) nowadays has significant history. In acoustic source localization systems multiple sensors (such as microphones or microphone arrays), placed at known positions and networked, are used to detect signals emitted from the source and perform localization of the source. In this paper, Discrete Probability Density (DPD) method, as a method for position determination, has been used to estimate location of acoustic sources such as artillery weapons. Wavelet decomposition of the acoustic signal enables to emphasize N-wave in the time domain. Time of Arrival (TOA) estimation is realized by a statistical analysis of narrow time segment which hold N-wave. Based on the estimated TOA Time Difference of Arrival (TDOA) is calculated. TDOA presents relative time difference of arrival of the sound signal between pairs of sensors. Localization of the acoustic sources is performed by time difference estimation of acoustic signal arrivals using Discrete Probability Density method. A number of experiments with fire off mortar projectiles have been conducted in order to verify the performance of the proposed approach.

Data-Driven, In Situ, Relative Sensor Calibration Based on Waveform Fitting Moment Tensor Inversion

This article presents a deconvolution-based methodology for the relative calibration of acoustic emission sensors. The method uses the observations from multiple acoustic emission arrivals to estimate correction functions that account for instrument response and site effects. The corrections act on the measurements reshaping the wave arrivals, in many cases allowing visible discrimination between compressional and shear phases. The methodology is illustrated with the application on a dataset obtained during the hydraulic fracturing at triaxial stress conditions of a Colton sandstone block. A workflow for the estimation of full moment tensor solutions, including the generation of quality control attributes complements the presentation. The moment tensor solutions display fracture geometries mostly aligned with the main hydraulic fracture’s plane. The style of activation can be associated with an extensive stress regime before breakdown and to a compressional stress regime after breakdown, when the pumps were reversed to withdraw fluid from the block. The source types commonly deviate from the pure double-couple type; however, the deviations are under 30° in the case of opening fractures, and under 20° in the case of closing fractures. Therefore, the acoustic emissions in this experiment are the result of predominantly shearing dislocations.

Localization and Subsurface Position Error Estimation of Gliders Using Broadband Acoustic Signals at Long Range

Broadband acoustic source transmissions recorded on Seagliders at ranges up to 700 km are used to estimate subsurface glider position. Because the sources transmitted at 9-min intervals the glider moved appreciably between source receptions. Source–glider ranges estimated from acoustic arrivals were combined using least squares analysis to estimate glider position and velocity during each reception period. The analysis was applied to 387 sets of source transmissions using three different flight models of glider subsurface motion for initial position input values. The offsets between the position estimated from the flight models and the acoustically derived position resulting from the inversions were 600–900-m root mean square (rms) depending upon the model and input parameters. The offsets were tripled if the positions from the flight models were not corrected for a dive-averaged current (DAC). Estimates of a posteriori errors ranged from 78–105-m rms and from 9.1–11.6-cm/s rms for glider position and velocity, respectively. Data residuals were on the order of 50-m rms, a dramatic reduction from 178-m rms, which was documented for the case neglecting the motion of the glider between subsequent source transmissions (Van Uffelen , J. Acoust. Soc. Amer., vol. 134, pp. 3260–3271, 2013). Overall horizontal glider speed was estimated to be approximately 21-cm/s rms.

Performance metrics for depth-based signal separation using deep vertical line arrays.

A recent publication by McCargar and Zurk [(2013). J. Acoust. Soc. Am. 133(4), EL320-EL325] introduced a modified Fourier transform-based method for passive source depth estimation using vertical line arrays deployed below the critical depth in the deep ocean. This method utilizes the depth-dependent modulation caused by the interference between the direct and surface-reflected acoustic arrivals, the observation of which is enhanced by propagation through the reliable acoustic path. However, neither the performance of this method nor its limits of applicability have yet been thoroughly investigated. This paper addresses both of these issues; the first by identifying and analyzing the factors that influence the resolution and ambiguity in the transform-based depth estimate; the second by introducing another, much simpler depth estimation method, which is used to determine the target trajectories required for observation of the interference pattern and the array requirements for accurate depth estimation.

Resolution, identification, and stability of broadband acoustic arrivals in Fram Strait

Fram Strait is the only deep-water connection between the Arctic and the world oceans. An acoustic system for tomography, glider navigation, and passive listening was installed in the central, deep-water part of the Strait during 2010–2012, with the primary objective of improving the estimates of transport through the Strait. Previous tomographic measurements have relied on the travel times of resolved, identified, and stable acoustic arrivals. The oceanographic conditions and highly variable sea ice in Fram Strait provide an acoustic environment that differs substantially from those in other tomographic experiments, however, and results in complex arrival patterns. Comparisons of the measured arrival patterns with predictions based on hydrographic sections show that it is difficult to resolve and identify individual arrivals in the early part of the arrival patterns. In addition, the early arrivals are unstable, with the arrival structure changing significantly over time. Later arrivals that are surface-reflected, bottom-reflected tend to be easier to resolve and identify, as well as more stable. The implication is that inverse methods need to use fluctuations in the overall structure of the early arrivals, which tend to sample the ocean similarly, in combination with the travel times of the later arrivals.

Deep ocean vector sensor array performance metrics

Recent work in passive sonar has drawn interest in the potential for vertical line arrays (VLAs) deployed below the critical depth—the depth inthe deep ocean at which the sound speed below the channel axis reaches the sound speed near the surface. Such arrays can take advantage of propagation via the reliable acoustic path (RAP), which has been shown to improve thesignal-to-noise ratio (SNR) of received signals from sources at or near the surface at moderate ranges. The potential of these deep ocean VLAs hasspawned further interest in the design of vector sensor VLAs that would allow azimuthal rejection and additional array gain over conventional pressure sensor VLAs. This work will present simulation results that explore and quantify the performance of such vector sensor VLAs deployed in thedeep ocean in terms of surface area coverage, detection probability, andoperational lifetime. The potential use of these deep ocean vector sensorarrays to estimate source depth using the depth-harmonic interferencebetween direct and surface-reflected acoustic arrivals will also be discussed.

Cross-correlation, triangulation, and curved-wavefront focusing of coral reef sound using a bi-linear hydrophone array.

A seven element, bi-linear hydrophone array was deployed over a coral reef in the Papahãnaumokuãkea Marine National Monument, Northwest Hawaiian Islands, in order to investigate the spatial, temporal, and spectral properties of biological sound in an environment free of anthropogenic influences. Local biological sound sources, including snapping shrimp and other organisms, produced curved-wavefront acoustic arrivals at the array, allowing source location via focusing to be performed over an area of 1600 m(2). Initially, however, a rough estimate of source location was obtained from triangulation of pair-wise cross-correlations of the sound. Refinements to these initial source locations, and source frequency information, were then obtained using two techniques, conventional and adaptive focusing. It was found that most of the sources were situated on or inside the reef structure itself, rather than over adjacent sandy areas. Snapping-shrimp-like sounds, all with similar spectral characteristics, originated from individual sources predominantly in one area to the east of the array. To the west, the spectral and spatial distributions of the sources were more varied, suggesting the presence of a multitude of heterogeneous biological processes. In addition to the biological sounds, some low-frequency noise due to distant breaking waves was received from end-fire north of the array.

On the spatial properties of ambient noise in the Tonga Trench, including effects of bathymetric shadowing.

In September 2012, the free-falling, deep-diving instrument platform Deep Sound III descended to the bottom of the Tonga Trench, where it resided at a depth of 8515 m for almost 3 h, recording ambient noise data on four hydrophones arranged in a vertical L-shaped configuration. The time series from each of the hydrophones yielded the power spectrum of the noise over the frequency band 3 Hz to 30 kHz. The spatial coherence functions, along with the corresponding cross-correlation functions, were recovered from all available hydrophone pairs in the vertical and the horizontal. The vertical coherence and cross-correlation data closely follow the predictions of a simple theory of sea-surface noise in a semi-infinite ocean, suggesting that the seabed in the Tonga Trench is a very poor acoustic reflector, which is consistent with the fact that the sediment at the bottom of the trench consists of very-fine-grained material having an acoustic impedance similar to that of seawater. The horizontal coherence and cross-correlation data are a little more complicated, showing evidence of (a) bathymetric shadowing of the noise by the walls of the trench and (b) highly directional acoustic arrivals from the research vessel supporting the experiment.

Evidence of three-dimensional acoustic propagation in the Catoche Tongue.

This paper presents observations of two classes of acoustic arrivals recorded on a sparsely populated vertical line array (VLA) moored in the center of the Catoche Tongue, a major reentrant in the Campeche Bank in the southeastern Gulf of Mexico. The acoustic signals were generated by signals underwater sound (SUS) located 50-80 km from the VLA. The first class of arrivals was identified as resulting from a direct (non-horizontally refracted) path. Then following a quiescent period, a second, more diffuse class of arrivals is observed and is believed to be the result of horizontal refraction from the margin of the Tongue. A spectral analysis of the measured data revealed that both classes of arrivals were characterized by the source spectrum associated with SUS. Additionally, the difference in time between the onset of the first and second class of arrivals observed as a function of range from the VLA is consistent with the relative difference in the length of the direct and refracted paths. The observations are further supported by a three-dimensional (3D) acoustic propagation computation that reproduces many of the features of the measured data and provides additional insight into the details of the 3D propagation.

Performance metrics for depth-based signal separation using deep vertical line arrays

A publication [McCargar & Zurk, 2013] presented a method for passive depth-separation of signals received on vertical line arrays (VLAs) deployed below the critical depth in the deep ocean. This method, based on a modified Fourier transform of the received signals from submerged targets, makes use of the depth-dependent modulation inherent in the signals due to interference between the direct and surface-reflected acoustic arrivals. Examination of the transform is necessary to determine performance of the algorithm in terms of the minimum target depth and range, array aperture, and temporal sampling. However, traditional expressions for signal sampling requirements (Nyquist sampling theorem) do not directly apply to the measured signal along a target trace due to uneven sampling in vertical angle imposed by the spatiotemporal evolution of the target track as observed on the VLA. In this paper, the effects of this uneven sampling on the ambiguity in the estimated depth (i.e., aliasing) are discussed, and expressions for the maximum snapshot length are presented and validated using simulated data produced with a normal-mode propagation model. Initial results are presented to show the requirements for snapshot lengths and target trajectories for successful depth separation of slow moving targets at low frequencies.

Acoustic noise interferometry in the Straits of Florida at 100 m depth: A ray-based interpretation

Cross-correlation function of ambient and shipping noise recorded simultaneously by two hydrophones provides an estimate of the acoustic Green's function, which describes deterministic sound propagation between the hydrophones and can be used to estimate physical parameters of the water column and seafloor. This paper presents results of an experimental investigation of acoustic noise interferometry in 100 m-deep water in the Straits of Florida. Acoustic noise was recorded continuously for about six days at three points near the seafloor. Coherent acoustic arrivals are successfully identified in the 20–70 Hz frequency band for pairs of hydrophones separated by ranges of 5.0 and 9.8 km. The measured noise cross-correlation functions are compared to ray-based simulations of Green's functions, with generally good agreement between correlation functions and simulations. Ray-based simulations are shown to reproduce multipath features of the measured correlation functions, which are due to multiple surface and bottom reflections. The feasibility of passive acoustic remote sensing using a few hydrophones in a shallow-water waveguide will be discussed. [Work supported by NSF, ONR, and NAVAIR.]

Bottom interacting acoustics in the north Pacific

During the 2004 Long-range Ocean Acoustic Propagation Experiment (LOAPEX), a new class of acoustic arrivals was observed on ocean bottom seismometers (OBSs) for ranges from 500 to 3200 km. The arrivals were called deep seafloor arrivals (DSFAs), because they were the dominant arrivals on the ocean bottom seismometers (OBSs), but were very weak on the deep vertical line array (Deep VLA), located above 750 m from the seafloor. Stephen et al. [JASA 134, 3307–3317 (2013)] attributed some of these arrivals to bottom-diffracted, surface-reflected (BDSR) energy that scattered from a seamount near the Deep VLA and subsequently reflected from the sea surface before arriving at the OBSs. In the Ocean Bottom Seismometer Augmentation in the North Pacific (OBSANP) Experiment in June to July 2013, we returned to the Deep VLA site with a near-seafloor Distributed Vertical Line Array (DVLA) that extended upward 1000 m from the seafloor and 12 OBSs. We transmitted to the instruments with a ship-suspended J15-3 acoustic source. The receiver locations and transmission program were designed to test the hypothesis that DSFAs correspond to BDSR energy, to further define the characteristics of the DSFAs, and to understand the conditions under which DSFAs are excited and propagate.

A comparison of measured and predicted broadband acoustic arrivals in Fram Strait

Fram Strait is the only deep-water connection between the Arctic and the world oceans. On the eastern side, the northbound West Spitsbergen Current transports warm Atlantic water into the Arctic, while on the western side the southbound East Greenland Current transports sea ice and polar water from the Arctic to the Nordic Seas and Atlantic Ocean. Significant recirculation and intense small-scale mesoscale variability in the center of the Strait make it difficult to accurately measure ocean transports through the Strait. An acoustic system for tomography, glider navigation, and passive listening was installed in the central, deep-water part of the Strait during 2010–2012. The integral measurements of temperature provided by tomography and the spatial resolution of the glider data are complementary to the data from the long-term array of oceanographic moorings at 780 50’N. The oceanographic conditions and highly variable sea ice in Fram Strait provide an acoustic environment that differs from both the high Arctic and the temperate oceans and that results in complex acoustic propagation. Improved understanding of the measured acoustic arrivals through comparison with predictions based on available environmental data is important for development of tomographic inversion and assimilation techniques, for glider navigation, and for acoustic communications.

The effect of seawater attenuation on the directionality and spatial coherence of surface-generated ambient noise

Acoustic attenuation in seawater is sufficiently weak that it usually has little effect on the spatial characteristics of ambient noise. However, at frequencies above 10 kHz and depths below 6 km, seawater attenuation may influence the directionality of surface-generated ambient noise. In a semi-infinite, homogeneous ocean, all the noise is downward-traveling, since there are no bottom reflections, and, in the absence of attenuation, the directional density function varies as the cosine of the polar angle measured from the zenith. When attenuation is present, the angular width of this noise lobe becomes narrower, because sound from distant surface sources is attenuated more than acoustic arrivals from overhead. This narrowing effect increases as frequency rises, since the attenuation is a rapidly increasing function of frequency. The spatial coherence of the noise is also modified by the attenuation. For a pair of horizontally (vertically) aligned sensors, the zeroes in the spatial coherence function are shifted to higher (lower) frequencies. These effects of attenuation on the noise field are sufficiently large to be detectable using the recently developed, deep diving instrument platform Deep Sound, which is capable of measuring broadband ambient noise on multiple hydrophones to depths of 11 km. [Research supported by ONR.]

Theory of the directionality and spatial coherence of wind-driven ambient noise in a deep ocean with attenuation

Acoustic attenuation in seawater usually has little effect on the spatial statistics of ambient noise in the ocean. This expectation does not hold, however, at higher frequencies, above 10 kHz, and extreme depths, in excess of 6 km, an operating regime that is within the capabilities of the most recently developed acoustic instrument platforms. To quantify the effects of attenuation, theoretical models for the vertical directionality and the spatial coherence of wind-generated ambient noise are developed in this paper, based on a uniform distribution of surface sources above a semi-infinite, homogeneous ocean. Since there are no bottom reflections, all the noise is downward traveling; and the angular width of the directional density function becomes progressively narrower with increasing frequency because sound from the more distant sources experiences greater attenuation than acoustic arrivals from overhead. This narrowing of the noise lobe modifies the spatial coherence, shifting the zeros in the horizontal (vertical) coherence function to higher (lower) frequencies. In addition, the attenuation modifies the amplitudes of the higher-order oscillations in the horizontal and vertical coherence functions, tending to suppress the former and enhance the latter. These effects are large enough to be detectable with the latest deep-diving sensor technology.

Time-angle ocean acoustic tomography using sensitivity kernels: The forward problem

Broadband acoustic signals around 1 kHz propagate through shallow water oceanic waveguides of ~100 m in depth and ~2 km in range as multiple ray-like wavefronts. These acoustic arrivals can be characterized by the following observables: travel-time (TT), direction-of-arrival (DOA), and direction-of-departure (DOD). By applying double-beamforming on the point-to-point signals recorded between two source-receiver arrays, the acoustic contribution of each arrival can be separated from the multi-reverberated data and the TT, DOA, and DOD observable variations are accurately measured. This study deals with the use of time-angle sensitivity kernels (TASK) to estimate the observable variations induced by sound speed perturbations in the waveguide. This approach is based on the first order Born approximation and takes into account the finite-frequency effects associated with wave propagation. The robustness the TASK approach is analyzed and compared to numerical parabolic equation simulations involving different sound speed perturbations. For example, parameters such as the perturbation location, the value and shape of the perturbation in the waveguide are modified. The combination of several perturbations and the influence of the source-receiver array apertures on the TT, DOA, and DOD estimates are also studied.

Acoustic observations of internal tides and tidal currents in shallow water

Significant acoustic travel-time variability and frequency shifts of acoustic intensity level curves in broadband signal spectrograms were measured in the East China Sea during the summer of 2008. The broadband pulses (270-330 Hz) were transmitted from a fixed source and received at a bottomed horizontal array, located at the 33 km range. The acoustic intensity level curves of the received signals indicate regular frequency shifts that are well correlated with the measured internal tides. Similarly, regular travel-time shifts of the acoustic mode arrivals correlate well with the barotropic tides and can be explained by tidal currents along the acoustic propagation track. These observations indicate the potential of monitoring internal tides and tidal currents using low-frequency acoustic signals propagating at long ranges.

Elastic coupled modes for range dependent propagation

The seismo-acoustic field in a range dependent fluid-elastic environment can be computed from elastic coupled modes. Range dependence breaks the strict mathematical orthogonality of the modes causing energy to be exchanged among the elements of the modal spectrum. The original theory is from Maupin (1988). Range-dependent propagation effects are illustrated for a 2-D model including a seamount. The mode coupling is directly proportional to the bathymetric slope of the range dependence. While the range-dependent environmental model is composed of vertical slices, no iteration is required to solve for both the transmitted and the reflected seismo-acoustic field, which is found by solving a matrix Riccati equation for the modal reflection and transmission matrices. A particular feature of the elastic coupled modes is the automatic inclusion of the interface Scholte wave modes, which are absent from any all-fluid environmental propagation model. The excitation of the Scholte modes may be important for observed deep shadow zone acoustic arrivals. The coupled mode results will be compared to the results from an elastic parabolic equation (PE). [Work supported by ONR.]