Acoustic Arrivals Research Articles

Sources depth estimation for a tonal source by matching the interference structure in the arrival angle domain.

A publication by McCargar and Zurk [J. Acoust. Soc. Am. 133(4), EL320-EL325 (2013)] introduced a passive source depth estimation method for a moving tonal source with a vertical line array (VLA), utilizing the depth-dependent modulation in the arrival angle domain caused by the interference between the direct and surface-reflected acoustic arrivals. Under the isovelocity approximation, this method can estimate the depth of sources at close ranges, but the depth estimation error will increase with the increase in source range, as the impact of the sound speed profile on sound propagation is ignored. This paper presents a theoretical formula for calculating the modeled interference structure in the arrival angle domain with the knowledge of the sound speed profile. By matching the measured interference structure obtained from the beamforming of the acoustic data received by the VLA with the modeled structure under different assumed source depths, the tonal source depth estimation is achieved, even for sources at the remote part of the direct arrival zone. The performance of this method is verified by simulation data, as well as experimental data radiated from a towed source and a non-cooperative passing ship.

Using ocean ambient sound to sense arrival time fluctuations due to temperature

Ambient noise interferometry is a technique that uses coherent ambient sound measured at two separate locations to estimate the Green’s function between the sensors. Specifically, if the ambient sound is diffuse, the cross-correlation between sensors converges to the Green’s function between the sensors. In this talk, we apply the technique of ambient noise interferometry to two Ocean Observatories Initiative hydrophones that are separated by 3.2km and bottom mounted at a depth of 1500 m. It has previously been shown that these hydrophones can passively estimate several multi-path propagation peaks reliably throughout the 8 years that they have been recording ambient sound. We present an algorithm that estimates the arrival time of these peaks using the empirical Green’s function and compare these estimated arrivals to simulated acoustic arrivals. We demonstrate that the arrival times estimated with ambient sound show good agreement with the simulated results and have clear annual fluctuations due to seasonal temperature changes in the water column. We will discuss the future possibilities of extending this work for inversion based oceanographic measurements. [work supported by ONR.]

Observations and simulations of caustic formation due to oceanographic fine structure.

An at-sea experiment in deep water was conducted to explore the impact of small-scale sound-speed variability on mid-frequency (1-10 kHz) acoustic propagation. Short-range (1-5 km) acoustic transmissions were sent through the upper ocean (0-200 m) while oceanographic instruments simultaneously measured the ocean environment within 2 km of the single upper turning points of the acoustic transmissions. During these transmissions, acoustic receptions over a 7.875 m vertical line array show closely spaced, sometimes interfering arrivals. Ray and full-wave simulations of the transmissions using nearby sound-speed profiles are compared deterministically to the received acoustic signals. The sensitivity of the acoustic arrivals to the vertical scales of ocean sound speed is tested by comparing the observed and simulated arrival intensity where the sound-speed profile used by the simulation is smoothed to varying scales. Observations and modeling both suggest that vertical fine-scale structures (1-10 m) embedded in the sound-speed profile have strong second derivatives which allow for the formation of acoustic caustics as well as potentially interfering acoustic propagation multipaths.

Deep ocean long range underwater navigation with ocean circulation model corrections.

An underwater navigation algorithm that provides a "cold start" (CSA) geographic position, geo-position, underwater while submerged using travel times measured from a constellation of acoustic sources is described in Mikhalevsky, Sperry, Woolfe, Dzieciuch, and Worcester [J. Acoust. Soc. Am. 147(4), 2365 - 2382 (2020)]. The CSA geo-position is used as the receive position in the ocean for acoustic modeling runs using an ocean general circulation model (GCM). A different geo-position is calculated using adjusted ranges from the travel time offsets between the data and modeled arrival times for each source. Because the CSA geo-position is close to the true position, the source to CSA position propagation model path and the source to true vehicle position data path of the acoustic arrivals are nearly coincident, enabling accurate measurement of travel time offsets. The cold start with model (CSAM) processing reduced the CSA geo-position errors from a mean of 58 to 25 m. A simulation is developed to estimate CSA and CSAM performances as a function of group speed variability between the source paths. The CSAM geolocation accuracy can be calculated from and is controlled by the accuracy of the GCM.

Surficial sediment sound speed estimation from wide-angle multipath arrivals.

Multipath acoustic arrivals on fixed receivers from a moving source in an underwater waveguide include wide-angle bottom reflections that are affected by sediment properties. Hence, one can use them as input data for seabed geoacoustic parameter estimation. In this study, the effects of seabed properties on such measurements are theoretically analyzed. The result shows that one can utilize time delays and source distances at the critical angle points of multipath arrivals to determine the sediment sound speed. The effect of sediment attenuation is also discussed, and the proposed method is demonstrated with experimental data.

Low-order acoustic mode arrivals in the Beaufort Duct

Significant changes in the stratification of the Beaufort Sea over the last few decades have produced a subsurface duct located between 100- and 300-meters depth, known as the Beaufort Duct. This subsurface duct allows for long-range acoustic transmission with little to no interaction with the sea surface or seafloor. In August and September of 2017, acoustic transmissions from five active moored tomography sources were collected at ranges up to 530 km by two Seagliders along with in-situ environmental measurements. Sound-speed profiles from the Seaglider data were used as input for parabolic equation and normal mode predictions. Both the predictions and recorded acoustic data show a peak acoustic arrival prior to the final cutoff. We refer to this as a “foldover” feature in the acoustic timefront, and it can be connected back to the unique ducting features in the input sound speed profiles. The relationship between the extent of the foldover and the shape of the sound-speed profile in the duct is explored using normal modes. Modal group speed predictions for the low-order modes are used to understand which modes make up the foldover feature present in the acoustic timefront and to interpret the acoustic arrival patterns measured on the Seagliders.

Acoustic receptions at close ranges measured on autonomous underwater vehicles in the Beaufort Sea

Rapid warming of the Pacific Summer Water layer strengthens a subsurface duct in the Beaufort Sea, allowing for long-range propagation at low frequencies. An array of tomography sources was deployed within the duct as part of the Canada Basin Acoustic Propagation Experiment (CANAPE) to study acoustic propagation in this environment. The moored transceivers provide measurements of acoustic propagation at several ranges from 176 to 285 km. Additionally, two Seaglider vehicles equipped with hydrophone receivers navigated in and around the CANAPE array and recorded the transmissions from the moored sources at ranges as far as 530 km and as close as 2 km. A spatially variable sound speed environment was generated from in-situ data measured by the Seagliders and CTD casts from research vessels. Acoustic arrivals measured on the vehicles were matched to range-dependent acoustic predictions made with a broadband Parabolic Equation model to estimate source-receiver range. Acoustic receptions from multiple moored sources were used to localize the Seagliders. Here, we examine the close range (2–25 km) receptions and their impacts on acoustic localization.

Automated matching of long range underwater acoustic arrivals to predictions using a minimum variance Approach

An automated method was developed to align underwater acoustic measurements taken at various depths and ranges to a reference model of acoustic arrival structure. Data used to demonstrate the method were collected by four autonomous underwater vehicles deployed in the Philippine Sea as part of an ocean acoustic tomography experiment. The arrivals were measured in the upper 1000 m of the ocean at ranges spanning several hundred kilometers from 5 moored acoustic tomography sources. The primary objective is to accomplish automatic source-receiver ranging by aligning measurements of long range acoustic arrivals to a single reference model. Acoustic arrival time structure for pulse compressed signals at long ranges is relatively stable, yet real ocean variability presents challenges in acoustic arrival matching. The presence of internal waves scatters the acoustic arrival structure and can introduce spurious arrivals in the measured data. This method takes advantage of simple projections of the measured structure onto the model space with constraints informed by scattering statistics consistent with the Garrett Munk internal wave energy spectrum. Compared to manual matching of the measured arrivals to eigenray models, more than 90% of the automatically obtained range estimates were within 150 m of the manually obtained range estimates.

Acoustic modeling in the Beaufort Sea using measurements and models of ocean sound speed

Environmental changes in the Arctic over the last few decades have resulted in a subsurface sound speed duct located between 100- and 300-meters depth, known as the Beaufort Duct, which allows for long range acoustic propagation with little or no interference from the ocean surface or bottom. In 2017, two Seagliders traversed between five moored active acoustic sources transmitting linear frequency modulated (LFM) sweeps with frequencies around 250 Hz. During the experiment the Seagliders recorded acoustic arrivals and measured temperature, salinity, and pressure. These environmental measurements are used to calculate sound speed for use as an input to forward acoustic propagation models, including rays and broadband parabolic equation predictions of acoustic time fronts. Results are compared with acoustic propagation predictions based on sound-speed profiles from ocean models as well as measured acoustic data. The measured and modeled acoustic arrival times along with the calculated sound-speed profiles of the region are used to explore the inverse problem.

Seismo-Acoustic Characterization of Mount Cleveland Volcano Explosions

Volcanic explosions can produce large, ash-rich plumes that pose great hazard to aviation, yet may often have few precursory geophysical signals. Mount Cleveland is one of the most active volcanoes in the Aleutian Arc, Alaska (United States) with at least 65 explosions between December 2011 and June 2020. We characterize the seismo-acoustic signals from explosions at Mount Cleveland over a period of 4 years starting in 2014 when the permanent local instrumentation was installed. While the seismic explosion signals are similar, the acoustic signals vary between explosions. Some explosion acoustic waveforms exhibit a single main compressional phase while other waveforms have multiple compressions. The time lag between seismic and acoustic arrivals varies considerably (up to 2.20 s) at a single station ∼3 km from the vent, suggesting a change in propagation path for the signals between explosions. We apply a variety of methods to explore the potential contributions to this variable time lag from atmospheric conditions, nonlinear propagation, and source depth within the conduit. This variable time lag has been observed elsewhere, but explanations are often unresolved. Our results indicate that while changes in atmospheric conditions can explain some of the variation in acoustic arrival time relative to the seismic signal arrivals, substantial residual time lag variations often still exist. Additionally, nonlinear propagation modeling results do not yield a change in the onset time of the acoustic arrival with source amplitudes comparable to (and larger) than Cleveland explosions. We find that a spectrum of seismic cross-correlation values between events and particle motion dip angles suggests that a varying explosion source depth within the conduit likely plays a dominant role in the observed variations in time lag. Explosion source depths appear to range from very shallow depths down to ∼1.5–2 km. Understanding the seismo-acoustic time lag and the subsequent indication of a variable explosion source depth may help inform explosion source modeling for Mount Cleveland, which remains poorly understood. We show that even with a single co-located seismic and acoustic sensor that does not always remain on scale, it is possible to provide meaningful interpretations of the explosion source depth which may help monitoring agencies understand the volcanic system.

Automated matching of measured long-range acoustic arrivals from autonomous gliders with acoustic predictions

The acoustic arrival matching localization method estimates the range of a mobile receiver from a fixed broadband acoustic source by matching the arrival with an acoustic prediction of the received arrival at a known range, calculating an offset from the known range, and combining the estimates of range from multiple sources using least-squares to estimate subsurface position. This is a labor-intensive task performed in post-processing, after recovery of the mobile platform. Automation of this matching is not straightforward due to the presence of spurious arrivals and other variations in the measured arrivals compared with predictions. Automated methods for estimating the range errors and performing ray identification for specific peak arrivals were developed using both programmatic and machine learning approaches. These methods take advantage of the temporal structure of the measured arrivals and simultaneously estimate range error and resolve measured peaks by comparing against a reference acoustic propagation model that describes the progression of the acoustic arrival structure in depth, range, and reduced travel time. The programmatic and machine learning approaches will be tested on range dependent ray simulations and evaluated in the context of a dataset collected using Seagliders during the North Pacific Acoustic Laboratory (NPAL) Philippine Sea Experiment.

Modeling and measurement of relative intensity of acoustic arrivals within the beaufort duct

The Beaufort Duct, located at approximately 175 m depth in the Canada Basin, allows for long-range acoustic propagation from sources moored within the duct. As part of the Canada Basin Acoustic Propagation/Glider Experiment (CANAPE/CABAGE), LFM sweeps with frequencies around 250 Hz were transmitted by broadband moored sources and received by two Seagliders traversing between them. In addition to receiving acoustic data, these Seagliders measured temperature, pressure, and salinity, providing measurements of the sound speed profile throughout the CANAPE study region. These profiles were used as inputs for broadband acoustic parabolic equation predictions. Measured data receptions within the duct show intense peak arrivals prior to the final cutoff that are higher in relative intensity than predicted. The duct properties of the measured sound speed profiles are parameterized and acoustic predictions using actual profiles as well as idealized profiles are used to study the predictability of these intense peaks within the duct.

The 12 December 2017 Baumgarten Gas Hub Explosion: A Case Study on Understanding the Occurrence of a Large Infrasound Azimuth Residual and a Lack of Seismic Observations

The Baumgarten explosion occurred on 12 December 2017 at a gas storage site about 30 km east of Vienna, Austria. Acoustic arrivals from this accidental surface explosion were detected at dozens of stations of the AlpArray seismic network to distances up to 150 km, mainly in easterly directions. Thus it was expected that the Hungarian infrasound array PSZI located about 230 km to the east-southeast of Baumgarten would detect this acoustic wave as well. Standard progressive multichannel correlation processing and frequency-wavenumber analysis identified a signal emerging at 7:57:55 UTC from an azimuth of 296°–300° and with trace-velocity > 400 m/s. The extraordinarily high trace-velocity and excessive backazimuth residual, relative to the explosion site direction of 282°, however cast strong doubts on the arrival’s connection to the Baumgarten event. Accounting for the effect of non-planar geometry of the infrasound array results in a reduction of the azimuth residual by half. Additionally, 2D and 3D raytracing methods are used including the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric model to further explain the remaining azimuth residual as well as to elucidate the large trace velocity estimates. The prevailing stratospheric winds in excess of 150 m/s are identified as the underlying cause. Including both factors the initial azimuth residual of up to 18° decreases to ~ 4°, allowing to associate the infrasound signal at PSZI with the Baumgarten event. Finally, the data from a seismic station at 30 km range is re-investigated for magnitude estimation. The local magnitude of ML < 1.0 explains well the scarcity of seismic observations within 50 km range, where three or four stations show signals, mainly consisting of Rg-type surface waves, but no body waves.

Passive broadband source depth estimation in the deep ocean using a single vector sensor.

For an acoustic receiver deployed at the bottom of the direct arrival zone of a submerged source at short horizontal ranges in deep ocean, the interference pattern of the direct and surface-reflected acoustic arrivals shows periodic modulation, which is directly related to the source depth, source frequency, and vertical arrival angle. In this work, the interference cycle presented in the frequency domain is used to extract the broadband source depth, with the vertical arrival angle obtained from the ratio of vertical acoustic intensity and horizontal acoustic intensity from the signal recorded by a single vector sensor. Experimental results demonstrate the source depth estimation without requiring knowledge of the ocean environment.

Using a regional ocean model to understand the structure and variability of acoustic arrivals in Fram Strait

A regional ocean model for Fram Strait provides a framework for interpretation of the variability and structure of acoustic tomography arrivals. The eddy-permitting model (52 vertical levels and 4.5 km horizontal resolution) was evaluated using long-term moored hydrography data and time series of depth-range averaged temperature obtained from the inversion of acoustic tomography measurements. Geometric ray modeling using the ocean model fields reproduces the measured arrival structure of the acoustic tomography experiment. The combination of ocean and acoustic models gives insights into acoustic propagation during winter and spring. Moreover, overlapping arrivals coming from different vertical angles can be resolved and explained. The overlapping arrival of purely refracted rays and surface-reflected/bottom-reflected (SRBR) rays has implications for the inversion of tomography data in Fram Strait. The increased knowledge about the ray-length variations of SRBR rays is valuable for choosing appropriate observation kernels for the data assimilation of acoustic tomography data in Fram Strait.

Operational ocean acoustic tomography

The estimation of sound speed fields using ocean acoustic tomography (OAT) in operational systems is limited because user intensive post processing and manual association of arrivals with ray paths are required. Furthermore, inversion techniques typically rely on CTD casts or historical data to create reference sound speed profiles and Gaussian statistical characterizations for the inversion, which may be either unavailable, out of date, or too approximate due to the intermittent ocean dynamics. A method is presented for real-time implementation of OAT, including the tracking of acoustic arrivals, model-data association of ray paths to arrivals (ray identification), and tomographic inversions. One- and two-dimensional tomographic inversions for sound speed fields are performed using stochastic ensemble forecasts of the sound speed field generated via the Multidisciplinary Simulation, Estimation, and Assimilation Systems (MSEAS) Primitive-Equation probabilistic ocean modeling system. The method is demonstrated on simulated and real data obtained from moored tomographic sources and receivers, deployed in deep water 100 km South of Nantucket, MA. [This research was developed with funding from the Defense Advanced Research Projects Agency (DARPA). ]

Analysis of the conditions for the occurrence of "the voice of the sea" using infrasound observations in marine waters

We study the conditions for the occurrence of "the voice of the sea" in the infrasound range can occur and determine its parameters from infrasound measurements in the Black Sea water area conducted in 2011 and 2016 and in the water area of the Sea of Okhotsk in 2017. To this end, we compared the observations of different parameters (mean correlation, spectra of acoustic arrivals, direction and phase velocity) of the high-frequency infrasound (1–10 Hz), recorded in Katsiveli (Crimea) in 2011 and 2016. We performed a detailed study of the wind conditions in the Black Sea water area during the measurement period, as well as the conditions for the propagation of acoustic waves along the direction of their arrivals.&#x0D; In both cases the atmospheric vortices in the direction of the arrival of infrasound were detected, which caused changes in wind velocity vector above the sea surface. The infrasound recorded in 2011 was caused by the interaction of two differently rotating atmospheric vortices over the sea. In 2016, a vortex was observed to the West from the registration point. The possibility of generating "the voice of the sea" due to the rotation of the wind velocity, which causes a nonlinear interaction of surface waves propagating in opposite directions to each other, and the formation of their 2nd harmonic in the form of a standing surface wave is discussed.&#x0D; From the analysis of the wind speed and wind direction profiles along the infrasound arrival path, as well as the acoustic pressure fields calculated by the parabolic equation method for the effective sound speed profiles in the direction of infrasound propagation, the most probable areas of infrasound generation were determined. In both cases these areas coincided with the zones in which the wind speed drops to zero, and the direction of the wind changes to the opposite.&#x0D; An example of simultaneous detection from one direction of the microbaroms with frequencies 0.2–0.3 Hz and "the voice of the sea" with a higher frequency of 5.5 Hz is given.

Localization of a mobile acoustic scatterer from sub-Rayleigh resolved acoustic arrivals in a refractive environment

Computational Bayesian inference on the state of an underwater mobile object from a continuous active acoustic transmission is presented. The challenge of sub-Rayleigh resolvable wave vectors in refractive environments with uncertainty in ambient acoustic noise power is addressed. The location and speed of the mobile scatterer are inferred under the challenging constraint of a small receive vertical aperture. The need for jointly inferring the vertical angles and Doppler offsets of the arrivals is addressed with a Gibbs sampling approach. The posterior density of the plane wave components is mapped to the object's range, depth, and speed through ray interpolation. A case scenario from an acoustic duct environment in the western Indian ocean is presented.

Travel-time correction and preliminary results for ocean acoustic tomography in South China Sea

An acoustic tomography trial experiment was conducted in South China Sea during May to August in 2016. Two moorings are installed apart from about 56.94 km, while each consists of one low frequency source, 20 hydrophones deployed from the depth of about 400 m to 1600 m, total 32 depth sensors and 3 compass and tilt sensors. Due to internal waves and currents in this area, as a typical value, horizontal drift of a mooring can reach 300 m, thus moorings drift need to be considered to correct ray travel-time. In this paper, the shape of a mooring is estimated firstly and locations of all hydrophone array elements are then calculated and finally used to determine travel-time perturbation of acoustic arrivals. The mooring is modelled as 2 curves, while the end of the mooring is fixed at the cement anchor on the sea floor. Optimization is used to acquire hydrophone location inferential solution. The inferred shape of hydrophone array and element locations are used to correct the travel-times measured in the experiment. We find that corrected travel-times match the trend of the change of sound speed profile better in the sea. Finally, the corrected travel-times are used to tomography of sound speed profile. AR (Autoregressive) process is used to describe the dynamic evolution of sound speed profile and Kalman filter is applied in the sequential estimation. The performances of the time-independent method and the method using AR process and Kalman filter are compared, reasonably the latter is better than the former in particular with abundant measured data.

Using a regional ocean model to understand the structure and sampling variability of acoustic tomography arrivals in Fram Strait

A regional ocean model for Fram Strait allows to understand the variability and structure of acoustic tomography arrivals. The eddy-permitting model (52 vertical layers and 4.5 km horizontal resolution) was evaluated using long-term moored hydrography data and time series of depth-range averaged temperature obtained from the inversion of acoustic tomography measurements. Geometric ray modelling on the ocean model fields reproduces the measured arrival structure of the acoustic tomography experiment. The combination of ocean and acoustic model gives insights into acoustic propagation during winter and spring. Overlapping arrivals coming from different vertical angles can be resolved and explained. The overlapping arrival of sound channel rays and bottom-reflected rays has implications for the inversion of tomography data in Fram Strait. The increased knowledge about the ray-length variations of bottom-reflected rays is valuable information for choosing appropriate observation kernels for the data assimilation of acoustic tomography data in Fram Strait.