Mapping of surface-generated noise coherence

Shallow water is a complex sound propagation medium, which is affected by the varying spatial–temporal ocean environment. Taking this complexity into account, the classical processing techniques of source localization and environmental inversion may be improved. In this work, a joint tracking approach for the moving source and environmental parameters of the range-dependent and time-evolving environment in shallow water is presented. The tracking scheme treats both the source parameters (e.g., source depth, range, and speed) and the environmental parameters (e.g., water column sound speed profile (SSP) and sediment parameters) at the source location as unknown variables that evolve as the source moves. To counter sample impoverishment and robustly characterize the evolution of the parameters, an improved particle filter (PF), which is an extension of the standard PF, is proposed. Two examples with simulated data in a slowly changing environment and experimental data collected during the ASIAEX experiment are utilized to demonstrate the effectiveness of the joint approach. The results show that we were able to track the source and environmental parameters simultaneously, and the uncertainties were evaluated in the form of time-evolving posterior probability densities (PPDs). The performance comparison confirms that the improved PF is superior to the standard PF, as it can reduce the parameter uncertainties. The tracking capabilities of the improved PF were verified with high accuracy in real-time source localization and well-estimated rapidly varying parameters. Moreover, the influence of different particle numbers on the improved PF tracking performance is also illustrated.

Estimating the sound speed profile by matching the acoustic intensity striation using a mounted horizontal line array is investigated. Due to constructive and destructive interference between normal modes, the acoustic intensity manifests range-frequency striation structure in shallow water. Using the environment and acoustic data measured during a tomography experiment conducted in the South China Sea in June, 2010, the possibility and validity of inverting for the water column sound speed profile by matching the acoustic intensity striations are studied. The results show that the fluctuation of the sound speed profile has obvious influence on the acoustic intensity striations. The acoustic data recorded in an experiment conducted in the South China Sea in June 2010 are analyzed to invert for the sound speed profiles. The sound speed profile is represented by Empirical Orthogonal Functions (EOFs) to reduce the unknown parameters. The estimates agree well with the measured sound speed profiles and most of the standard deviations are less than 1 m/s.

A quasi-analytical three-dimensional (3D) normal mode model for longitudinally invariant environments can be used to compute vertical noise coherence in idealized ocean environments. An examination of the cross modal amplitudes in the modal decomposition of the noise cross-spectral density shows that the computation can be simplified, without loss of fidelity, by modifying the vertical and horizontal mode sums to exclude non-identical mode numbers. In the special case of a Gaussian canyon, the across-canyon variation of the vertical wave number associated with each mode allows a set of horizontally trapped modes to be generated. Full 3D and Nx2D parabolic equation sound propagation models can also be used to calculate vertical noise coherence and horizontal directionality. Intercomparison of these models in idealized and realistic canyon environments highlights the focusing effect of the bathymetry on the noise field. The absolute vertical noise coherence increases, while the zero-crossings of the real component of the coherence are displaced in frequency when out-of-plane propagation is accounted for.

Generally, most inversion approaches model the seabed as a stack of range-independent homogeneous layers with unknown geoacoustic parameters and layer numbers. In our previous study, we established a layered geoacoustic seabed model based on sub-bottom profiler data to characterize low-frequency (100–500 Hz) airgun signal propagation at short ranges (0–20 km). However, when applying the same model to simulate high-frequency (500–1000 Hz) explosive sound signal propagation, it failed to adequately reproduce the observed significant transmission loss phenomenon. Through systematic analysis of transmission loss (including water column sound speed profiles, seabed topography, and sediment properties), this study proposes a range-dependent layered geoacoustic model using the Range-dependent Acoustic Model–Parabolic Equation (RAM-PE). Stepwise inversion implementation has successfully explained the observed experimental phenomena. To generalize the proposed model, this study further introduces a trans-dimensional inversion framework that automatically resolves sediment property interfaces along propagation paths. The method effectively combines prior information with trans-dimensional inversion techniques, providing improved characterization of range-dependent seabed environments.

Measurements of the vertical coherence of ambient noise in shallow water off the east coast of Korea are presented. The noise was received by a 15-m-length vertical line array covering the water column from 96.5 to 111.5 m, which was deployed in waters 160 m deep. The level of the power spectral density for frequencies less than 300 Hz was relatively low, suggesting that the contribution of the shipping noise to the overall noise level was negligibly small. The results of the measured coherence imply that the noise field was fairly symmetrical about the horizontal but not isotropic. The vertical coherence of the wind-generated sea-surface noise is compared to theory, without the consideration of shipping noise. The comparison suggests that the primary source of ambient noise in the experimental site was wind-driven sea-surface noise. A small discrepancy is found in frequencies above approximately 3 kHz. Possible causes for this are discussed.

In December 2013, an experiment using an autonomous underwater vehicle (AUV) as a moving source for water column sound speed profile (SSP) inversion was conducted in Mogan Lake, China. With in-situ conductivity, temperature, and depth (CTD) measurements, the empirical orthogonal functions were first constructed to represent the SSP. Radiated noises of the AUV were received on a vertical line array and processed via acoustic field matching to obtain estimates of the range-independent SSP, along with the position and velocity of the AUV and the water column depth. The frequency of the radiation noise of the AUV was approximately 14 kHz. At such high frequency, even though the source motion was slow, a significant Doppler shift/broadening was observed. To incorporate the Doppler effects to better match the measured data, a forward acoustic model is derived based on the waveguide Doppler and normal mode theory. An analytical solution of the forward model is obtained for arbitrary signal integration intervals with a monochromatic source, moving radially relative to the receiver. Through simulations and experimental data processing, the feasibility of using an AUV source for water column SSP inversion has been demonstrated; it is also shown that the waveguide Doppler model is more effective compared with the model that does not consider the waveguide Doppler effect.

This work attempts to exploit the difference in sound speed between oil and seawater to determine the location of large underwater oil plumes. Two perturbative inversion schemes, both based on the work of Rajan, et al. [JASA 82, 998–1017 (1987)] will be presented. Both methods can be used to estimate sound‐speed in the water column as a function of range and depth. The first technique uses horizontal wavenumber data which are estimated from the complex pressure field of a narrow band signal. The inversion result provides an estimate of sound speed profile along a two‐dimensional slice through the environment. The method is demonstrated using data from the Shallow Water 2006 (SW06) experiment. The second technique utilizes modal travel time data. When applied in a multi‐sensor deployment, the outputs of this inversion scheme provide a three‐dimensional characterization of the coastal environment. Furthermore, because sound propagates at a much faster speed than oil spill spreads, this technique can be used to track changes in the size and shape of the oil plume over time. [Work supported by ARL:UT IR&D]

Buoyancy-driven underwater gliders are autonomous underwater vehicles that were originally developed to collect oceanographic data. CMRE is studying the use of this technology for the characterization of denied areas, including alternate sensor payloads and applications. During the Rapid Environmental Assessment phase of the Noble Mariner 2012 NATO exercise, Conducted in Gulf of Lions in September 2012, an omnidirectional hydrophone was mounted on a shallow water glider to sample the spatial distribution of the acoustic and oceanographic fields at different ranges and depths. This paper presents a study of the potential to localize acoustic sources by using the acoustic and environmental data collected by the glider. During the experiment, a bottom moored acoustic source was deployed in an area with benign bathymetry. Continuous wave and frequency modulated pulses were broadcast for approximately 6 h. The glider was flying along predefined tracks and the distances from the source were typically from 5 to 9 km. A Ray tracing model is used to evaluate the arrival structures of the acoustic signal, and to estimate the source location. The impact of the range dependent water column sound speed profile on the uncertainty of the source localization is also discussed.

Noise due to ship traffic is a pervasive component of the global and local ocean noise from 10 to 500 Hz, but little is known of its effects on marine mammals because such effects depend on the sound exposure levels at a given location. The approach presented here consists of modeling the sound field due to various spatial distributions of noise sources located 5–10 m below the sea surface. We approximate a three‐dimensional sound field by computing the sound propagation from each source along a prescribed number of two‐dimensional vertical radial slices of a range‐dependent environment, which includes bathymetry, seafloor geoacoustic properties, water column sound speed profiles, and sea surface roughness. Propagation results from all sources are incoherently summed to form the overall sound exposure map. By varying the density of ships in a given area we can simulate the effects of shipping lanes and assess how the characteristics of the underlying seafloor alter the results.

Seismic study of transition zones in Arctic regions in summer is troublesome because of the presence of large areas covered by shallow waters like bays, lakes, rivers, their estuaries and so on. The winter is more convenient and essentially facilitates logistic operations and implementation of seismic acquisition. But in winter there is a complicating factor - intensive seismic noise generated for acquisitions installed on the ice covering shallow waters. It is well-known that this noise is connected with flexural waves generated in ice by seismic sources. These waves are one of the strongest known coherent noises. At the same time they are much slower than surface waves well known for onshore acquisition and seem to be easy avoided by f-k filtration. However, this type of filtration fails to suppress such noise. To understand the matter the representative series of numerical experiments are conducted and prove that the main impact to noise is multiple conversions of flexural waves to the body ones and vice versa. Ways to reduce this noise are proposed and discussed.

The transverse structure in the current and salinity distributions across the mouth of Delaware Bay are examined over the tidal and subtidal time scales. Results show that the across-estuary variation in bathymetry, in the form of a channel-shoal configuration, has a very significant impact on the characteristics of transverse variability. The mean current over the channel is characterized by a strong outflow of low salinity water in the upper layer and a strong inflow of high salinity water in the lower layer, consistent with the density-induced gravitational circulation. The mean flow pattern in the shallow waters over the shoals is marked by transverse rather than vertical variation. The subtidal current and salinity fluctuations are primarily driven by the effect of local atmospheric forcing. The subtidal current fluctuations in the upper layer of the channel are frictionally coupled to the local wind, resulting in downwind currents. The subtidal current fluctuations in the lower layer of the channel, however, flow in the direction of local setup and against the wind. With a wind blowing down the estuary, the wind-induced current tends to reinforce the two-layer structure of the gravitational circulation and substantially enhance the vertical shear and surface to bottom salinity difference. The reverse occurs with a wind in the up-bay direction. The subtidal currents in the shallow areas to the right of the channel exhibit largely depth-independent response to the effect of local wind, with downwind currents at both the surface and the bottom. At tidal frequencies the currents show only a modest variation across the bay mouth. Tidal currents are highly deterministic, but the characteristics of the tidal variability in salinity exhibit significant changes over long time scales. These long-term changes in the intratidal salinity variability are caused by the nonlinear interactions between the tidal and subtidal motions. The residual salt flux through the bay mouth shows significant subtidal fluctuations. The leading factor responsible for producing such subtidal fluctuations is the advection of salt by the wind-induced subtidal currents, but the effect of tidal pumping also contributes significantly to the overall residual salt flux into the estuary.

To study current tomography in very shallow water regions a simultaneous oceanographic and broadband (1-25 kHz) acoustic experiment was conducted in the Delaware Bay. The mean water depth was 15 m and the source-receiver range was 760 m. In this paper, we discuss the feasibility of using reciprocal acoustic transmission for current tomography applications. A beamforming technique is used to resolve the arrival time of direct and surface-bounced rays since in shallow water the received acoustic signals are more complicated due to multiple interactions with bottom and sea surface. Using the experimental data, the accuracy of travel time measurements for variable environmental conditions is examined for different center frequencies and bandwidths. The current velocity prediction results are compared with ADCP measurements to determine the feasibility of current tomography in shallow water.

The Arctic Ocean is undergoing dramatic changes in both ice cover and ocean structure. The Canada Basin Acoustic Propagation Experiment (CANAPE), which includes both deep and shallow water components, was designed to understand the effects of changing Arctic conditions on low-frequency propagation and ambient noise. The deep-water component, which is reported on here, includes a yearlong experiment in the Canada Basin during 2016–2017, preceded by a short Pilot Study during July–August 2015. During 2016-2017, a Distributed Vertical Line Array (DVLA) receiver with 60 Hydrophone Modules was moored within a six-element acoustic transceiver array with a 150-km radius. Environmental measurements on the DVLA include 28 Sea-Bird MicroCATs and upward- and downward-looking Acoustic Doppler Current Profilers (ADCPs) located below the hydrophone array. The acoustic transceivers had sources at 175-m depth and 15 Hydrophone Modules located above the sources. Environmental measurements on the transceiver moorings include ice-profiling sonars, upward-looking ADCPs on the subsurface floats, and 10 temperature sensors located below the acoustic transceivers. The one-year deployment provides measurements at least partially in open water during summer, in the marginal ice zone (MIZ) as it transitions across the array during the spring and autumn, and under complete ice cover during winter.

The aim of this paper is to estimate possibilities of matched-field geoacoustic inversion (MFI) in shallow water, and to recommend optimal arrangements of signal source and hydrophone array in variety of particular environments. We assumed ocean to be range independent with bottom consisting of homogeneous liquid layers. Sound fields were calculated as superposition of normal modes and continuous spectrum for tonal point source and vertical line array. MFI based on Bartlett processor was used. Possibilities of MFI were characterized by MFI penetration depth, sensitivity to various bottom parameters and non-uniqueness of inverted data. These characteristics were analysed as functions of frequency and the source depth and range for different values of sound attenuation in sediments and for various sound-speed profiles. To estimate possibilities of MFI in real ocean conditions, influence of array tilt and inadequacy of geoacoustic model were analysed. The influence of continuous spectrum was also discussed. Among the major results, optimal source ranges corresponding to the maximum penetration depth of MFI were calculated, and high influence of attenuation in sediments on possibilities of MFI was revealed.

Influence of internal wave induced sound speed variability on acoustic propagation in shallow waters of North West Bay of Bengal