Isospeed Research Articles

Direct measurements of sediment geoacoustic properties in the New England Mud Patch and shelf breaka).

This paper reports on an original set of direct sound speed measurements collected with the acoustic coring system in the New England Mud Patch (NEMP) and shelf break area to the south. Cores collected within the NEMP show range-dependence of the mud with slower sound speed and lower attenuation on the west side. In the shelf break region, the highest sound speeds are observed between the 200- and 350-m isobaths. The depth-dependence of the mud layer in the NEMP includes a surficial layer with a negative sound speed gradient of 28 s-1. The remainder of the mud column has a weak positive sound speed gradient of 6.2 s-1 over an isovelocity layer. Comparison between in situ and ex situ sound speed measurements provides an assessment of the effects of sediment disturbance from gravity coring operations. Small differences in the upper 2.5 m were attributed to the changes in the geoacoustic properties caused by disturbance from the coring process. Below 2.5 m, the average difference is close to zero, suggesting that these sediments were minimally disturbed. Finally, an in situ measurement of shear speed was obtained near the depth of maximum penetration. The shear speed was well correlated with sound speed from approximately the same depth interval.

On the Dependence of Acoustic Modes on Media Parameters in the Pekeris-Airy Waveguide

A waveguide consisting of an isospeed water layer overlying a halfspace with a sound speed gradient is considered. The dependence of horizontal wavenumbers of normal modes in such a waveguide on the sound frequency, water depth and geoacoustic parameters of the bottom are investigated. It is shown that these dependencies are described by ordinary differential equations that can be solved numerically at very low computational cost providing a substantial increase in the efficiency of broadband modelling of sound propagation. The explicit formulae for the derivatives of horizontal wavenumbers with respect to the bottom parameters can be also used in dispersion-based geoacoustic inversion methods. This approach can be extended to the case of a waveguide consisting of several layers of Airy-type media. We also propose a new and concise proof that the spectrum of the Sturm-Liouville problem from which the modes are found for this waveguide is purely discrete.

Representation and interpretation about underwater sound speed gradient field in the GNSS-A observation

SUMMARY The Global Navigation Satellite System—Acoustic ranging combination technique (GNSS-A) is a seafloor geodetic observation technique that achieves an accuracy of centimetres by combining high-rate GNSS data with acoustic ranging. The technique determines the seafloor position by acoustic ranging between the sea surface and multiple seafloor stations, using GNSS data from the sea surface station. Here, the gradient state of the underwater sound speed structure (SSS) is a significant source of error. The open-source software GARPOS can reduce the effect from underwater gradient structures by retrieving the underwater disturbance as a parameter projected onto the sea surface and seafloor. To evaluate the effects of underwater disturbances, a quantitative comparison of the model parameters is necessary. In this study, we developed a representation method to evaluate features of the ocean field. Here, the expression method was described in the order of a formulation and an interpretation in the case of a 2-D cross-section and extension to the case of an actual 3-D field. This method makes it possible to evaluate SSS states in the GNSS-A observations. As an example, we showed the correlation between the anomaly of the expressed ocean state and the anomaly of the seafloor station position, showing that this expression method is an effective index for correcting bias errors. Additionally, we used the data from sites located in the Kuroshio area, a strong current near Japan, to show that the ocean state can be quantitatively interpreted using this expression method.

Subsurface acoustic ducts in the Northern California current system.

This study investigates the subsurface sound channel or acoustic duct that appears seasonally along the U.S. Pacific Northwest coast below the surface mixed layer. The duct has a significant impact on sound propagation at mid-frequencies by trapping sound energy and reducing transmission loss within the channel. A survey of the sound-speed profiles obtained from archived mooring and glider observations reveals that the duct is more prevalent in summer to fall than in winter to spring and offshore of the shelf break than over the shelf. The occurrence of the subsurface duct is typically associated with the presence of a strong halocline and a reduced thermocline or temperature inversion. Furthermore, the duct observed over the shelf slope corresponds to a vertically sheared along-slope velocity profile, characterized by equatorward near-surface flow overlaying poleward subsurface flow. Two potential duct formation mechanisms are examined in this study, which are seasonal surface heat exchange and baroclinic advection of distinct water masses. The former mechanism regulates the formation of a downward-refracting sound-speed gradient that caps the duct near the sea surface, while the latter contributes to the formation of an upward-refracting sound-speed gradient that defines the duct's lower boundary.

Overflow waters in the western Irminger Sea modify deep sound speed structure and convergence zone propagation

Deep sound speed structure in the western Irminger Sea is found to be highly dynamic in comparison to the adiabatic (uniform) sound speed gradient underpinning data assimilation and modeling efforts around the globe. A beamed source parabolic equation model is used to illustrate how the resulting non-uniform sound speed structure at 1 to 1.5 km in depth and sound speed inversion near the seafloor produce observable effects on acoustic signals between a shallow source and shallow vertical line array at convergence zone ranges. Beamforming analysis shows that a uniform sound speed gradient leads to “ideal” interference patterns that do not capture or represent modeled convergence zone properties, such as location, strength, and sharpness. Overall findings suggest that in situ information about sound speed below 1 km is necessary for low frequency, long-range propagation studies, particularly in areas of complex thermohaline circulation.

Anomalous reflection from a two-layered marine sediment.

This paper concerns the theory of acoustic reflection from a two-layered marine sediment, the upper layer of which consists of a fine-grained material (mud). The seawater above and basement below the layer are treated as homogeneous half-spaces. Within the mud layer, the density is taken to be constant, and three sound speed profiles are considered: uniform, linear, and inverse-square. The reflection coefficient exhibits a background component that is similar in all three cases, exhibiting only a weak sensitivity to the gradient of the profile, the frequency, and the depth of the layer. Additionally, the two profiles with a non-zero gradient, linear and inverse-square, exhibit a sequence across grazing angle of narrow spikes of total reflection. The angular distribution of this acoustic glint is highly sensitive to the frequency and depth of the layer, and mildly so to the gradient. As the gradient approaches zero, the glint vanishes and the reflection coefficient reduces identically to the form of a uniform sound speed profile. If it were detectable, the angular distribution of the glint, observed at several frequencies, could constitute a unique, sensitive set of "fingerprints," allowing the depth and sound speed gradient of the mud layer to be inferred.

Influence of the sound speed profile below the isothermal surface duct on the duct mode leakage rates

The isothermal surface duct is an important feature in the sound speed profile (SSP) in ocean, as it allows acoustic signals to propagate to long distances. Usually, such a duct is modeled as two intervals of sound speed with respect to depth with N 2-linear SSP approximating linear SSP. The interval above the duct boundary has a positive sound speed gradient over depth, while the interval below has a negative gradient. At the duct boundary the gradient normally has a discontinuity. In this paper, based on the author’s previously derived solution for the depth-separated Helmholtz equation in the environment with the N 2-exponential sound speed profile, he shows how this SSP can be used to model the smooth transition between the two intervals. Also, three SSPs with nearly identical sound speed between the duct boundary and the surface but with different profiles below the duct are considered, and energy leakage from the duct is calculated for the first three duct modes in a range of frequencies. It is shown that the shape of the sound speed profile below the duct can significantly affect the modal leakage rates.

Full-Bayes GNSS-A solutions for precise seafloor positioning with single uniform sound speed gradient layer assumption

A systematic analysis methodology for precise seafloor positioning using the GNSS-A has been constructed and implemented in the open-source software GARPOS. It introduces a linearized perturbation field model for extraction of the 4-dimensional sound speed variation, and solves the perturbation parameters simultaneously with the seafloor position based on the empirical Bayes approach. Although it can provide the solutions stably and almost analytically, it has less expandability when imposing additional nonlinear constraint parameters in the observation equation. Even though such parameters can be optimized by applying some information criteria, information on the details of the joint posterior probability would be lost and only the conditional posterior can be estimated. To overcome the above limitations, we implemented full-Bayes estimation using the Markov-Chain Monte Carlo algorithm. This approach can not only help evaluate the dependency of the existing constraint parameters on the seafloor position, but also let us discuss the effects of the additionally imposed constraints. We imposed a constraint under the assumption that a temporally-variable gradient layer steadily lies at a certain depth in the observation scale (typically < 10 km × 10 km, < 1 day). This models the cases with temperature gradients due to a large-scale structure such as the Kuroshio current or internal waves with long-wavelength. The constraint narrows the posterior of the horizontal position and provides a better solution for many datasets, especially in the Nankai Trough region. For the other datasets, the constraint emphasized bias errors, which can also provide information on the possibility of instrumental and modelling errors.

ACOUSTIC PROBING OF THE ANISOTROPIC STRUCTURE OF THE ATMOSPHERE

A new decomposition method (decomposition into N and U-waves) of infrasonic signals corresponding to partial reflection of N-wave sounding pulses from anisotropic atmospheric layers and recorded in areas of geometric shadow at large distances from explosions and volcanic eruptions is presented. The decomposition method makes it possible to determine the vertical gradients of the effective sound speed (sound speed plus wind speed in the direction of propagation) that are not available for determination by other methods.

Inferences of seabed characterization in a variable ocean environment

Dimensional reduction methods based on feature extraction are critical for statistical inference application in variable ocean environments. Presented are analyses for selected acoustic measurements made during the Seabed Characterization Experiment in 2021-2022. Broadband sources were deployed on the New England continental slope between the 200 and 400 m depth contours. The method of analysis includes first analyzing short range (higher grazing angles) acoustic measurements provided by both a broadband towed source and explosive sources. Features utilized for short-ranged data include those associated with seabed interferometry. Then, longer range data (lower grazing angles) are utilized to provide improved estimates of such quantities as the attenuations and sound speed gradients. Features utilized for long-ranged data include those associated with the dispersion of signals generated by explosive sources. As a general guide the mid-frequency band is utilized to infer characteristics about the upper portions of the sediment whereas the lower frequencies are utilized to infer the deeper portions of the seabed. [Work supported by Office of Naval Research.]

Mid-frequency sound propagation over a mud seabed with a compressional sound speed depth gradient

During Seabed Characterization Experiment 2017 in the New England Mudpatch under near isospeed conditions of the water column, observations were made of well-defined intensity striations in time-frequency spectrograms in the 1.5–4.0 kHz band. These observations provided the basis for the hypothesis that the compressional sound speed profile for a homogenous fine-grained sediment possessed a gradient in the upper portions of the seabed. This hypothesis was tested with a statistical inference method that extracts probability density functions for parameter values representing an empirical-based nonlinear profile whose origins go back to Hamilton. Inferred was a surface sound speed that is less than the sound speed of the water at the seafloor and a surface sound speed gradient of about 6 1/s. In 2022 the experiment was repeated during non-isospeed conditions of the water column. The results for 2017 and 2022 are compared with the intent of understanding the response of the geoacoustic properties of the upper portions of a fine-grained sediment to changes in the bottom water temperature and salinity of the water column. [Work supported by Office of Naval Research.]

Measuring camera information capacity with slanted-edges (Invited)

As Machine Vision (MV) and Artificial Intelligence (AI) systems are incorporated to an ever-increasing range of imaging applications, there is a corresponding need for camera measurements that can accurately predict the performance of these systems. At the present time, the standard practice is to separately measure the two major factors, sharpness and noise (or Signal-to-Noise Ratio), along with several additional factors, then to estimate system performance based on a combination of these factors. This estimate is usually based on experience, and is often more of an art than a science. Camera information capacity (C), based on Claude Shannon's ground-breaking work on information theory, holds great promise as a figure of merit for a variety of imaging systems, but it has traditionally been difficult to measure. We describe a new method for measuring camera information capacity that uses the popular slanted-edge test pattern, specified by the ISO 12233:2014/2017 standard. Measuring information capacity requires no extra effort: it essentially comes for free with slanted-edge MTF measurements. C has units of bits per pixel or bits per image for a specified ISO speed and chart contrast, making it easy to compare very different cameras. The new measurement can be used to solve some important problems, such as finding a camera that meets information capacity requirements with a minimum number of pixels, important because fewer pixels mean faster processing as well as lower cost.

Improvement of the flare evaluation for cameras and imaging applications when using near-infrared lighting

Near-infrared (NIR) light sources have become increasingly present in our daily lives, which led to the growth of the number of cameras designed for viewing in the NIR spectrum (sometimes in addition to the visible) in the automotive, mobile, and surveillance sectors. However, camera evaluation metrics are still mainly focused on sensors in visible lights. The goal of this article is to extend our existing flare setup and objective flare metric to quantify NIR flare for different cameras and to evaluate the performance of several NIR filters. We also compare the results in both visible and NIR lighting for different types of devices. Moreover, we propose a new method to measure the ISO speed rating in visible light spectrum (originally defined in the ISO standard 12232) and an equivalent ISO for NIR spectrum with our flare setup.

Adaptable Distributed Sensing in Coastal Waters: Design and Performance of the μFloat System

Buoyancy-controlled underwater floats have produced a wealth of in situ observational data from the open ocean. When deployed in large numbers, or “distributed arrays,” floats offer a unique capacity to concurrently map 3D fields of critical environmental variables, such as currents, temperatures, and dissolved oxygen. This sensing paradigm is equally relevant in coastal waters, yet it remains underutilized due to economic and technical limitations of existing platforms. To address this gap, we developed an array of 25 μFloats that can actuate vertically in the water column by controlling their buoyancy, but are otherwise Lagrangian. Underwater positioning is achieved by acoustic localization using low-bandwidth communication with GPS-equipped surface buoys. The μFloat features a high-volume buoyancy engine that provides a 9% density change, enabling automatic ballasting and vertical control from fresh to salt water ( 3% density change) with reserve capacity for external sensors. In this paper, we present design specifications and field benchmarks for buoyancy control and acoustic localization accuracy. Results demonstrate depthholding accuracy within ±0.2 m of target depth in quiescent flow and ±0.5 m in energetic flows. Underwater localization is accurate to within ±5 m during periods with sufficient connectivity, with degradation in performance resulting from adverse sound speed gradients and unfavorable spatial distributions of surface buoys. Support for auxiliary sensors (&lt;10% float volume) without additional control tuning is also demonstrated. Overall performance is discussed in the context of potential use cases and demonstrated in a first-ever array-based three-dimensional survey of tidal currents.

Sound speed profile estimation in a thick mud layer from travel time data

The sound speed profile in a surficial mud layer is of importance because it controls what is acoustically illuminated within and below the mud layer. A variety of estimates have been made at the New England Mud Patch (NEMP) with some near-surface sound speed gradients as large as 10 s−1 and larger. Here, single bounce seabed reflection travel time data are used to infer what mud sound speed gradients are and are not plausible near the central NEMP region where the mud thickness is ∼10 m. Time series data were collected on a moored hydrophone from a broadband source with a 1 second repetition rate as the tow ship transited on a radial from the hydrophone to a distance of ∼1 km at a speed of 2 m/s. The water column was essentially isovelocity and grazing angles ranged from 4.5° to 84°. It is known that at the base of the mud, there is a mud-sand transition layer (of order 1 m thick) where sound speed gradients can be large. However, above this layer, the travel time data clearly indicate that a mud 10 s−1 sound speed gradient is too large. [Research supported by ONR Ocean Acoustics Program.]

Scanning Doppler LIDAR wind profiles to inform near shore atmospheric acoustic propagation modeling

This work presents a comprehensive experimental system to measure concurrent atmospheric acoustic transmission loss and meteorological conditions. A three-dimensional scanning Doppler lidar wind profiler captures real-time wind speed gradients at many locations along the acoustic propagation path of a simple pitch catch style study. A long-range acoustic device on an anchored pontoon sends known chirp sequences to a seven-channel receiver array at the water’s edge at ranges up to approximately one kilometer. Additional synchronized meteorological observations include temperature, humidity, and wind measured with anemometers. The meteorological data stream is used to inform the sound speed gradient implemented in a parabolic equation based numerical model of atmospheric acoustic propagation. The model can account for sea surface roughness and accommodate a sound speed profile that changes along the propagation range. Model predictions are compared to measured transmission losses. An assessment of the value of the computational cost of incorporating the varying sound speed profiles in the model is presented.

Coupled effects of nonlinear internal gravity waves and seabed properties on underwater sound propagation

Underwater sound propagation can be affected by strong sound speed gradients induced by nonlinear internal waves in the ocean. Meanwhile, geoacoustic properties of the seabed control acoustic reflections. Experimental data collected at the shelfbreak on the northeast of the South China Sea were analyzed to investigate the joint effects. Both two-dimensional (2D) and three-dimensional (3D) numerical sound propagation models with realistic seafloor and oceanographic inputs have been established to study the nonlinear internal wave effects observed on hydrophone vertical line array moorings. Compared to the 3D model, the prediction errors and uncertainties of the 2D scheme neglecting the transversal/azimuth energy propagation are discussed. In addition, the impact of sediment properties on 3D propagation effects affecting underwater soundscapes in the area is investigated. [Work supported by the Office of Naval Research.]

Noninvasive acoustic time-of-flight measurements in heated, hermetically-sealed high explosives using a convolutional neural network

We present a data-driven technique for measuring the time-of-flight through material sealed within a container. Time-of-flight measurement provides a noninvasive means of quantifying the sound speed profile within a material by transmitting an acoustic burst and then measuring the time required for the burst to arrive at an opposing receiver. In a hermetically-sealed cylindrical container, a portion of the acoustic energy propagates through the material as a bulk wave, while the remainder of the acoustic energy propagates around the container walls as guided waves. As a result, interference from the guided waves obscures the bulk arrival, inhibiting measurement of the sound speed. The technique uses a Convolutional Neural Network (CNN) to identify critical features in the measured waveforms and identify bulk wave arrivals. We demonstrate this time-of-flight measurement technique on high explosive-filled containers as they are heated from room temperature to detonation. This is a particularly challenging application for acoustic time-of-flight measurements as the high explosives have significant sound speed gradients as they undergo heating, and they lead to significant attenuation of the bulk wave, as opposed to the guided waves, which do not suffer significant attenuation. We characterize the performance of the CNN as a function of the high explosive temperature and as a function of the CNN hyperparameters. We then provide physical insight into the error trends.

Statistical Inference of Sound Speed and Attenuation Dispersion of a Fine-Grained Marine Sediment

Acoustic recordings of signals in the 1.5–4.0-kHz band were analyzed for information about the sound speed and attenuation frequency dispersion of a fine-grained sediment found in the New England Mudpatch. Analysis of piston cores established prior bounds for a geophysical parameterization of a seabed model that predicts Kramers–Kronig dispersion relations. Sediment layers are described by the Buckingham viscous grain shearing (VGS) model that accounts for the effects of overburden pressure of compressional and shear speeds and attenuations. A statistical inverse problem was solved by using multiple samples of received levels recorded on two vertical line arrays as a function time and hydrophone depth for six frequencies in the 1.5–4.0-kHz band. A statistical inference model that assumed both model parameters and data samples are random variables quantified information content from marginalization of a conditional posterior probability distribution for the geophysical parameters that characterize the mud layer. From the inferred geophysical parameter point estimates the sediment sound speed and attenuation frequency dispersion are predicted and compared to previously reported direct measurements. Also, the predicted sound-speed gradient in the mud sediment from the VGS model is compared to a previous inference that utilized explosive sources.

Monitoring Tidal Currents and Macroturbulence in a High-Flow Tidal Channel Using a Kilometer-Scale Acoustic Travel-Time Instrument

A challenge in exploiting tidal energy resources is to determine the most suitable sites for development, and as the industry grows, to monitor the progressive impacts of tidal farms on the flow field and sediment transport regimes. A feasibility experiment was carried out with the fluvial acoustic tomography (FAT) system to assess the combined effects of irregular bathymetry, horizontal and vertical sound-speed gradients, and macroturbulence on the quality of FAT-estimated velocities. The experiment involved reciprocal transmissions at 30-s intervals for four days with two 7-kHz broadband transceivers. To determine the origin and potential interpretive value of the frequent multiple arrivals in the unstratified, highly-turbulent flow conditions, we performed a series of simulations with the Bellhop3D model. We demonstrate that the multiple arrivals mainly arise from out-of-plane sound propagation paths resulting from the effects of the local bathymetry. The maximum mean difference between FAT-estimated currents and the coincident moving-vessel acoustic Doppler current profiler (ADCP) transects is found to be less than 10%. Comparisons are also made between velocity spectra during the full flood and the ebb determined using the FAT velocities and the vertically-averaged velocities from a bottom-mounted ADCP deployed in a later experiment. The spectra exhibit similar shapes in the frequency and wave number overlap regions, indicating that FAT is able to detect macroturbulence. Good agreement is also obtained between the sound-speed estimates based on the travel time and those from <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> temperature and salinity data. Overall, the results demonstrate the potential of shallow acoustic tomography for monitoring environmental conditions in high-flow tidal channels.