Isospeed Research Articles

3-D acoustic propagation through an estuarine salt wedge

The estuarine salt wedge presents a dynamic and highly refractive waveguide, the acoustic propagation characteristics of which are controlled by the water column sound speed gradient and boundary interactions. Acoustically, the salt wedge consists of two isospeed layers separated by a thin, three-dimensional, high-gradient layer. The behavior of a broadband (500–2000 Hz) acoustic field under the influence of an estuarine salt wedge in the Columbia River estuary is explored using two 3-D acoustic propagation models: 3-D rays and 3-D parabolic equation (3DPE). These model results are compared to data collected during a field experiment in 2013. Results demonstrate that the dominant physical mechanism controlling acoustic propagation in this waveguide shifts from 3-D bottom scatter in a non-refractive waveguide (before the entrance of the salt wedge) to 3-D acoustic refraction with minimal bottom interaction in a refractive waveguide (when the salt wedge occupies the acoustic transect). Vertical and horizontal refraction in the water column and out-of-plane scattering by the bottom are clearly evident at specific narrowband frequencies; however, these mechanisms contribute to, but do not account for the total observed transmission loss.

Levin's collocation method for predicting sound fields in the presence of sound speed gradients

Sound propagation in a stratified medium has been studied for decades. The Ray tracing method, which is one of the most popular methods, calculates the ray path and propagating angle of a sound ray. However, the ray method is not accurate under many circumstances compared with the wave-based numerical schemes. The Fast field program (FFP), an efficient method for numerical evaluation of the Fourier integral, is accurate but requires significant computational resources for high frequency sound fields because of the highly oscillatory functions found in the integrand. In this paper, a Levin’s collocation method is introduced and used in the evaluation of the sound fields above a locally reacting ground with the presence of the sound speed gradient. The method is far more efficient than most other available numerical techniques such as the FFP and Parabolic Equation method.

3D acoustic propagation through an estuarine salt wedge at low-to-mid-frequencies: Modeling and measurement

The estuarine salt wedge presents a dynamic and highly refractive waveguide, the acoustic propagation characteristics of which are controlled by the water column sound speed gradient and boundary interactions. Acoustically, the salt wedge consists of two isospeed layers separated by a thin, three-dimensional (3D), high-gradient layer. The behavior of a broadband (500-2000 Hz) acoustic field under the influence of an estuarine salt wedge in the Columbia River estuary is explored using two 3D acoustic propagation models: 3D rays and 3D parabolic equation. These model results are compared to data collected during the field experiment. Results demonstrate that the dominant physical mechanism controlling acoustic propagation in this waveguide shifts from 3D bottom scatter in a non-refractive waveguide (before the entrance of the salt wedge) to 3D acoustic refraction with minimal bottom interaction in a refractive waveguide (when the salt wedge occupies the acoustic transect). Vertical and horizontal refraction in the water column and out-of-plane scattering by the bottom are clearly evident at specific narrowband frequencies; however, these mechanisms contribute to, but do not account for, the total observed broadband transmission loss.

Maximum Entropy Derived Statistics of Sound-Speed Structure in a Fine-Grained Sediment Inferred From Sparse Broadband Acoustic Measurements on the New England Continental Shelf

Marginal probability distributions for parameters representing an effective sound-speed structure of a fine-grained sediment are inferred from a data ensemble maximum entropy method that utilizes a sparse spatially distributed set of received pressure time series resulting from multiple explosive sources in a shallow-water ocean environment possessing significant spatial variability of the seabed. A remote sensing seabed acoustics experiment undertaken in March 2017 off the New England Shelf was designed so that multiple independent analyses could infer the statistical properties of the seabed. The current analysis incorporates the measured horizontal variability from interpretations of a subbottom profiling survey of the experimental area. An idealized range- and azimuth-dependent parameterization of the seabed is derived from identification of horizons within the seabed that define multiple sediment layers. A sparse set of explosive charges were deployed on circular tracks with radii of about 2, 4, and 6.5 km with an acoustic array at the center to correlate a set of random measurements to physical acoustic processes that characterize the seabed. The mean values of a surface sound speed ratio and a linear sound speed gradient for the fine-grained sediment layer derived from 12 data samples processed in the 25-275-Hz band provide an estimate of the effective sound-speed structure in a 130-km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area. The inferred sediment sound speed values are evaluated by predicting measured time series data not used in the statistical inference, and are also compared to historical measurements. Finally, the low-frequency maximum entropy estimate of the sediment sound speed along with physical measurements derived from piston core measurements are utilized to estimate the sediment grain bulk modulus.

Estimates of Low-Frequency Sound Speed and Attenuation in a Surface Mud Layer Using Low-Order Modes

Whereas there have been numerous theoretical and experimental studies on the properties of marine granular sands, there are significantly fewer studies on sediments classified as muds. The validity of geoacoustic models for muddy sediments has not been successfully tested due to the lack of inverted low-frequency sound speed and attenuation values. The geoacoustic properties of a surface fine-grained mud layer, overlaying three sand transition layers, and a half-space basement within the New England Mud Patch, were studied using explosive signals from long-range along-shelf sound propagation tracks. The sound-speed profile of the mud layer in the low-frequency band (100-500 Hz) was estimated using acoustic normal mode characteristics including the mode shapes and the modal dispersion curves of low-order modes, which mainly propagated in the water column and the surface mud layer. The ambiguity of sound speed at the top of the mud layer and sound-speed gradient was approximately removed. It was found that the resultant sound-speed ratio at the water-sediment interface was close to unity and the sound-speed gradient was 1.8 1/s with a standard deviation of 1.0 1/s. The attenuation in the mud layer was inverted using the attenuation coefficient of the first mode extracted from explosive signals at three source locations. The estimated attenuation at 150 Hz had a mean of 0.006 dB/m and a standard deviation of 0.003 dB/m.

Standardized spectral and radiometric calibration of consumer cameras.

Consumer cameras, particularly onboard smartphones and UAVs, are now commonly used as scientific instruments. However, their data processing pipelines are not optimized for quantitative radiometry and their calibration is more complex than that of scientific cameras. The lack of a standardized calibration methodology limits the interoperability between devices and, in the ever-changing market, ultimately the lifespan of projects using them. We present a standardized methodology and database (SPECTACLE) for spectral and radiometric calibrations of consumer cameras, including linearity, bias variations, read-out noise, dark current, ISO speed and gain, flat-field, and RGB spectral response. This includes golden standard ground-truth methods and do-it-yourself methods suitable for non-experts. Applying this methodology to seven popular cameras, we found high linearity in RAW but not JPEG data, inter-pixel gain variations >400% correlated with large-scale bias and read-out noise patterns, non-trivial ISO speed normalization functions, flat-field correction factors varying by up to 2.79 over the field of view, and both similarities and differences in spectral response. Moreover, these results differed wildly between camera models, highlighting the importance of standardization and a centralized database.

Gradient field of undersea sound speed structure extracted from the GNSS-A oceanography: GNSS-A as a sensor for detecting sound speed gradient

The Global Navigation Satellite System-Acoustic ranging combination technique (GNSS-A) is a key observation method for detecting geodetic events such as co- and postseismic effects following a megathrust earthquake and the distributions of slip-deficit rate along the subduction zone. For capturing detailed slip and slip-deficit event information, Yokota et al. (Mar Geophys Res, 2018. https://doi.org/10.1007/s11001-018-9362-7 ) improved the accuracy of GNSS-A seafloor positioning using a method for extracting the gradient field of an undersea sound speed structure (SSS). In this paper, we examined how the extracted gradient parameters reflect a real ocean field. A comparison with the JCOPE2, a data assimilated ocean circulation model, shows that the extracted gradient parameters reflect the structure generated by a strong ocean current and can also be used for determination of the location and horizontal structure of the Kuroshio flow path. The results are also consistent with direct SSS observations. GNSS-A can thus serve as a sensor for detecting undersea sound speed gradient fields rather than surface condition.

Altered Lower Extremity Joint Work Profiles During Inclined Running At Iso-efficiency Speeds

Inclined running is a common component of many runners’ training programs in the form of hill repeats. Runners are often forced to slow their running pace, iso-efficiency speed (ISO), during inclined running to maintain their metabolic effort. Though commonly used in training, little is known regarding the changes in joint kinetics associated with running on an inclined treadmill at an ISO speed. PURPOSE: To evaluate changes in lower extremity joint work when running on increasing inclines at ISO speeds. METHODS: Eleven NCAA Division I runners performed five 5-second treadmill running trials during each of three ISO speed inclined conditions: 0% incline, 4% incline, and 8% incline. ISO speed was confirmed using oxygen consumption (VO2) using indirect calorimetry. Kinematics and ground reaction forces were recorded simultaneously using an 8-camera motion capture system (100 Hz, Qualisys Inc.) and instrumented treadmill (1000 Hz, Bertec), respectively. Visual 3D was used to calculate ankle, knee and hip joint powers while custom software (MATLAB, MathWorks) calculated positive and negative joint work as joint power integrated with time. A repeated measures ANOVA with Tukey’s post-hoc was used to determine the effect of treadmill incline on joint work. RESULTS: Negative ankle joint work increased while negative knee and hip joint work decreased with increasing treadmill incline (Table 1). Positive ankle and hip joint work increased while positive knee work decreased with increasing treadmill incline (Table 1). CONCLUSIONS: Our findings suggest that even at ISO speeds (i.e., reduced speeds to maintain effort), inclined treadmill running increases eccentric demands on plantarflexors, and concentric demands on knee and hip extensors. These data may support altered training prescriptions for athletes recovering from joint-specific muscular injuries to minimize training-related stresses placed on those tissues.

Seabed properties on the Chukchi Shelf observed during the 2016–-2017 canada basin acoustic propagation experiment

Previous work examined seabed geoacoustical properties near the 150 m isobath on the Chukchi shelf as observed during the 2016–2017 Canada Basin Acoustic Propagation Experiment (CANAPE) [Sagers and Ballard, JASA 144(3), 1666]. That work reported a water/sediment compressional sound speed ratio near 0.98 and a compressional sound speed gradient in the upper sediment around 6 s-1. This work extends the prior analysis by investigating additional locations throughout the larger shallow-water experimental site. Data from subbottom profile surveys, measurements from an in situ acoustic coring system and inferences made using ship-radiated noise received on Autonomous Multichannel Acoustic Recorders (AMARs) deployed by the Defence Research and Development Canada (DRDC) are examined to understand seabed layering and sediment properties throughout the experimental region. Particular interest is given to understanding whether geoacoustical properties on the Chukchi shelf exhibit range-dependence. [Work sponsored by ONR.] Previous work examined seabed geoacoustical properties near the 150 m isobath on the Chukchi shelf as observed during the 2016–2017 Canada Basin Acoustic Propagation Experiment (CANAPE) [Sagers and Ballard, JASA 144(3), 1666]. That work reported a water/sediment compressional sound speed ratio near 0.98 and a compressional sound speed gradient in the upper sediment around 6 s-1. This work extends the prior analysis by investigating additional locations throughout the larger shallow-water experimental site. Data from subbottom profile surveys, measurements from an in situ acoustic coring system and inferences made using ship-radiated noise received on Autonomous Multichannel Acoustic Recorders (AMARs) deployed by the Defence Research and Development Canada (DRDC) are examined to understand seabed layering and sediment properties throughout the experimental region. Particular interest is given to understanding whether geoacoustical properties on the Chukchi shelf exhibit range-dependence. [Work sponsored by...

Influence of surface duct on the vertical spatial characteristics of wind-generated noise in deep ocean

Vertical spatial characteristics of the wind-generated noise include the noise vertical directionality and the noise vertical coherence, which seriously affect the performance of sonar devices and play an important role in ocean environment parameters inversion. In this paper, we investigate the influence of surface duct on the noise vertical spatial characteristics at the depths below the duct. The Kuperman-Ingenito (K/I) model is employed to describe the distribution of the noise sources, and Pekeris-branch-cut-based normal modes are used to represent the Green's functions between the noise sources and the receivers. Both the noise vertical directionality and the noise vertical coherence are expressed as a function of the normal modes, so that we can investigate the physical reason for the variance of the noise vertical spatial characteristics by analyzing the variance of the normal modes. The numerical simulations on the noise vertical spatial characteristics show that the influence of surface duct above the critical depth is different from that below the critical depth. Above the critical depth, there exists a peak in the noise vertical directionality at the edge of the horizontal notch close to the bottom side. In the presence of surface duct, this peak significantly rises up, and the noise vertical coherence deviates from that in the absence of surface duct and tends to be perfect positive coherence and perfect negative coherence periodically as the vertical distance between the two receivers increases. On the other hand, below the critical depth, the noise power comes from the horizontal direction becomes stronger and the noise vertical coherence tends to be perfect positive coherence in the presence of surface duct as compared with the case in the absence of surface duct. Moreover, the influence will become severer as the thickness of the surface duct increases, while almost keep unchanged when the sound speed gradient in the surface duct varies. The modal analysis indicates that the noise source excites more refracted normal modes with stronger modal intensity in the presence of surface duct, and the excited refracted normal modes become more and stronger if the surface duct is thicker. As the result, the increase of the refracted mode number and the enhancement of their modal intensity cause the vertical spatial characteristics of noise to change.

A noval method of constructing shallow water sound speed profile based on dynamic characteristic of internal tides

In order to provide constraint to the number of inversion parameters, sound speed profile is often modeled by empirical orthogonal functions (EOFs). However, the EOF method, which is dependent on the sample data, is often difficult to apply due to insufficient real-time &lt;i&gt;in-situ&lt;/i&gt; measurements. In this paper, we present a novel basis for reconstructing the sound speed profile, which can be obtained by using historical data without real-time sample. By deducing the dynamic equations and the state function of water particle, the hydrodynamic mode bases (HMBs) can be calculated from historical data without real-time in-situ measurement, and a method of constructing the sound speed profile is established by using the dynamic characteristics of seawater. The use of the World Ocean Atlas 2013 (WOA13) can obtain the seasonal profiles of temperature and salinity, and then the HMB which represents the dynamic characteristic of internal tides is obtained and analyzed. Unlike EOF, the HMB and its projection coefficients are directly related to the sea dynamic features and have a more explicit physical meaning. According to the orthogonality analysis of hydrodynamic mode, the first-order coefficient can be used to describe the depth change of sound speed iso-lines and the second-order coefficient can be used to describe the change of sound speed gradient. Based on the conductance-temperature-depth profiles and broadband data from underwater explosion collected in the East China Sea experiment of the Asian Seas International Acoustic Experiment, the HMB is tested and compared with the EOF in the sound speed profile reconstruction and matched field tomography. The results show that the sound speed profile in shallow water area can be expressed by the HMB with proper precision. By means of the conventional matched field tomography, the valid sound speed profile can also be obtained in the form of HMB coefficients. The result of transmission loss prediction and tomography from HMB are as good as those from EOF, while the HMB has less dependent on real-time &lt;i&gt;in-situ&lt;/i&gt; measurement. The HMB is easy to obtain and closely related to the physical characteristics of seawater, it can be used as an efficient alternative to EOF for monitoring the marine dynamic phenomena in sea areas with insufficient real-time in-situ measurement.

Comparison of block-based stereo and semi-global algorithm and effects of pre-processing and imaging parameters on tree disparity map

One method of creating a 3D model of a tree is stereo vision technique. Tree geometric characteristics have been used to predict yield, fertilizer application in citrus crops, water consumption or biomass. So, a disparity map and 3D model of the tree can be useful. There are different algorithms to match the stereo pair for calculating disparity maps. Many parameters affect the disparity map, such as the lighting that affects camera and imaging parameters, the calibration that needed to rectify the image, etc. In this study, the block-based stereo and semi-global algorithms with 5 implementations were compared to calculate tree disparity maps. Also, the effects of ISO speed, exposure time, metering mode (imaging parameters) and image un-distortion and rectification (pre-processing) were investigated. The results showed that the semi-global algorithm was better than the block-based stereo algorithm. Windows size affected on disparity maps and best results obtained when this parameter was set to default. Maximum disparity value limits search space. Best results obtained in real value. For smaller value than real value, disparity map was not calculated outside that maximum disparity. Therefore, disparities or pixels missed. It observed that image rectifying, ISO speed, exposure time and metering mode were affected on disparity map. When pictures did not rectify, disparity map calculated but with some errors. Camera parameters were affected on light intensity of pictures. Best disparity map obtained in conditions that ISO speed and exposure time had fewer value and metering mode was set to pattern mode and these parameters were similar for both left and right images.

A three-dimensional bistatic reverberation model of underwater explosion by interface scattering at variable sound speeds

Underwater reverberation is a main limitation of the sonar performance and thereby the reverberation level estimation becomes crucial. In this study, based on the Lambert law and ray theory, a model for predicting 3D bistatic reverberation performance by interface scattering at linear sound speed profile is established and then verified through the underwater explosion experiments. The Influences of source and receiver positions, and relative sound speed gradient on reverberation performance are further investigated. The results indicate that: (1) the proposed model can predict the short-range mean reverberation level effectively, with the deviation 2–4 dB and describe the whole reverberation level distribution in some details; (2) at the early reverberation phase, the interference effects between the interface scattering sounds are considerable and a dominating interface exists; the counterbalance between the losses by scattering, spreading and medium absorption results in the local high-intensity zones close to corresponding interfaces, respectively; (3) as the sound source moves towards some interface, associated local high-intensity zone gradually expands, while the other one shrinks; if the sound speed approaches are constant, an extra local high-intensity zone will appear between the previous two but with a lower magnitude.

Speed of sound and geo-alpha in clayey silt derived from concurrent inversion of geo and bio-acoustic parameters from transmission loss data and co-located chirp sonar measurements

Application of the concurrent geo and bio-acoustic inversion method (Diachok and Wales, 2005) to broadband (0.3–5 kHz) transmission loss (TL) measurements, and co-located normal incidence chirp sonar measurements provided measures of geo-acoustic properties of clayey-silt in the Santa Barbara Channel. Inversion calculations assumed that the geological environment may be characterized by the interfacial sound speed, unconsolidated layer thickness, sound speed gradient, g, and geo-alpha, αG; and that the biological environment may be characterized by the layer depth, layer thickness, and bio-alpha (attenuation coefficient within the layer). Co-located cores provided ground truth; echo sounder and trawl measurements provided biological truth. Both the concurrent inversion and the chirp-based methods yielded approximately the same value of g, 6.5/s, in the top 9 m thick layer of clayey-silt. The value of αG, based on TL measurements, was quite low, approximately 0.02 dB/λ, in good agreement with Holland and Dosso’s (2013) only previously reported in-situ estimate of geo-alpha in silty-clay in this frequency-depth range. These results have important implications for estimation of geo parameters from TL measurements in biologically intense environments. Refinements to envisioned follow-on experiments will be discussed. [This research was supported by the Office of Naval Research Ocean Acoustics Program.]Application of the concurrent geo and bio-acoustic inversion method (Diachok and Wales, 2005) to broadband (0.3–5 kHz) transmission loss (TL) measurements, and co-located normal incidence chirp sonar measurements provided measures of geo-acoustic properties of clayey-silt in the Santa Barbara Channel. Inversion calculations assumed that the geological environment may be characterized by the interfacial sound speed, unconsolidated layer thickness, sound speed gradient, g, and geo-alpha, αG; and that the biological environment may be characterized by the layer depth, layer thickness, and bio-alpha (attenuation coefficient within the layer). Co-located cores provided ground truth; echo sounder and trawl measurements provided biological truth. Both the concurrent inversion and the chirp-based methods yielded approximately the same value of g, 6.5/s, in the top 9 m thick layer of clayey-silt. The value of αG, based on TL measurements, was quite low, approximately 0.02 dB/λ, in good agreement with Holland and D...

Estimating North Pacific Right Whale calling depths in the Bering Sea via modal dispersion

Calling depth distributions and ranges are estimated for two types of calls produced by critically endangered eastern North Pacific right whales (NPRW) in the Bering Sea, using passive acoustic data collected with bottom-mounted single-hydrophone recorders at the 50 m isobath. Nonlinear time resampling of 12 NPRW “upcalls” and 20 broadband “gunshots” typically isolated 3 to 4 individual mode arrivals below 200 Hz. Matched-mode processing (MMP) methods were then used to estimate range, depth, and propagation environment. When plotted as a function of range and frequency, MMP ambiguity surfaces reveal the existence of large sound speed gradients in the sediment, along with strongly downward-refracting sound speed profiles during certain summer/fall seasons. Refractive propagation effects from these profiles can induce dramatic changes in signal structure that required new developments in “warping” techniques. Gunshot sounds were generally produced at a few meters depth, while upcall depths clustered between 10 and 25 m, consistent with previously published bioacoustic tagging results from North Atlantic right whales. Current work is examining whether consistent differences exist in the calling depths of bowhead, humpback, and right whales, which could potentially provide a feature for species classification of ambiguous calls. [Work sponsored by NPRB.]

Acoustic Ray Tracing Comparisons in the Context of Geodetic Precise off-shore Positioning Experiments

ABSTRACTThe GNSS-Acoustics (GNSS-A) method couples acoustics with GNSS to allow the precise localization of a seafloor reference in a global frame. This method can extend on-shore GNSS networks and allows the monitoring of hazardous oceanic tectonic phenomena. The goal of this study is to test the influence of both acoustics ray tracing techniques and spatial heterogeneities of acoustic wave speed on positioning accuracy. We test three different ray tracing methods: the eikonal method (3D sound speed field), the Snell-Descartes method (2D sound speed profile), and an equivalent sound speed method. We also compare the processing execution time. The eikonal method is compatible with the Snell-Descartes method (by up to 10 ppm in term of propagation time difference) but takes approximately a thousand times longer to run. We used the 3D eikonal ray tracing to characterize the influence of a lateral sound speed gradient on acoustic ray propagation and positioning accuracy. For a deep water (⩾ 3,000 m) situation, frequent in subduction zones such as the Lesser Antilles, not accounting for lateral sound speed gradients can induce an error of up to 5 cm in the horizontal positioning of a seafloor transponder, even when the GNSS-A measurements are made over the barycenter of a seafloor transponder array.

Sound propagation effects of near-seabed internal waves in shallow water

Computational modeling is used to examine effects of near-seabed nonlinear internal waves. Waves of this type have been observed many times on the South New-England Shelf and elsewhere. The waves appear to be common when the dominant vertical density gradient zone is near the seabed. Many observations have been with profilers that alias the waves, or with instruments fixed to the seafloor that provide snapshots of passing waves. We have recently observed these features in greater detail using acoustic remote sensing from a ship, and we are now in a position to model their effects on sound propagation. Results are presented for many frequencies and many source/receiver/wave geometries. An important question remains about along-crest coherence of the waves, which would govern ducting and other anisotropic propagation effects. These waves are not as well studied as waves linked to a near-surface pycnocline, which can be studied remotely with EM via surface signature. The strong density gradients found in many shallow water areas allow the internal waves to have high frequencies, creating rapidly changing acoustics thereby shortening acoustic coherence time. The related strong sound-speed gradients, found both near the surface and near the seabed, are responsible for the important acoustic effects of the waves.

Domain Adaptation in Steganalysis for the Spatial Domain

A scenario of domain adaptation (DA) in machine learning occurs when training and test data are drawn from some population with different distributions. In steganalysis, this scenario can arise when images used for training and testing come from different cameras, especially in blind detection. Although there has been some work in this area, it is still not clear that one can design a feasible detection scheme for all devices from one camera model. In this research, Spatial Rich Models (SRM) and ensemble classifiers have been applied for feature extraction and classification, respectively. After carefully collecting images from several camera models from mobile phones, with at least two devices for each model, we identify two measurable factors that affect detection: ISO speed and exposure time. This allows us to adapt the classifier from one device to a different one of the same model, even when images from the two devices are significantly different in visual appearance, by choosing specific training data. Our experiments show that a well-trained stego detector based on data from one source shows more adaptability to new target data if the training images have similar distributions of ISO speed and exposure time as the target images.

Laboratory measurements of sound speed and attenuation dispersion in calcareous sediments from coral reefs

The calcareous sediment is an important type seafloor sediment exist in coral reef island sea areas. The acoustic properties of calcareous sediment are significant for modeling sound propagation and underwater reverberation. In order to analyze the frequency dependence of sound speed and attenuation in calcareous sediments, the sediments were firstly screened and remodeled in plexiglass tubes in laboratory, and the sound speed and attenuation were measured at the frequency range of 27–247 kHz. The grain sizes of sediment samples were <0.075 mm, 0.075-0.5 mm, 0.5-1 mm, 1–2 mm, and 2–4 mm, respectively. The sound speeds of different grain-size sample were 1564.83–1607.36 m/s, 1564.82–1607.36 m/s, 1527.76–1553.69 m/s, 1529.40–1594.72 m/s, and 1541.87–1596.51 m/s, respectively. The attenuation were 24.94–206.35 dB/m, 20.78–208.75 dB/m, 9.66–271.94 dB/m, 12.33–310.36 dB/m, and 12.60–293.60 dB/m, respectively. The sound speeds and attenuation were found to increase remarkable with frequency. The fine grained sediments have higher sound speed than the coarser sediments, which due to the higher bulk density and lower porosity of fine grained sediments. The dispersion gradients of sound speed and attenuation in coarser sediments were more remarkable than that in fine-grained sediments, which may due to the higher tortuosity of coarser sediments.

Path losses in sandy sediments with upward refracting sound speed profiles

In this work, receiver-to-receiver path losses calculated from measurements of underwater explosions in shallow water (water depth ∼15 m) off the coast of Virginia Beach, VA will be presented. Within the approximate frequency range of 40 to 70 Hz, these data have path losses that suggest energy increasing with range by up to 10 dB. Above this frequency range, these data show energy decreasing with range as expected. An analytical model [Frisk, George V., “Determination of sediment sound speed profiles using caustic range information,“ Bottom-Interacting Ocean Acoustics. Springer, US, 1980, 153–157] and numerical propagation models will be used to show that these path losses can be attributed to an upward refracting sound speed gradient in the sediment. This upward refraction of sound waves propagating in the seabed results in the formation of caustics that define shadow zone boundaries. Below the cutoff frequency of the waveguide, receivers in the shadow zone will exhibit lower sound levels than receivers positioned farther from the source and outside of the shadow zone. While frequencies above cutoff will also undergo refraction in the seabed, the observability of this effect will be masked by trapped modes propagating in the water.