Shallow-water Variability Research Articles

Azimuthal dependence of sound propagation due to seabed variability in shallow water

In shallow water regions, the environment has complicated temporal and spatial variability including changes of bathymetry, sediment layer structure, bottom property, and physical oceanographic spatial and temporal changes due to processes like internal waves. All these effects can influence sound propagation in the waveguide. The azimuth angle dependence of sound propagation has been studied using the broadband acoustic signals measured at the Atlantic Generation Station site on the New Jersey continental Shelf, where two distinctive geologic/geoacoustic regions exist [J. Acoust. Soc. Am. 96(6), 1994]. The current paper revisits this idea by analyzing the modal dispersion of broadband acoustic signals deployed along circular tracks at the site of the Seabed Characterization Experiment 2017, where the seabed shows strong azimuthal dependent sub-mud layer ridges overlaid by a relatively uniform mud layer with variable thickness along different directions. The results of this paper can be utilized to assess the azimuthal dependence of the sound propagation in the mud patch region. [Work supported by ONR Ocean Acoustics.]

Parameter dependence of acoustic quantities in a nonlinear internal wave duct

Parameter dependence of acoustic quantities in a nonlinear internal wave duct BJD, Matt Milone, YTL Ocean features with 3-D spatial variability in shallow water can significantly affect acoustic propagation. One example is a curved front modeled with a discontinuous sound speed change over a sloping shelf [Lin and Lynch, JASA-EL (2012)], which has an extension to a continuous sound-speed change. An approach using normal modes and perturbation approximations yields convenient formulas that show how acoustic quantities depend on environmental parameters [DeCourcy et al., ASA, Salt Lake City (2016)]. Another common 3-D example is nonlinear internal waves, with wave fronts that pairwise can produce acoustic ducting, radiating, and scattering effects often observed in field data. The previous approach is applied to this feature, using a well model with two sound-speed jumps for such a duct [Lin et al., (2013)]. Approximate formulas for acoustic wavenumbers and phase speeds are determined in order to estimate sensitivity to changes in environmental parameters. All mode types will be considered (whispering gallery, fully bouncing, and leaky), highlighting differences from those in the single-front example. [Work supported by ONR Grants N00014-14-1-0372 and N00014-11-1-0701.]

Use of acoustic frequency shifts for monitoring oceanographic processes in shallow water.

Waveguide invariant theory is used to calculate the frequency shifts of the acoustic intensity spectral levels caused by environmental variability in shallow water. In winter conditions, long-term acoustic monitoring of tide and slope water intrusions has been demonstrated using measured frequency shifts of the acoustic intensity spectral levels [Turgut et al., J. Acoust. Soc. Am. 121, 2534–2541 (2007)]. Acoustic frequency shifts caused by other oceanographic processes such as background internal waves and solitary internal waves are also investigated. Additional broadband acoustic data from several summer experiments indicate the feasibility of extracting background internal wave parameters as well as internal solitary wave parameters when they propagate almost perpendicular to the acoustic propagation path. [This work was supported by the ONR.]

Hafnium and neodymium isotope variations in NE Atlantic seawater

The 176Hf/177Hf ratio of seawater is measured directly with 143Nd/144Nd in a composite vertical profile in the NE Atlantic. The value of ɛHf of intermediate and deep water exhibits little variation and averages +1.0 ± 0.8; surface water shows a wider range, −5.7 to +3.3. While intermediate and deep water samples plot on, or close to, the seawater Hf‐Nd isotope array, defined by ferromanganese deposits, near‐surface seawater samples plot between the seawater and terrestrial rock ɛHf − ɛNd arrays. It is likely that the shallow water variability in ɛHf arises from a short residence time of Hf in surface water and isotopically heterogeneous inputs. The difference between surface and intermediate‐deep water ɛHf may be maintained by the release of surface scavenged Hf at depth from a subset of sinking particles; the composition of εNd at middepths reflects overlying surface water and water advected along isopycnals from areas where they outcrop in winter. Uniform ɛHf values below 1000 m suggest the residence time of Hf may be longer than that of Nd. The radiogenic isotopic composition of seawater Hf relative to Nd and terrestrial rocks suggests that minerals with high Lu/Hf ratios may be preferentially weathered, providing a source of radiogenic Hf to the oceans.

Mapping seabed variability: Rapid surveying of coastal regions

Directional broadband reverberation can be exploited for rapidly probing seabed variability over large areas O (103–104)km2; the data need not be calibrated. Seabed variability thus mapped inherently combines spatial variability in geoacoustic properties that control reflection and those that control scattering. It is shown that in some cases, the variability can be separated. The method is applied to measurements in the Straits of Sicily where the data indicate the existence of two provinces where the statistics of the geoparameters are distinct. Three attributes of each province are quantified: the province boundaries, measures of spatial variability and/or uncertainty, and the location of discrete scattering features (that may lead to sonar clutter). One province is quite large and exhibits remarkably little spatial variability. The variability is probed on lateral scales ∼200×400m, yet the ratio of the seabed geoparameters varies less than 3 dB from 300−1600Hz over ∼1000km2. While this result seems contrary to prevailing notions of extreme seabed variability in shallow water, it indicates that certain kinds of geophysical (and concomitant geoacoustic) variability may be averaged in such a way that the seabed appears approximately homogeneous to long-range acoustic systems.

Predictability of shallow water sound transmission in selected shallow water experiments

This paper summarizes our efforts to compare calculations in several shallow water areas using range dependent models with results from coherent sound transmission experiments. The approach is to use detailed environmental and geophysical measurements to determine acceptable ranges of input parameters for the range dependent calculations, and then to determine within these bounds the ability to calculate the pressure amplitude, the phase of the signal, and the modal wave number spectrum. In addition, the coherence of the transmitted signal and the measurement of relative array signal gain were estimated. Although shallow water is known for its variability, these investigations clearly demonstrate predictable metrics of sound transmission, provided a sufficient type and an amount of environmental data are known. These investigations also show the key factors in this variability for acoustic predictability and highlight the areas of uncertainty. These results further suggest that shallow water variability and sound transmission can be calculated deterministically with dynamic changing environmental oceanographic variables.

Mode coupling in the presence of random sediments and shallow water variability

The continental shelf west of Florida has a relatively flat bottom over a range of tens of kilometers, and the bottom and water column have fairly well characterized acoustic parameters. Parabolic equation models are used for acoustic propagation in both elastic and purely acoustic cases. The calculated effect of shear contribution is comparatively small. The variation in mode coupling over a frequency range of about 100–400 Hz is evaluated. Even in this flat-bottom case, modal coupling due to the environment occurs, especially at lower frequencies. The water column tends to be downward refracting (especially the bottom half), so that the variation in the penetrability of the sediment leads to strongly changing amounts of mode coupling. In the case of softer bottoms, this can lead to very significant coupling. [Work supported by the Office of Naval Research.]

Effects of internal waves and turbulence on a horizontal aperture sonar

Random variability in the water column will affect the operation of a horizontal aperture sonar. Two sources of variability in shallow water are turbulence and internal waves. In a modeling study, the effects of turbulence and internal waves on a shallow-water imaging system are compared. The operational principles of a large aperture imaging system are first reviewed. A shallow-water internal wave model is developed by modifying the Garrett-Munk model. The internal waves are assumed to dissipate and drive the small-scale turbulence. The two phenomena are predicted to have markedly different effects on a system. Turbulence has short spatial correlation scales whose primary effects will be manifested in the variance of the acoustic phase. By contrast, internal waves will have much larger scattering but also a longer correlation scale. The primary acoustic quantity of interest for internal waves is shown to be the curvature of the phase as observed along the aperture. Properties of shallow-water internal waves are shown to preclude the use of standard acoustic calculations based on the Markov approximation. Using archival environmental data, sample calculations are presented for the site of a planned August 1996 experiment.

The Naval Research Laboratory’s high-frequency multimode towed vehicle

To obtain high-resolution measurements of acoustic spatial variability in shallow water (SW) and very shallow water (VSW) over a wide range of environmental conditions, the Naval Research Laboratory has developed a highly stable multimode underwater vehicle that can be towed and its depth and attitude controlled via commands from the towing vessel. The vehicle was configured with manual and semiautonomous control systems, data acquisition and communications systems, and an acoustic sensor system for measuring acoustic variability in water depths from 20 to 100 ft. Two high-frequency (20- to 600-kHz) arrays were mounted flush on the sides of the tow body and operated in either broad-beam mode or ‘‘phased-beam’’ mode using the latest ‘‘Doily’’ technology developed at ARL Penn State. In ‘‘phased-beam’’ mode the arrays form sine and cosine shape functions, with the beam steer angle dependent on frequency of operation. Through selection of tow speed and pulse repetition rate, the scattering characteristics of the same area of ocean bottom are simultaneously measured over a wide range of frequencies. Technical details of vehicle design and operation and some results from a high-frequency experiment conducted during 1996 are presented.

Naval Research Lab’s multi-mode towed vehicle inertial navigation data acquisition system

High resolution measurements of acoustic spatial variability in shallow water (SW) and very shallow water (VSW) over a wide range of environmental conditions, were obtained using the towed vehicle described in the following abstract, 3aAO8. To obtain real-time vehicle attitude and current measurements a Doppler velocity log from EDO Corporation, Motion PAK accelerometer from Systron Donner were integrated with pitch, roll, and heading sensors to provide a real-time three-axis navigation system. Real-time data acquisition software was developed by the Naval Research Laboratory utilizing National Instruments LABView object oriented programming language to record vehicle attitude and optionally provide real-time pitch and roll corrections based upon changes in vehicle velocity. The system is designed to measure, record, and monitor up to 16 bins of current velocity between vehicle and bottom surface. Technical detail of data acquisition design, operation, and some results from a high-frequency experiment conducted during 1996 are presented.

Measurements on the origin of the wind-dependent ambient noise variability in shallow water

Contributions of environmental quantities related to the noise source field and the propagation path are derived from the comparison of measurements of the wind-dependent noise by omnidirectional receivers at a fixed North Sea station with shipborne measurements in the Baltic Sea. The influence of propagation loss on the wind-dependent shallow water noise appears to be only marginal, even at extremely different sea areas. The quantity governing the noise spectrum level under uncontaminated conditions is the wind speed at the sea surface for which the second power law relation has been verified between 50 Hz and 20 kHz and above a ‘‘threshold’’ wind speed of ≊5 kts. Neither the characteristic height of the sea waves nor the wind turbulence at the reference height are relevant to the noise production, but both may indicate wind profile changes which originate an essential portion of the noise variability for a given wind speed. Further deviations from the second power law are attributed to a bubble layer effect under storm conditions reducing or enhancing the high frequency noise level thus yielding a spread of the average spectrum level of more than 20 dB.