Sound Speed Field Research Articles

Ocean weather and climate observations using the IUSS

For more than a decade, long-range acoustic propagation measurements have been used to remotely infer characteristics of the oceans’ interior structure. The most prominent efforts have been those focused on the application of tomographic methods to measure the sound speed and current fields, and by inference the temperature field. To a large degree, the genesis of this activity was earlier measurements taken on the U.S. Navy’s undersea surveillance system, and IUSS remains a central part of many experiments being conducted today. This paper will illustrate the importance of the Navy’s assets to several recent experiments and will highlight the role of IUSS in the multi-institutional Acoustic Thermometry of Ocean Climate (ATOC) Program. [Work supported by ONR and ARPA.]

Oceanographic–topographic interactions in acoustic propagation in the Iceland–Faeroes front region

Effects of oceanographic variation with distance on long-range, low-frequency acoustic propagation in the Iceland–Faeroes front region of the ocean are considered in the presence of realistic topographic variations. A numerical model using a parabolic approximation to the Helmholtz equation, a fluid sediment parametrization and variable topography, is used to calculate acoustic propagation. Oceanographic sound-speed fields output from the Harvard Open Ocean Model, supplemented by climatology in deep regions, provide input sound-speed profiles. Two different propagation transects are considered, both running from shallow to deep water across a developing eddy and across the front. Source depths near the surface, middle, and bottom of the shallow starting profile are studied. Some cases of near invariance to oceanographic changes are found, as are other cases of locally large oceanographic effects (≳30 dB).

Void-fraction measurements and sound-speed fields in bubble plumes generated by breaking waves

Recent field and laboratory experiments have confirmed that low-frequency sound (10 to 300 Hz) is generated under breaking waves. It has been proposed that collective oscillations of the bubble plume generated by breaking may be the mechanism responsible for the generation of this sound. Confirmation of this process requires independent measurement of the void fraction, and therefore sound speed, in the bubbly mixture. Detailed measurements are presented of the evolution of the void-fraction field in bubble plumes generated by large-scale three-dimensional (3-D) laboratory breaking waves. Various moments of the void-fraction field are calculated and compared with results from two-dimensional (2-D) laboratory breaking waves [Lamarre and Melville, Nature 351, 469–472 (1991)]. The kinematics of the bubble plume reveals that the initial horizontal velocity of the plume is approximately 0.7C, where C is the wave phase speed. The centroid of the bubble plume is found to deepen at a speed of approximately 0.2H/T, where H and T are the wave height at breaking and the wave period, respectively. The radial dependence of the void-fraction and sound-speed field is characterized in terms of simple functions of time. Finally, the void-fraction measurements described here, along with independent measurements of the pressure fluctuations under breaking waves [Loewen and Melville, J. Acoust. Soc. Am. 95, 1329–1343 (1994)], support the hypothesis that low-frequency sound is generated by the collective oscillations of the bubble plume.

Acoustic characteristics of seawater in the area of the coastal discharge front off the Guinean shore

Regularities in the structure and variability of the acoustic characteristics of Guinean shelf waters during the rainy season have been identified as a result of the generalization of the sound speed fields derived from temperature and salinity observations. Three areas with qualitatively different hydroacoustic properties have been revealed, namely, (i) near-shore shallow waters, where a near-surface haline acoustic channel occurs by virtue of intensive seawater freshening; (ii) a discharge front accommodating an intermediate acoustic waveguide in the temperature jump layer; and (iii) off-shore shelf areas displaying a characteristic alteration of the sound speed vertical gradient sign at thec(z) profile.

Moving ship tomography in the north atlantic

Moving Ship Tomography (MST) is a method of obtaining high‐resolution, nearly synoptic three‐dimensional maps of the ocean temperature field over large areas [Cornuelle et al., 1989]. We measure acoustic travel times along a multitude of paths crossing at many different angles and then reconstruct the sound speed (temperature) field in a manner analogous to a medical CAT‐scan. A large number of crossing ray paths are generated by using a moving receiver. This report compares preliminary maps of sound speed obtained using acoustic data (Figure 1) with maps obtained with air expendable bathythermograph (AXBT) and conductivity, temperature, depth (CTD) profile data. Given such maps, we will be able to study advecting fronts and interacting eddies with high resolution and test how well numerical models can predict the evolving fields.

Ocean acoustic tomography at 1000‐km range using wavefronts measured with a large‐aperture vertical array

Broadband acoustic signals transmitted from a moored 250‐Hz source to a 3‐km‐long vertical line array of hydrophones 1000 km distant in the north central Pacific Ocean were used to determine the amount of information available from tomographic techniques used in the vertical plane connecting a source‐receiver pair. A range‐independent, pure acoustic inverse to obtain the sound speed field using travel time data from the array is shown to be possible by iterating from climatological data without using any information from concurrent environmental measurements. Range‐dependent inversions indicate resolution of components of oceanic variability with horizontal wavelengths shorter than 50 km, although the limited spatial resolution of concurrent direct measurements does not provide a strong cross‐validation, since the typical cast spacing of 20–25 km gives a Nyquist wavelength of 40–50 km. The small travel time signals associated with high‐wavenumber ocean variability place stringent but achievable requirements on travel time measurement precision. The forward problem for the high‐wavenumber components of the model is found to be subject to relatively large linearization errors, however, unless the sound speed field at wavelengths greater than about 50 km is known from other measurements or from a two‐dimensional tomographic array. The high‐ocean‐wavenumber resolution that is in principle available from tomographic measurements is therefore achievable only under restricted conditions.

Synoptic variability of the acoustic characteristics in tropical Atlantic waters

Synoptic variability of the sound speed and parameters of the SOFAR channel in the tropical Atlantic are considered. The synoptic component of the temporal and spatial dispersions of sound speed is singled out. The synoptic processes are shown to essentially affect the vertical hydroacoustic structure on a 1-month scale. It has been found that synoptic variations in the sound speed field penetrate as far as the SOFAR channel axis and deform it considerably.

Acoustic characteristics of the north-eastern Atlantic Ocean

The vertical structure of the climatic and seasonal fields of the estimated speed of sound in the north-east Atlantic Ocean is considered. Zonation according to typicalc(z) profiles allowing for seasonal variations was carried out. It is shown that the water areas with a dual-channel vertical hydroacoustic structure vary essentially throughout the year. The features of the structure of the sound speed field in the lenses of the Mediterranean Sea waters are investigated. The effect produced by a lens on the characteristics of the sound propagation is studied.

On equations for the speed of sound in seawater

Long-range acoustic transmissions made in conjunction with extensive environmental measurements and accurate mooring position determinations have been used to test the accuracy of equations used to calculate sound speed from pressure, temperature, and salinity. The sound-speed fields computed using the Del Grosso equation [V. A. Del Grosso, J. Acoust. Soc. Am. 56, 1084–1091 (1974)] give predictions of acoustic arrival patterns which agree significantly better with the long-range measurements than those computed using the Chen and Millero equation [C. Chen and F. J. Millero, J. Acoust. Soc. Am. 62, 1129–1135 (1977)]. The predicted ray travel times and travel time error have been calculated using objectively mapped sound-speed fields computed from conductivity, temperature, depth (CTD) and expendable bathythermograph (XBT) data. Using the measured and predicted ray travel times, a negligible correction to Del Grosso’s equation of +0.05±0.05 m/s at 4000-m depth is calculated.

Variability of heat content in the Northcentral Pacific in summer 1987 determined from long-range acoustic transmissions

The evolution of the depth integrated heat content in the Northeast Pacific during summer 1987 has been measured using long-range acoustic transmissions between transceivers deployed in a triangle approximately 1000 km on a side. Transmissions at 4-day intervals gave travel times that were inverted to obtain estimates of the range-average sound-speed fields along the sides of the triangle. The estimated sound-speed fields are converted to temperature fields and integrated to obtain depth and range averaged heat content. The acoustically determined sound-speed and temperature fields agree with conventional CTD and XBT data. The accuracy of acoustical measurements of heat content are comparable to estimates from CTD and XBT data and can be made much more frequently. The acoustically measured variability of heat content at short-time time scales limits the accuracy of studies using infrequent XBT sections. Ocean/atmosphere heat exchange from bulk formulas cannot fully explain the observed heat content change; the excess heat change is calculated to be of order 50–150 W m−2. [Work supported by NSF and ONR.]

Test of nonperturbative ocean acoustic tomography inversion

This work is part of an effort to compare various ocean acoustic tomography inversion methods given the same hypothetical sound-speed field (specified in a 1000- by 1000- by 5-km region). To test the WPL nonperturbative inversion method [Jones etal., NOAA Tech. Memo. ERL WPL-217 (1991)], the authors simulated tomographic data using ray tracing, performed an inversion, and compared the results with the sound-speed field provided. The simulation sampled the region in several vertical slices, and the inversion yielded the range average of the symmetric part of the sound-slowness profile for each slice.

Relative arrival tomographic reconstruction using drifting receivers and a ship-board source

Rather than testing ocean imaging with accurately located and timed acoustic transmission equipment, a study of the quality of images produced with sonobuoy-type drifting receivers and ship-lowered sources is presented. The imprecise location and timing information means that the only information available is the relative timing of multipath (ray-type) arrivals. Accuracy of relative arrival time measurement is presumed to be 5 ms and positioning is to 1 km. The forward model is a ray trace through the background, and the basis set has various vertical forms. Three array sizes are considered, 100, 350, and 1000 km. Deployments on these different scales will have different oceanographic goals. The distribution of successfully mapped variance at all scales smaller than the array dimension will be shown. The appropriate array dimension, relative to the sound-speed field correlation length, will be discussed. Comparisons with more standard quick-survey methods (e.g., AXBT drops) will be considered.

Travel time tomography simulations for the test ocean constructed for the 124th Meeting of the Acoustical Society of America

Geometric rays were traced for a variety of source–receiver configurations in the test ocean sound-speed field constructed for the 124th Meeting of the Acoustical Society of America. A stochastic inverse incorporating the statistical information distributed with the test ocean was performed using only one-way travel times as data. Realistic errors in travel times and source–receiver positions were allowed for. The travel times were assumed to be contaminated with internal-wave-induced travel time fluctuations. The fluctuation variance increased linearly with range, with a magnitude of 10 ms rms at 1000 km. This internal wave noise can be reduced by a factor of N−1/2 by assuming that the mesoscale sound-speed structure remains static over N internal-wave-induced decorrelation times. Decorrelation times shorter than 10 min have been observed in the North Pacific at 1000-km range, so a daily average can yield a net contamination below 1 ms, neglecting the effects of longer period fluctuations like internal tides. The simulation results are sensitive to the size of the assumed error, and using values below 1 ms can lead to unrealistically good performance. The size of the source–receiver position errors can vary dramatically depending on the navigational techniques used and can have significant effects on the array design problem. These sensitivities will be explored in the process of choosing a source–receiver array adequate to resolve 96% of the sound-speed variance at 700-m depth in the test ocean.

Void-fraction measurements in bubble plumes generated by 2-D and 3-D breaking waves.

Recent experiments have confirmed that low-frequency sound (10 to 300 Hz) is generated under breaking waves. It has been suggested that volume oscillations of the bubble plume generated by breaking is the mechanism responsible for the generation of this sound. Confirmation of this process requires independent measurement of the void fraction, and therefore sound speed, in the bubbly mixture. Detailed measurements of the evolution of the void-fraction field in bubble plumes generated by 2-D and large scale 3-D laboratory breaking waves are presented and compared. Various moments of the bubble plume void-fraction field give information on the (a) volume of air entrained, (b) cross-sectional area of the plume, (c) mean void fraction and sound speed, (d) bubble plume resonant frequencies, (e) potential energy of the bubble plume, (f) horizontal and vertical displacement and velocity of the bubble plume. These measurements along with independent measurements of the sound radiated (Loewen and Melville, this meeting) support the hypothesis that low-frequency sound is generated by the volume oscillation of the bubble plume. The radial dependence of the bubble plume’s void-fraction (and sound speed) field and its effect on low-frequency sound generation is investigated and the influence of the void-fraction instrumentation detection threshold in estimating the above moments is also examined. [Work supported by ONR (Ocean Acoustics) and NSF (Physical Oceanography).]

On equations for the speed of sound in seawater.

Long-range acoustic transmissions made in conjunction with extensive environmental measurements and accurate mooring position determinations have been used to test the accuracy of equations used to calculate sound speed from pressure, temperature, and salinity. The sound-speed field computed using the Del Grosso equation [V. A. Del Grosso, J. Acoust. Soc. Am. 56, 1084–1091 (1974)] give predictions of acoustic arrival patterns which agree significantly better with the long-range measurements than those computed using the Chen and Millero equation [C. Chen and F. J. Millero, J. Acoust. Soc. Am. 62, 1129–1135 (1977)]. The predicted ray travel times and travel time error have been calculated using objectively mapped sound-speed fields computed from CTD and XBT data. Using the measured and predicted ray travel times, a negligible correction to Del Grosso’s equation of 0.05±0.05 m/s at 4000-m depth is calculated. Small errors of about 50 m in the GPS determination of mooring positions lends a depth-independent error of 0.1 m/s to the sound-speed equation correction. [Work supported by NSF and ONR.]

Passive localization of calling animals using acoustic tomography.

The temporal and spatial patterns in the distribution of animals are poorly understood primarily because of inadequate observation techniques. A new passive localization technique (acoustic tomography) is experimentally demonstrated which automatically localizes the positions of calling animals with rms errors of a few centimeters from measurements of the call’s travel-time difference at several pairs of microphones [J. L. Spiesberger and K. M. Fristrup, Am. Naturalist 135, 107–153 (1990)]. Tomographic localization is significantly more precise (by factors from about 2 to 100) than passive acoustic techniques used previously because tomography accounts for and estimates the sound speed and wind fields which significantly modify the travel-time differences. Experimental results will be presented of tomographic localization and tomographic mapping of the sound-speed field in the atmosphere.

A ray variability analysis of sound transmission from Heard Island to California.

In support of the Heard Island experimental effort, the influence of mesoscale and seasonal ocean variability on three-dimensional acoustic ray paths from Heard Island through the Indian and Pacific Oceans to the California coast is investigated. For the simulation of these global acoustic ray paths, a three-dimensional Hamiltonian ray tracing code is used. The input sound-speed fields to the acoustic ray model are interpolated from gridded temperature and salinity output data from the Semtner–Chervin eddy-resolving, global, general circulation model. A previous ray analysis was conducted using ocean fields containing only the mesoscale variability. This analysis provided input to experimental planners for receiver placement off the California coast [C. S. Chiu and A. J. Semtner, J. Acoust. Soc. Am. Suppl. 1 88, S92–S93 (1990)]. Here, a new analysis is presented that includes the acoustic effects of ocean seasonal cycles. In particular, the variability of the paths, arrival azimuthal angles, and travel times are discussed.

Acoustic propagation and oceanic fine structure.

Internal waves distort the sound-speed field in the sea. This time variable distortion causes fluctuations in acoustic intensity and phase in forward transmission experiments. Initial efforts to predict the spectrum of internal wave-induced phase fluctuations were successful. Corresponding success was not achieved in the prediction of acoustic intensity, which is sensitive to smaller vertical scales than phase. Are the oceanic environmental models inaccurate or are the relevant scattering models in error? Here, a statistical picture of the fine-scale wave field is presented. The data consist of 10 000 profiles of density and sound speed, from the sea surface to 560 m, collected over a 20-day period. Vertical wave-number-frequency spectra of sound-speed anomaly are estimated from these profiles. Of relevance is the frequency dependence of the spectrum at the vertical wave number corresponding to the Fresnel scale of a given acoustic experiment. In an Eulerian frame, where vertical advection influences the observations, the frequency dependence varies significantly with vertical wave number. The form, as well as level, of the power spectrum of log intensity is dependent on the Fresnel scale of the experiment. Since estimation of the Fresnel scale is sometimes associated with significant uncertainty, the shape of the log amplitude spectrum might prove a valuable guide to the scattering physics.

Time domain analysis of normal mode, parabolic, and ray solutions of the wave equation

Estimation of sound speed and temperature fields from tomographic data requires knowledge of the accuracy of the forward model. For this reason, time domain results using three models derived from the acoustic wave equation are compared here. The models chosen for the analysis are normal modes (NM) and the parabolic equation (PE) and ray approximations. (The NM and PE time series are obtained via Fourier synthesis). The analysis is done using a depth-dependent sound-speed profile where only refracted and surface-reflected/refracted energy is allowed to propagate. Because the NM representation is theoretically exact in this environment, it is used as the comparative base. Travel-time sequences generated using the PE and ray approximations are compared to those generated using NM. For the angular aperature analyzed (±9 to ±16.5 deg), phase errors in the PE models investigated limit reliable predictions of travel time to distances of about 1000 km. However, the ray approximation is found to be adequate for estimating arrival times at ranges out to 4000 km for frequencies of 100 Hz and above.

Propagation in atmospheric convective‐boundary‐layer turbulence

Turbulence in the atmospheric convective boundary layer is inhomogeneous and anisotropic. The vertical correlation length of the sound speed field is typically proportional to height from the ground, whereas the horizontal correlation length is proportional to boundary‐layer thickness. Solutions for the turbulence strength and diffraction parameters, valid for this particular case, are presented. The solutions differ significantly from those for homogeneous, isotropic turbulence. An intercomparison is made between the theoretical solutions, numerical propagation simulations, and experimental data. For the numerical simulations, rays were traced through wind and temperature fields generated by a large‐eddy simulation.