Examples and applications in long-range ocean acoustics

Acoustic travel-time tomography of the atmosphere is a nonlinear inverse problem which attempts to reconstruct temperature and wind velocity fields in the atmospheric surface layer using the dependence of sound speed on temperature and wind velocity fields along the propagation path. This paper presents a statistical-based acoustic travel-time tomography algorithm based on dual state-parameter unscented Kalman filter (UKF) which is capable of reconstructing and tracking, in time, temperature, and wind velocity fields (state variables) as well as the dynamic model parameters within a specified investigation area. An adaptive 3-D spatial-temporal autoregressive model is used to capture the state evolution in the UKF. The observations used in the dual state-parameter UKF process consist of the acoustic time of arrivals measured for every pair of transmitter/receiver nodes deployed in the investigation area. The proposed method is then applied to the data set collected at the Meteorological Observatory Lindenberg, Germany, as part of the STINHO experiment, and the reconstruction results are presented.

Acoustic travel time tomography uses the dependence of sound speed on temperature and flow properties along the propagation path to measure these parameters. An algorithm is introduced that is capable of resolving the two-dimensional flow field within a specified measuring area. Different flow fields have been simulated to explore the reconstruction properties of the algorithm. Furthermore, an experiment within the Barrel of Ilmenau has been carried out, which demonstrates that the acoustic tomographic method is able to detect flow fields in a closed convection chamber. The simulations reveal that the vector tomographic algorithm can resolve the flow field characteristics speed and direction. Furthermore it is demonstrated that the distribution of sound sources and receivers has to be homogeneous around the measuring site. For the reconstruction of flow distributions, this requirement is even more important compared to temperature reconstructions, since information along inclined sound ray paths within each grid cell are needed to reconstruct the flow vector properly.

Acoustic travel-time tomography of the atmosphere is a nonlinear inverse problem which attemps to reconstruct temperature and wind velocity fields in the atmospheric surface layer (ASL) using the dependence of sound speed on temperature and wind velocity fields along the propagation path. A new statistical-based algorithm is introduced in this paper based on fixed-point unscented Kalman filter (UKF) which is capable of reconstructing and tracking temperature and wind velocity fields within a specified investigation area.

An acoustic tomographic system has been used to estimate area averaged wind and temperature data within an area of 97 m × 12 m considering the dependence of sound speed on meteorological conditions To obtain information about vertical structure of meteorological data, eight sound sources and receivers were placed in two different heights above the ground (0.5 m and 2.7 m). Spatially, the acoustic measurements correspond to line integrated N2O concentration measurements (98 m) using FTIR-spectrometers Taking stability of atmospheric layering into account, acoustic tomographic measurements serve as basis for estimating vertical fluxes of momentum and sensible heat.

Shallow water acoustics has flourished over the past fifty years, but most especially over the last 25 years. After World War II, the Cold War between the USSR and the West focused the emphasis in ocean acoustics on deep, “blue water.” After the Cold War waned in 1990, the emphasis changed to “brown water” coastal acoustics studies. In this paper, we will look at the advances over the past half-century in shallow water acoustics, with emphasis on the experimental results from acoustics, oceanography, marine geology, and marine biology.

Veritable snapshots-in-time of the vector fields in the classic problem of plane wave refraction at a planar interface are drawn simply and directly using a contour-plotting routine. These field pictures, together with the antecedent mathematics, give students and teachers a panoramic view of the pertinent wave physics. Classroom examples and computer assignments, based on the methodology of this paper, help to underscore the real physical nature of the subject called electromagnetics.

Marine sediments exist universally as the lower boundary for sound propagation in ocean waveguides, and knowledge of the properties of these sediments is important for accurate modeling of sound propagation and reverberation. In order to test theory predictions of the frequency dependence of sound speed and attenuation, it is necessary to have accurate information on the sediment properties, which is most easily done in a laboratory environment. Initial results reported here were done at high frequency in a small tank, as a preliminary step before making similar low frequency measurements in a much larger tank. A sandy sediment was used and the sound speed and attenuation were measured through different thicknesses of the sample. In the frequency range of 90–170 kHz, the measured sound speed was 1757–1767 m/s, and the attenuation was 22–30 dB/m. The sound speed dispersion was found to be very weak, as expected, and much smaller than the measurement uncertainty. The attenuation was found to increase approximately linearly with frequency. The measured sound speed agrees well with Biot theory predictions, while the measured attenuation is higher than Biot predictions, most likely because the measurement include effects such as volume scattering not taken into account in the theory.

This presentation investigates the impact of atmospheric turbulence on sound propagation using a beamforming array. Experimental measurements were conducted at the North Hartland Dam in Hartland, VT with a Nor848B acoustic camera and a dual-diaphragm compression driver serving as the sound source playing simultaneous frequencies at 225, 450, 900, 1800, and 3600 Hz. The source and acoustic camera were positioned approximately 100 m apart, with the source approximately elevated 55 m and the camera 1.5 m above ground level. A weather station, situated 2 m above the ground, was placed along the propagation path to monitor atmospheric conditions. Measurements were carried out at various times of the day to capture variability in different weather conditions. Observations using the acoustic camera revealed significant acoustic image wander and scintillation. This paper presents a statistical analysis of the acoustic image wander over time, and correlations between these variations and weather conditions.

The sound-ray model SMART (Sound propagation model of the atmosphere using ray-tracing) simulates the modified sound propagation due to sound-ray refraction in a stratified and moving atmosphere. The study was focussed on the influence of refraction on the sound propagation because this effect can result in a significant deviation from the straight-ray approximation used by the acoustic tomography and by the SODAR technique. For validation of the simulated data a measuring campaign was carried out over flat terrain in autumn 2004. The comparison of the modelled with measured data during clear night conditions with strong temperature inversion shows a satisfactory agreement, which leads to the conclusion that the important effect of refraction on outdoor sound propagation is reliably described by the model SMART.

Time-Reversal Invariance and the Relation between Wave Chaos and Classical Chaos.- Acoustic Time-Reversal Mirrors.- Ocean Acoustics, Matched-Field Processing and Phase Conjugation.- Time Reversal, Focusing and Exact Inverse Scattering.- Detection and Imaging in Complex Media with the D.O.R.T. Method.- Ultrasound Imaging and Its Modeling.- Nondestructive Acoustic Imaging Techniques.- Seismic Anisotropy Tomography.- Elastic-Wave Propagation in Random Polycrystals: Fundamentals and Application to Nondestructive Evaluation.- Imaging the Viscoelastic Properties of Tissue.- Estimation of Complex-Valued Stiffness Using Acoustic Waves Measured with Magnetic Resonance.- A New Approach for Traveltime Tomography and Migration Without Ray Tracing.- Simple Models in the Mechanics of Earthquake Rupture.

In 2015, the Graduate Program in Acoustics at Penn State will be celebrating 50 years as the only program in the United States offering the Ph.D. in Acoustics as well as M.S. and M.Eng. degrees in Acoustics. An interdisciplinary program with faculty from a variety of academic disciplines, the Acoustics Program is administratively aligned with the College of Engineering and closely affiliated with the Applied Research Laboratory. The research areas include: ocean acoustics, structural acoustics, signal processing, aeroacoustics, thermoacoustics, architectural acoustics, transducers, computational acoustics, nonlinear acoustics, marine bioacoustics, noise and vibration control, and psychoacoustics. The course offerings include fundamentals of acoustics and vibration, electroacoustic transducers, signal processing, acoustics in fluid media, sound-structure interaction, digital signal processing, experimental techniques, acoustic measurements and data analysis, ocean acoustics, architectural acoustics, noise control engineering, nonlinear acoustics, ultrasonic NDE, outdoor sound propagation, computational acoustics, flow induced noise, spatial sound and 3D audio, marine bioacoustics, and the acoustics of musical instruments. Penn State Acoustics graduates serve widely throughout military and government labs, academic institutions, consulting firms, and industry. This poster will summarize faculty, research areas, facilities, student demographics, successful graduates, and recent enrollment and employment trends.

A underwater acoustic imaging system capable of working in extremely turbid conditions and producing high resolution dimensioned images has been developed. It is suitable for inspecting pipelines and offshore structures. A large development program has been carried out in Australia as a collaborative effort between a number of organisations to overcome the traditional problems with underwater imaging. The system uses megahertz frequency sound, sparse array technology, one-bit digitisation and chirp illumination. Acoustic three dimensional images with range and orthogonal dimensions have been obtained from a prototype underwater acoustic imaging system and examples are presented. Resolutions of millimeters have been obtained. This high resolution, large image (103x103x103 voxels) system is a new-to-the-world capability. This paper is a step in diffusing the new technology so that underwater acoustic imaging may become an innovation in offshore inspection and measurement. Introduction A new technology has been demonstrated for generating three dimensional near video quality images using underwater acoustics. There are two technologies for producing focused acoustic images, lenses and beam forming from twodimensional arrays. The lens has been demonstrated for two dimensional images by Belcher (1998) using a onedimensional array in the focal plane. The lens system places an array of acoustic sensors at the focal plane of the system and avoids the processing load of beam forming. Erikson (1997) discusses a more capable two-dimensional focal plane array. Beam formed images rely on the ability of the signals from an appropriately spaced array of acoustic sensors to be delayed in time and added to make the reflected pressure signals in phase for a particular voxel. For a large imaging volume this can become a very large processing load. The Underwater Acoustic Imaging technology is founded on five elements. Sparse arrays, one-bit digitisation, chirp ensonification, economical acoustic tiles and correlation flattening. While all of these elements have been used in some context before, it took the implementation of them all in a single system to produce a potential innovation. The sonar equation sets limits to performance that are determined by the source level and the system noise. While the source level is felt to be limited by the cavitation pressure, even before that magnitude the acoustic signal is nonlinear. Non linear acoustic signals distort during propagation and make replicate correction more difficult. System noise limits the range at which an object of given target strength can be seen above this noise. Arrays do not focus exactly on a single voxel but allow acoustic energy from adjacent voxels to leak into the picture. Replicate correlation of the received signals with the chirp ensonification, which provides focusing in the range direction, does the same. The clutter from elements of the target limits the size of objects that can be viewed clearly by restricting the contrast. Technology For an active acoustic imaging system such as the present, the acoustic pressure on the face of the receive array depends on the source level, the range of the target, its target strength and the absorption and scattering of water.

The observed upper ocean zonal current, 20°C isotherm depth, and zonal wind stress, calculated from the European Centre for Medium Range Weather Forecasts (ECMWF) model output, were examined in the equatorial Pacific during the period of frequent Westerly Wind Bursts (WWBs) in the period October-April from 1988 (La Nina year) to 1992 (El Nino year). WWBs increased in strength and number, shifting eastwardly towards El Nino, and causing the frequent reversal of surface current, and its vertical gradient within the western basin. Similar features were also observed in the west-central basin, but only during the El Nino event. The reversal of surface zonal current in the central and eastern basins was generally accompanied by surfacing of the Equatorial Undercurrent (EUC), though the vertical gradient of zonal current velocity in the upper ocean never reversed. Fluctuations in upper ocean zonal currents in response to the WWBs were largest in the western basin, while fluctuations in 20°C isotherm depths were largest in the central and eastern basins. Discernible peaks in ocean responses due to WWBs were modulated in their propagation away from the forced region. Since WWBs occurred in succession at various locations, ocean responses to each individual WWB were interfered with by other WWB events. A simple analytical model was therefore devised to describe the temporal and spatial evolution of the interference patterns in relation to various locations and WWB events. Pattern evolutions for both upper ocean zonal current and thermal structures were further modulated by easterly wind fluctuations east of the dateline. Tracing individual wave modes using peakto- peak comparisons is therefore a subjective process.

Persistent multiyear cold states of the tropical Pacific Ocean drive hydroclimate anomalies worldwide, including persistent droughts in the extratropical Americas. Here, the atmosphere and ocean dynamics and thermodynamics of multiyear cold states of the tropical Pacific Ocean are investigated using European Centre for Medium-Range Weather Forecasts reanalyses and simplified models of the ocean and atmosphere. The cold states are maintained by anomalous ocean heat flux divergence and damped by increased surface heat flux from the atmosphere to ocean. The anomalous ocean heat flux divergence is contributed to by both changes in the ocean circulation and thermal structure. The keys are an anomalously shallow thermocline that enhances cooling by upwelling and anomalous westward equatorial currents that enhance cold advection. The thermocline depth anomalies are shown to be a response to equatorial wind stress anomalies. The wind stress anomalies are shown to be a simple dynamical response to equatorial SST anomalies as mediated by precipitation anomalies. The cold states are fundamentally maintained by atmosphere-ocean coupling in the equatorial Pacific. The physical processes that maintain the cold states are well approximated by linear dynamics for ocean and atmosphere and simple thermodynamics.

A flexible sound generator based on the thermoacoustic effect was proposed for use with thin and flexible devices. The proposed sound generator was composed of three thin films made of aluminum, polyimide, and graphite for effective interaction between acoustic energy and heat. The aluminum functions as an electrode for heat radiation into the air, polyimide as a heat insulator, and graphite as a heat sink. The shape of the electrode was a rectangle. Relationship between sound pressure, applied voltage and temperature change of the electrode was modeled. Radiated sound pressure was analytically described as a function of the input signal's frequency. Experimental measurements were carried out and they showed the validity of the model in terms of radiated sound pressure estimation from the proposed structure with selected materials. Surface vibration was also observed, and it was confirmed that the proposed sound generator is vibration-free.