Showing papers on "Ocean acoustic tomography published in 2013"

Measuring the Kuroshio Current with ocean acoustic tomography

Abstract: Ocean current profiling using ocean acoustic tomography (OAT) was conducted in the Kuroshio Current southeast of Taiwan from August 20 to September 15, 2009. Sound pulses were transmitted reciprocally between two acoustic stations placed near the underwater sound channel axis and separated by 48 km. Based on the result of ray simulation, the received signals are divided into multiple ray groups because it is difficult to resolve the ray arrivals for individual rays. The average differential travel times from these ray groups are used to reconstruct the vertical profiles of currents. The currents are estimated with respect to the deepest water layer via two methods: An explicit solution and an inversion with regularization. The strong currents were confined to the upper 200 m and rapidly weakened toward 500 m in depth. Both methods give similar results and are consistent with shipboard acoustic Doppler current profiler results in the upper 150 m. The observed temporal variation demonstrates a similar trend to the prediction from the Hybrid Coordinate Ocean Model.

Measuring Ocean Currents: Tools, Technologies, and Data

Abstract: Measuring Ocean Currents: Tools, Technologies, and Data covers all major aspects of ocean current measurements in view of the implications of ocean currents on changing climate, increasing pollution levels, and offshore engineering activities. Although more than 70% of the Earth is covered by ocean, there is limited information on the countless fine- to large-scale water motions taking place within them. This book fills that information gap as the first work that summarizes the state-of-the-art methods and instruments used for surface, subsurface, and abyssal ocean current measurements. Readers of this book will find a wealth of information on Lagrangian measurements, horizontal mapping, imaging, Eulerian measurements, and vertical profiling techniques. In addition, the book describes modern technologies for remote measurement of ocean currents and their signatures, including HF Doppler radar systems, satellite-borne sensors, ocean acoustic tomography, and more. Crucial aspects of ocean currents are described in detail as well, including dispersion of effluents discharged into the sea and transport of beneficial materials-as well as environmentally hazardous materials-from one region to another. The book highlights several important practical applications, showing how measurements relate to climate change and pollution levels, how they affect coastal and offshore engineering activities, and how they can aid in tsunami detection. It includes: coverage of measurement, mapping and profiling techniques; descriptions of technologies for remote measurement of ocean currents and their signatures; and, reviews crucial aspects of ocean currents, including special emphasis on the planet-spanning thermohaline circulation, known as the ocean's "conveyor belt," and its crucial role in climate change.

Passive acoustic tomography of the ocean using arrays of unknown shape

Abstract: It was recently established that ocean acoustic tomography based on an inversion of ray travel times can be implemented without use of any dedicated sound sources by cross-correlating the ambient noise recorded on two line arrays, the shapes of which are known. In contrast to active tomography, the amount of useful information from noise interferometry is proportional to the product of the numbers of receivers in the two arrays. In our study based on the 2D and 3D numerical experiments, we examine a hypothesis concerning the feasibility of simultaneous performance of a passive ray tomography and passive positioning of arrays through cross-correlation of ambient or shipping noise. The numerical experiments are conducted under conditions close to those of a field experiment on passive ocean tomography. It is demonstrated that, when using arrays of 20–40 hydrophones, the sound velocity profile and the array shape can be found from noise correlation to an accuracy adequate for oceanological and acoustic applications.

Shallow-water acoustic tomography from angle measurements instead of travel-time measurements

Abstract: For shallow-water waveguides and mid-frequency broadband acoustic signals, ocean acoustic tomography (OAT) is based on the multi-path aspect of wave propagation. Using arrays in emission and reception and advanced array processing, every acoustic arrival can be isolated and matched to an eigenray that is defined not only by its travel time but also by its launch and reception angles. Classically, OAT uses travel-time variations to retrieve sound-speed perturbations; this assumes very accurate source-to-receiver clock synchronization. This letter uses numerical simulations to demonstrate that launch-and-reception-angle tomography gives similar results to travel-time tomography without the same requirement for high-precision synchronization.

Inversions of statistical parameters of an acoustic signal in range-dependent environments with applications in ocean acoustic tomography

Abstract: The paper presents an application of a method for the characterization of underwater acoustic signals based on the statistics of the wavelet transform sub-band coefficients in range-dependent environments. As it was illustrated in previous work, this statistical characterization scheme is a very efficient tool for obtaining observables to be exploited in problems of ocean acoustic tomography and geoacoustic inversion, when range-independent environments are considered. Now the scheme is applied in range-dependent environments for the estimation of range-dependent features in shallow water. A simple denoising strategy, also presented in the paper, is shown to enhance the quality of the inversion results as it helps to keep the signal characterization to the energy a significant part of it. The results presented for typical test cases are encouraging and indicative of the potential of the method for the treatment of inverse problems in acoustical oceanography.

Parallel computing method of seeking 3D eigen-rays with an irregular seabed

Abstract: In the ocean environment, the internal wave and irregular seabed can cause the ray trace direction reverse in vertical and deflect in horizontal. Therefore the propagation of ray in three dimensional (3D) should be taken into account for long range sound propagation, such as ocean acoustic tomography. The 3D eigen-ray calculation usually is very complex problems and cost long computation time. Though some optimization methods can be applied to decrease computation time, they may lose some computational accuracy. Parallel programming is an important method to improve the speed of multiple data operation at low-cost. A parallel algorithm for 3D eigen-rays searching in waveguide with an irregular seabed is realized based on OpenMP. According to this model, seeking eigen-rays and computation ray tracing and transmission time of eigen-rays are completed. Computing results show that this parallel algorithm performs higher precision and speed with 3.76 speedup ratio in a quad-core CPU. It is hoped that this 3D sound ray model can provide some help in acoustical oceanography research.

Time-angle ocean acoustic tomography using sensitivity kernels: Numerical and experimental inversion results

Abstract: In shallow water acoustic tomography, broadband mid-frequency acoustic waves (1 to 5 kHz) follow multiple ray-like paths to travel through the ocean. Travel-time (TT) variations associated to these raypaths are classically used to estimate sound speed perturbations of the water column using the ray theory. In this shallow water environment, source and receiver arrays, combined with adapted array processing, provide the measurement of directions-of-arrival (DOA) and directions-of-departure (DOD) of each acoustic path as new additional observables to perform ocean acoustic tomography. To this aim, the double-beamforming technique is used to extract the TT, DOA and DOD variations from the array-to-array acoustic records. Besides, based on the first order Born approximation, we introduce the time-angle sensitivity kernels to link sound speed perturbations to the three observable variations. This forward problem is then inverted with the maximum a posteriori method using both the extracted-observable variations and the proposed sensitivity kernels. Inversion results obtained on numerical data, simulated with a parabolic equation code, are presented. The inversion algorithm is performed with the three observables separately, namely TT, DOA and DOD. The three observables are then used jointly in the inversion process. The results are discussed in the context on ocean acoustic tomography.

Time-angle ocean acoustic tomography using sensitivity kernels: The forward problem

Abstract: Broadband acoustic signals around 1 kHz propagate through shallow water oceanic waveguides of ~100 m in depth and ~2 km in range as multiple ray-like wavefronts. These acoustic arrivals can be characterized by the following observables: travel-time (TT), direction-of-arrival (DOA), and direction-of-departure (DOD). By applying double-beamforming on the point-to-point signals recorded between two source-receiver arrays, the acoustic contribution of each arrival can be separated from the multi-reverberated data and the TT, DOA, and DOD observable variations are accurately measured. This study deals with the use of time-angle sensitivity kernels (TASK) to estimate the observable variations induced by sound speed perturbations in the waveguide. This approach is based on the first order Born approximation and takes into account the finite-frequency effects associated with wave propagation. The robustness the TASK approach is analyzed and compared to numerical parabolic equation simulations involving different sound speed perturbations. For example, parameters such as the perturbation location, the value and shape of the perturbation in the waveguide are modified. The combination of several perturbations and the influence of the source-receiver array apertures on the TT, DOA, and DOD estimates are also studied.

Full waveform inversion applied to hydroacoustic velocity mapping

Abstract: Water temperature, salinity concentration, current velocity distribution, etc. in the ocean, are very important physical properties to understand the thermal structure of the ocean. The monitoring of these physical properties could be achieved by either of two kinds of measurement schemes, i.e., one the direct measurements using instruments such as ARGO floats and the other the indirect method using some other parameters such as the sound speed. Ocean acoustic tomography (OAT) was invented to verify the possibility of the latter. Although analysis based on the travel time of sound inversion using the inversion technique (TTI) has been attempted based on the ray theory, it is expected to improve both the resolution and the accuracy of the measurements. In the ocean, sound waves travel through complex structure where the contrast in the velocity could be of the order of less than a percent. Therefore, the application of TTI may not produce satisfactory results. We first state the following two hypotheses to propose a method to improve OAT: 1) full waveform inversion (FWI) could improve the resolution, 2) attenuation is likely to be considered in the velocity inversion. The application of FWI to synthetic examples to test these hypothesis provided us pretty satisfactory results and the validity of our hypothesis have been verified.

Path planning of autonomous underwater vehicle for optimal acoustic tomography

Abstract: Ocean acoustic tomography (OAT) can infer the state of the interior of the survey domain from measurements along the periphery. Traditionally, a network of mooring platforms with projector/hydrophone array is deployed, sometimes combined with moving vessel nodes. In this paper we look at the OAT problem with autonomous underwater vehicle (AUV) equipped with a hydrophone as a moving receiver and study the AUV path-planning techniques to optimize the tomographic measurements. The path planning problem is to find the optimal AUV depths that minimize the sound speed profile posterior uncertainty along its path under the AUV navigation constraint. Numeric simulation results are shown to confirm the effectiveness of the approach.

Inversion of ocean currents using NTU-RAY

Abstract: Ocean Acoustic Tomography is a well-known technique for acoustic remote sensing of the temperature, water velocity, and current today. This technique makes it possible to probe the ocean where is large scale or difficult to deploy instruments. In this study, we build an inverse method to estimate the ocean current from the travel time perturbation of eigenrays which was calculated by NTU-RAY. NTU-RAY is a numerical model which is included in NTU-GBM (Gaussian Beams Model). The purpose of this study is to estimate the feasibility of an experiment in the north-eastern sea of Taiwan in the future. The method is tested first in an idealized environment, such as Pekeris waveguide, and then applies to the real ocean scenario. Results show that the inverse method works well generally but with limitations. (This work is sponsored by National Science Council of Taiwan under project “Physical Oceanography and Acoustic Applications at the Marine Observatory in the Northeastern Taiwan (MONET) coordination project and subproject 1: Study of Underwater Acoustic Signature Extraction and its Data Base Automation” No. NSC 101-2221-E-002-028-MY2).

Climate Change and Tomography

Abstract: Ocean acoustic tomography was first developed by Munk and Wunsch (1979), Munk et al. (1995) and who used it to generate temperature profile of sea water using acoustics. Cornuelle (1985, 2008) have used tomographic methods to generate temperature profiles. It has been already reported in literature that a temperature difference of 1 °C in sea water results in a difference of 4 m/s in speed of acoustic waves passing through it. This indicates that waves passing through different temperatures will have different time of flight to reach the same distance. This phenomenon can be used to regenerate the temperature profile of the path traversed by the wave (ray) from projection data. Sea water acts as a good medium for sound waves to travel long distances. These waves get destructed by many ways such as noise produced by naval ships, noise produced by sea mammals and other similar sounds.