Ocean acoustic tomography: Past, present, and maybe future

Ocean Acoustic Tomography (OAT) is an efficient and economical marine acoustic observation technique. Targeted observation is an appealing procedure to reduce the uncertainty of ocean environment prediction through additional observation. This study aimed to assess the validity of OAT as an observation method for targeted observation. OAT based on Niche Genetic Algorithm was employed to extract sound speed and temperature profiles from acoustic transmission time, utilizing data from the 2019 Yellow Sea experiment. The inversion results were compared with measurement data, which are found to be accurate and reliable. To further evaluate OAT as targeted observation method, the vertical bias structure of OAT was added on synchronous measurement data in the sensitive area of targeted observation to simulate OAT observation in sensitive area. This simulated data was then incorporated into a 3D-Var assimilation system to improve the short-term prediction of the target region. Comparing the predictions derived with the measurement data at the verification time, it shows that the simulated OAT observation improved the quality of target region prediction, indicating that OAT can be an effective observation method for targeted observation. An Observing System Simulation Experiment was conducted to assess the impact of OAT characteristics on prediction improvement. The results show that both adding observation nodes and extending the observation duration have positive effects, while extending the observation duration performs better.

Chapter 1 - Fundamental Knowledge

Accurate knowledge of the spatial and temporal evolution of the three-dimensional ocean sound speed profile (SSP) is crucial for underwater source localization. Ocean acoustic tomography (OAT) methods aim at reconstructing SSPs variations based on acoustic measurements between multiple source-receiver pairs (e.g., eigenrays arrival times). This study investigates the estimation of range-dependent SSPs using a classical model-based OAT method (i.e., ray-based ocean acoustic tomography), for various configurations of source and receiver configurations using autonomous platforms in a highly-dynamic upper ocean environment. A regional oceanographic simulation of the De Soto Canyon circulation in the Gulf of Mexico is used to construct 3D sound speed variations spanning a month long which exhibits significant sub-mesoscale variability. Two main aspects affecting OAT performance in the presence of high 3D SSPs variability are investigated: (1) The influence of the input acoustic data (i.e., source-receivers configuration and platform motion, arrival-times accuracy...) and (2) the actual implementation of the OAT scheme (i.e., selection of complexity reduction basis, linearized forward model assumptions as well as the use of iterative solvers) on SSPs estimations errors. Practical implications for the design of OAT experiments in the dynamic upper ocean will be discussed. [Work supported by the Office of Naval Research.]

Early attempts at understanding scintillations in sound transmission through the ocean assumed that ocean fine structure is homogeneous and isotropic. It is, in fact, dominated by internal waves, which are neither homogeneous nor isotropic. Sound Transmission Through a Fluctuating Ocean (Flatté et al., Cambridge University Press, Cambridge, England 1979) combined recently developed internal-wave models with path-integral methods to predict the fluctuations of resolved acoustic multipaths. At that time, Ocean Acoustic Tomography (Munk et al., Cambridge University Press, Cambridge, England 1995), which uses ray travel times to determine large-scale ocean structure, had just been proposed. The wideband acoustic sources and receivers required to make travel-time measurements for tomography also provided the technology needed to quantify acoustic fluctuations. Conversely, internal-wave-induced scattering limits travel-time measurement precision. Tomography experiments at 25-km range provided some of the earliest tests of path-integral predictions. Subsequent tomography experiments provided data at ever-increasing ranges and decreasing frequencies. The Acoustic Thermometry of Ocean Climate project made measurements at megameter ranges with 1400-m vertical line array receivers. The next step is a modular distributed VLA capable of spanning the full water column that is under development to enable separation of acoustic modes using spatial filtering and to fully characterize deep-water acoustic time fronts.

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 inversed by 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.

On the basis of the stream function field computed using a diagnostic model in the South China Sea in the summer 1998, the acoustic ray travel time tomography algorithm for reconstruction of two-dimensional flow field was tested by the numerical simulation in the region east to Vietnam. Error or noise contaminates the coefficients of both transfer matrix and measure data of the inverse equation of ocean acoustic tomography (OAT). Previous work on ill-posed problem in OAT commonly used Tikhonov regularization technology considering error only in observation data. Here, the truncated total least squares method (TTLS), which simultaneously considers error and noise on both sides, is applied for inverse solutions and its performances is compared with conventional Tikhonov method. The investigation suggests that the truncated total least squares method can substantially improve reconstruction of stream function field and is a promising alternative method for the solution of ocean acoustic tomography inverse problem.

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.

The travel time measurements used in ocean acoustic tomography are based on the use of wideband waveforms. Because of this, the effects of source or receiver motion must be modeled as time compression/expansion and not as simple frequency shifts. Equivalent situations occur when there is a frequency error between the clocks used to generate an ocean acoustic tomography wideband signal and the clock used to digitize the associated reception, and when the clock used to digitize the reception is not “properly” matched to the structure of the transmitted waveform. The common thread is that you have uniformly spaced samples at one set of times but you really need them at a different uniform spacing at a different set of times. The obvious solution is to interpolate. This paper describes the effects of some simple interpolaton techniques on the processing of the wideband waveforms used in making ocean acoustic tomography travel time measurements.

Getting oceanographic data on a large scale and with sufficient time and space resolution is generally very difficult. However, these data are essential when studying with accuracy the propagation of sound, especially when simple analytic models are not available due to the complexity of the ocean variability at the required distance and time scale. This is for example the case of oceanic acoustic tomography (OAT) in the North Eastern Atlantic (study area of the French national program POMME). In OAT, the classical stratified assumption has to be checked against realistic simulations of long range wave propagation. For the North Eastern Atlantic where the Mediterranean mid waters and the mixed upper layer induce complex effects on temperature profiles, numerical models tend to be more and more accurate. This is the case for the CLIPPER model developed by the LEGI(*) in Grenoble. The authors show in this work how this set of high resolution and realistic numerical data can be used to simulate the acoustic wave propagation with new insights. They focus their analysis to the POMME area: [-15 W, -22 W] longitudes, [35 N, 48 N] latitudes. They first describe the numerical oceanographic data as given by the CLIPPER model and show their high resolution quality. They present then the results of systematic ray tracing calculations that has been made between pairs of transmitter and receiver (receiver is a long-line vertical array) in the North-South direction.

As ocean insonification for basin scale Ocean Acoustic Tomography (OAT) appears to be a rather complicated and expensive matter, it is then interesting to get as much as possible information from each insonification. One possibility is then to increase the number of reception points. In that context, IFREMER started the design of a new autonomous receiver which could fulfill OAT requirements.

Ocean acoustic tomography (OAT) is a method proposed by Munk and Wunsch (1979) to observe the ocean using acoustic transmissions. Although the ocean is nearly opaque to most electromagnetic radiation, acoustic waves propagate well, and low frequencies (less than 100 Hz) can propagate usefully for more than 10 megameters (Munk, Worcester, and Wunsch, 1995). The term "tomography" was chosen to highlight the analogy to medical computed axial tomography (CAT), although because ocean instrumentation and access are expensive, the number of data available in OAT has been far smaller than in medical applications. For this reason, the reconstruction of the unknown field depends critically on utilizing all information, including prior guesses of first and second moment statistics of the data and unknowns and, in some cases, dynamical models of the ocean.

Ocean acoustic tomography (OAT) technique can obtain oceanographic information and has been received a lot of interest. High frequency OAT (in few kHz range) can be used for small and confined areas such as estuaries and bays with complicated hydrological conditions. In this study, we investigate the application of the high-frequency reciprocal transmission OAT to assess the temperature and current in Xiamen sea area using computer simulations and sea experiments. Based on the temperature data obtained from remote sensing, high frequency OAT is employed to reconstruct two-dimensional temperature, sound speed, and current field of the 1.2 km × 1.2 km region. The results show that increasing the number of acoustic stations decreases the travel-time errors of high frequency OAT; however, excessively increasing stations can not significantly improve the inversion accuracy. Furthermore, this method has also been examined by a sea experiment on monitoring the current and water temperature of Wuyuan Bay. High frequency OAT might provide an effective method on temperature and current observation at coastal estuary areas. (This work was financially supported by the National Science Foundation of China (Grant No. 11174240) and the Fundamental Research Funds for the Central Universities (2011121010))

A data assimilation scheme for ocean acoustic tomography (OAT) data is developed and evaluated by an identical twin experiment to understand the dynamical processes and the evolution of flow pattern in the Kuroshio extension region. The scheme consists of three components, i.e., the two-layer quasi-geostrophic (QG) oceanic numerical model, the travel time data obtained from OAT, and the data assimilation technique which consists of the Kalman filter and observation equations relating the model variables (stream function and interface displacement) to the OAT-derived reciprocal travel time. The identical twin experiment using two different model fields made from the same model demonstrates both the stability and the availability of the scheme developed in the present study.

We analyzed the effect of a sound speed structure near the ocean surface on sound propagation in a deep sound channel in the ocean by ocean acoustic tomography (OAT), using the normal mode theory. The time structure of the received pulse train in the deep sound channel varies according to the sound speed structure near the ocean surface. In the analysis of the inverse problem in OAT, attention must be paid to the sound speed structure near the ocean surface in the ocean area of interest.

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.