Showing papers on "Ocean acoustic tomography published in 2006"

An experimental demonstration of blind ocean acoustic tomographya)

Abstract: Despite the advantages clearly demonstrated by ocean acoustic tomography (OAT) when compared to other ocean monitoring techniques, it suffers from several technical-related drawbacks. One is the requirement for rather expensive equipment to be maintained and operated at several locations in order to obtain sufficient source–receiver propagation paths to cover a given ocean volume. This paper presents the preliminary feasibility tests of a concept that uses ships of opportunity as sound sources for OAT. The approach adopted in this paper views the tomographic problem as a global inversion that includes determining both the emitted signal and the environmental parameters, which is a similar problem to that seen in blind channel identification and was therefore termed blind ocean acoustic tomography (BOAT). BOAT was tested on a data set acquired in October 2000 in a shallow-water area off the west coast of Portugal, including both active and passive (ship noise) data. Successful results show that BOAT is abl...

Mode-1 internal tides in the western North Atlantic Ocean

Abstract: Mode-1 internal tides were observed the western North Atlantic using an ocean acoustic tomography array deployed in 1991–1992 centered on 25°N, 66°W. The pentagonal array, 700-km across, acted as an antenna for mode-1 internal-tides. Coherent internal-tide waves with O (1 m) displacements were observed traveling in several directions. Although the internal tides of the region were relatively quiescent, they were essentially phase locked over the 200–300 day data record lengths. Both semidiurnal and diurnal internal waves were detected, with wavenumbers consistent with those calculated from hydrographic data. The M 2 internal-tide energy flux was estimated to be about 70 W m −1 , suggesting that mode-1 waves radiate 0.2 GW of energy, with large uncertainty, from the Caribbean island chain at this frequency. A global tidal model (TPXO 5) suggested that 1–2 GW is lost from the M 2 barotropic tide over this region, but the precise value was uncertain because the complicated topography makes the calculation problematic. In any case, significant conversion of barotropic to baroclinic tidal energy does not occur in the western North Atlantic basin. It is apparent, however, that mode-1 internal tides have very weak decay and retain their coherence over great distances, so that ocean basins may be filled up with such waves. Observed diurnal amplitudes were an order of magnitude larger than expected. The amplitude and phase variations of the K 1 and O 1 constituents observed over the tomography array were consistent with the theoretical solutions for standing internal waves near their turning latitude. The energy densities of the resonant diurnal internal waves were roughly twice those of the barotropic tide at those frequencies.

Fluctuation of long-distance sound propagation in the central pacific

Abstract: Ocean acoustic tomography (OAT) is a useful method for observing oceanographic phenomena over wide regions. A tomography experiment using seven 200 Hz transceivers was executed in the central equatorial Pacific Ocean from January to December 2000 to monitor ocean phenomena related to El Nino and southern oscillation (ENSO). Arrival times of the maximum amplitude of received signals were used to evaluate fluctuations through out the year. Tidal effects and semidiurnal currents were observed from the spectral analysis of arrival times through out the experimental period. From the comparison of reciprocal arrival time in several paths, reciprocal arrival time differences between transceiver no. 4 (T4) and no. 6 (T6) changed during mid-September to November. This change may be caused by ocean changes in the oceanic environment. Various changes through out an entire year could be detected using the sound propagation data at a depth of 1,000 m.

Backpropagation techniques in ocean acoustic inversion: Time reversal, retrogation and adjoint model-A review

Abstract: In light of recent interest in adjoint modelling in underwater acoustics, we present a selective review which is mainly focused on the underlying concept of backpropagation. The different implementations of that concept to date are compared and discussed in the framework of experimental acoustic inversion in shallow water with application to source localisation, ocean acoustic tomography, geoacoustic inversion and underwater communications. Well established inversion or focalisation methods based on matched field processing, model-based matched filter and time reversal mirror are related to less popular ones such as acoustic retrogation and other variants of backpropagation. In contrast to the latter, adjoint-based, variational inversion approaches make use of the adjoint of a forward model to backpropagate the model-data mismatch at the receiver toward the source. The paper describes in greater detail adjoint methods and applications in underwater acoustics. We also present results using environmental data obtained during a geoacoustic inversion experiment in the Mediterranean. © 2006 Springer.

Ocean acoustic tomography using high-frequency systems

Abstract: New system concepts have emerged in which large numbers of acoustic and environmental sensors are networked together in littoral areas. The spatial scales for these networks are much finer—features such as linear and nonlinear internal waves/tides can affect both network and sensor performance. A vision readily comes together in which ocean acoustic tomography is used to monitor the finescale, using the network to relay travel‐time information in real time and also to probe the environment. Further, travel‐time information may be assimilated into an ocean circulation model together with direct current and temperature measurements from fixed and mobile platforms to provide nowcasts and forecasts of the local ocean weather. The acoustic tomography in this vision uses a much higher frequency band (e.g., 8–16 kHz) than has traditionally been used and there has been some skepticism about the feasibility of doing tomography in this band. Using data from two large experiments in Hawaii (KauaiEx and MakaiEx), we show that (1) HF signals can be used to clearly measure the channel impulse response, (2) changes in the impulse response are directly linked to changes in the oceanography, and, therefore, (3) HF signals can be used to invert for the ocean structure.

Adjoint-based shallow water tomography with partially known bottom geoacoustic properties

Abstract: Recently, a numerical extension of the wide-angle PE (WAPE) adjoint-based inversion method has been presented that allows direct inversion of the geoacoustic parameters that are embedded in a discrete representation of nonlocal boundary conditions at the water-sediment interface [Hermand et al., ICTCA 2005]. In contrast to conventional ocean acoustic tomography (OAT), correct modeling of sound interaction with the bottom is particularly important for shallow water acoustic tomography (SWAT). Uncertainty or partial knowledge of the bottom acoustic parameters can significantly degrade SWAT performance. In this context the paper discusses an adjoint-based tomography approach that includes the geoacoustic parameters as additional unknowns in the parameter space. A priori information about the bottom characteristics, e.g., the approximate thickness and composition of sediment layers and hard rock basement are often available from site surveys with sub-bottom profiling systems or in the form of previously archived in situ data. This information can be used as an initial solution to start the inversion process or be included in an augmented cost function through the use of regularization. By means of adjoint modeling exact gradient information can be obtained to determine the sound speed profile of the water column and concomitantly adjust the partially known sediment and bottom halfspace parameters. A case study based on environmental data obtained in Mediterranean shallow waters is used to present first results.

A multistation coastal acoustic tomography system with coherent operation

Abstract: A multistation coastal ocean acoustic tomography (CAT) system with coherent operation has been developed and applied to field experiments over ten times in the last 8 years. The system is designed to measure both temperature and current velocity fields in the coastal ocean. As is well known, an ocean acoustic tomography is generally based on travel time measurements. In the CAT system, the clock accuracy for travel time measurement is most important to realize coherent transmission from multistations and to measure especially current velocity. For highly accurate coherent transmission from multistations, each station is equipped with a timing module synchronized by the clock signals of global positioning system (GPS). As a result, time error is maintained less than 0.1 μs. In the sound transmission from multistations, the code division multiple access (CDMA) and spread spectrum technology are applied. The coded signals also improved remarkably the signal‐to‐noise ratio of the CAT system. With detecting not only the amplitude, but also the phase of received signals, the accuracy of travel time measurement is improved within a sampling interval of data. This multistation CAT system served to map 2‐D time‐varying tidal current structures in Tokyo Bay (Kaneko et al., 2005).

A new concept of ocean acoustic tomography

Abstract: Ocean Acoustic Tomography (OAT) uses powerful active emissions of repetitive signals causing problems when acoustic discretion is required as in military operations. In this paper, we propose to develop a new concept of OAT, called Discreet Acoustic Tomography (DAT), which is based on a stealthy acoustic signals emission. An appropriate global procedure to synthesize a signal waveform in accordance with the compromise between interception probability and accuracy in channel parameters estimation, is proposed. Finally, this procedure was applied to a realistic scenario of which objective consists in hiding a synthetic signal in ship noise with the constraint to have an accurate estimation of the channel parameters. Results obtained illustrate the interest and the potential of the proposed method.