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Summary of Results

The most important scientific achievements of our project are:

Successful processing of raw acoustic data and tomographic reconstruction of the sound speed field for two particular sections prove that the experimental background was created for adequate comparison of three different approaches to the Ocean Acoustic Tomography under the same conditions and making recommendations on their use in future regular tomographic observational systems.

The to date results of the project were summarized in 4 articles in Russian scientific journals [P. 1, P. 2, P. 3, P. 4] (the first one was already brought out). Two more articles are currently under preparation in cooperation with European participants of the project. The project achievements were reported at 4 international conferences on acoustics and oceanography hold in Europe and the U.S. in 1995-96. The 6-page report with the first results on tomographic inversions was published in the Proceedings of 3rd European Conference on Underwater Acoustics [P. 5]. At the other meetings the abstracts of the respective reports were published [P. 6, P. 7, P. 8, P. 9]. Copies of all publications are attached in the Part ii of the report.

The experimental data processing and the inversions, started during the present project, were further supported by the Russian Foundation for Basic Research (RFBR), project # 95-05-14616 entitled ``Acoustic Tomography of the Western Mediterranean from a Moving Ship''. Development of the innovative inversion techniques contributed also to the project RFBR # 96-05-65880 devoted to acoustic tomography and thermometry of Arctic ocean.

The present state of data processing and inversions was reported and discussed at three workshops hold in Kiel, Germany (February 1995) and Heraklion, Crete, Greece (September 1995 and June 1996). For better coordination with the major experiment, the first 2 meetings were combined with regular workshops of the THETIS-2 team. The basic challenges in both experiments are quite similar. The discussions hold at the workshops were fruitful for both projects. They initiated or promoted the study of:

Well-grounded judgements on use of Dynamic Tomography in future long-term observational systems are possible only after completion of the inversions and their interpretation. However, the experience gained in at-field implementation of Dynamic Tomography technique allows to provide some methodological advices for other researchers interested in this subject.

  1. For sound speed field reconstruction in Dynamic approach one can apply a differential scheme, e.g., use the differences in times-of-flight of several eigenrays instead of the times-of-flight themselves.
  2. Formally speaking, differential tomography does not require the precise knowledge of absolute propagation time and source-receiver separation. However, it is quite useful to have accurate positioning systems for the source(s) and receiver(s) and precise time synchronization between all of them. Even if the total error in measuring the absolute propagation time does not allow to use it directly for inversion, such a priori information helps to choose out the unique solution of the inverse problem.
  3. Relative position of the ship-borne source or receiver with respect to the R/V can be controlled by high-frequency acoustic navigation systems, using the R/V as observation base. It is desirable to monitor the device position permanently over all time of the record and not only between the tomographic receptions, as it was done in our experiment.
  4. If a ship-borne receiving system is applied (cf. a ship-borne source), special precations should be taken to decrease the ship-generated noise. If possible, the measurements should be carried out with (most of) the R/V engines turned off.
  5. Significant receiver drift unavoidable in Moving Ship Tomography is no hindrance to application of M-sequence based probing signals. Special data-processing technique allows to estimate the drift velocity with 0.5-1 cm/s accuracy. Then the negative impact of the Doppler frequency shift is eliminated by, first, changing the demodulation frequency, and, second, adjusting the time scale of the recorded signal. Both effects proved to be important.
  6. One can use the receiver drift to synthesize a horizontal receiving aperture and carry out angular decomposition of the recorded signal at such array. For significant drift velocity and signal duration (in the case considered about 0.7 m/s and 200 s, respectively) it allows to resolve simulteneously arriving rays with different incident angles. Under certain conditions one can try to artificially increase the R/V velocity sacrificing the SNR due to flow past a receiver to longer aperture and better angular resolution. Vertical displacement of the receiver may give even better results [37] (regrettably, not implemented in our experiments).
  7. The dynamic range of the receiving system should be large enough - no less than 80 dB - to account for variations both in the external noise level due to the sea state and the transmission loss for different propagation ranges / different sources.

next up previous contents
Next: Application of Results Up: Results Previous: Prediction of SSP variations

Dmitry Mikhin
Sun Dec 8 11:32:03 GMT+0300 1996