ACOUSTIC MONITORING OF OCEAN CURRENTS IN COASTAL REGIONS

Principal Investigator: David R. Palmer
Collaborating scientist(s):
Oleg A. Godin, P.P. Shirshov Oceanography Institute of the Russian Academy of Sciences
D. Yu. Mikhin, P.P. Shirshov Oceanography Institute of the Russian Academy of Sciences

L. M. Brekhovskikh, P.P. Shirshov Oceanography Institute of the Russian Academy of Sciences

Objective: To develop, implement, and verify a new approach to acoustic monitoring of ocean currents in the coastal environment and to develop plans for a permanent coastal ocean monitoring system based on this new approach. This system will provide real-time, three-dimensional maps of the coastal current field having horizontal scales ranging up to 100 km. The system will be affordable; likely consisting of a number of moored transceivers and a single transceiver array, all cabled to shore.

Rationale: The only acoustical systems now available for measuring currents in coastal regions are high-frequency ones that, at most, measure vertical profiles at a single location. They are completely inadequate for three- dimensional real-time monitoring of the current field. There are compelling reasons for such monitoring. Real-time knowledge of currents in coastal areas is of value to shipping and fishing interests, and for fisheries management. It is also of value in performing search and rescue missions and in tracking oil spills, sewage-discharge plumes, and other potential pollutants. Current maps can serve as a tool for conservation management by guiding activities that minimize negative influences on the coastal environment. In addition, they can be used to verify general circulation models and the assumptions made in their derivation. Ocean acoustic tomography exploits the transparency of the ocean to low-frequency sound and the sensitivity of ray-path travel times to ocean temperature and currents. It is the only viable technique for producing synoptic temperature and current maps of the ocean's interior over horizontal scales large compared to the ocean depth. In "temperature tomography" the temperature field is inferred from one-way transmissions between acoustic sources and receivers. In "current tomography" the current field is inferred from reciprocal transmissions between pairs of acoustic transceivers. A number of sea trials conducted since the late 1970's have shown both temperature and current tomography are feasible in the deep ocean for propagation paths up to about 1000 km. These techniques are now ready for routine operational use by the oceanographic community. This situation is to be contrasted with that in shallow, coastal regions. Experiments conducted in the Straits of Florida demonstrated that the deep- water approach is not suitable in shallow water. Many of the rays cannot be resolved or identified -- to speak nothing of the identification and resolution of pairs of eigenrays in reciprocal transmissions. This situation exists because of the extreme sensitivity of bottom-interacting ray paths to small-scale topography and results in poor resolution in the vertical plane. The failure of traditional current tomography in coastal regions motivated researchers at Shirshov Oceanography Institute of the Russian Academy of Sciences (OIRAS) to develop matched non-reciprocity tomography (MNT), a new approach that could be a practical tool for monitoring ocean currents in coastal regions. It is based on recent progress in the theory of acoustic propagation in moving media achieved at OIRAS.

Method: This effort focuses on two experiments to take place in the Straits of Florida to test and verify MNT. The first experiment, will validate and extend existing theoretical and numerical feasibility studies and will define optimal engineering parameters. The second experiment will use additional transceivers in order to produce a three-dimensional data set. In support of these two experiments, an efficient full-field mathematical model of sound propagation in unsteady, three-dimensional coastal environment will be developed. This model will be the basis for the development of an optimum inversion scheme for constructing maps of the current field from the acoustic data.

Accomplishment: A detailed, draft proposal was submitted to NOAA's Office of Oceanic and Atmospheric Research for consideration for funding under the U.S.-Russian Ocean Studies Agreement. At the recent meeting in St. Petersburg of the U.S.-Russian Joint Committee on Cooperation in Ocean Studies, the Project was determined to be of an advanced character and to have a highly substantiated scientific base. The Committee agreed to continue to seek financial resources for its implementation. Analytic studies have been completed that indicate non-reciprocity of the phase of a continuous wave (CW) signal propagating in opposite directions does not depend, to first order, on fluctuations in sound speed and on uncertainties in transceiver location (see references below). Numerical simulations carried out for a model ocean resembling the conditions in the Straits of Florida have confirmed stability and sensitivity of the phase non-reciprocity of CW signals and the superiority of the MNT scheme to other possible schemes( see references). Presentations have been given at NOAA's Office of Oceanic and Atmospheric Research, the Naval Research Laboratory, the University of Miami, the NOAA Environmental Technology Laboratory, and the 129th Meeting of the Acoustical Society of America. An informal agreement has been made with the Ocean Pollution Research Center, RSMAS/University of Miami to cooperate on computer simulations of MNT in the Straits of Florida.

Key reference:
O. A. Godin and D. Yu. Mikhin, A full field approach to acoustic tomography of ocean currents, J. Acoust. Soc. Am. 97, 3264 (1995).

O. A. Godin and D. Yu. Mikhin, Simulations of acoustic tomography of ocean currents in a coastal region, J. Acoust. Soc. Am. 98, No. II(2) (1995).

R. V. Smart and A. Voronovich (eds.), Acoustic Techniques for Measuring Ocean Variables, A Review of Existing and Emerging Opportunities, prepared jointly by NOAA, the Advanced Research Projects Agency, the Naval Research Laboratory, the National Science Foundation, the Office of Naval Research, and the Strategic Environmental Research and Development Program, (1995). Available by request from Captain Robert Smart, NOAA/OAR, bsmart@rdc.noaa.gov

O. A. Godin, D. Yu. Mikhin, and A. V. Mokhov, "A full field inversion method for acoustic tomography of oceanic currents," Proc. of the NATO Conference on Full Field Inversion Methods in Ocean and Seismic Acoustics, Lerici, Italy, 27 June - 1 July, 1994 in Full Field Inversion Methods in Ocean and Seismo Acoustics, O. Diachok, Caiti, P. Gertstoft, and Schmidt, (eds.) (Kluwer Academic Press, 1995)

D. R. Palmer. M. Lawson, D. A. Seem, and Y. H. Daneshzadeh, "Ray-path identification and acoustic tomography in the Straits of Florida," J. Geophys. Res. 90, 4977 (1985).

D. R. Palmer, R. M. Jones, and T. M. Georges, "Classical chaos and the sensitivity of the acoustic field to small-scale ocean structure," Computer Phys. Commun. 65, 219 (1991).

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