We have been developing Ocean Acoustic Tomography System at Japan Marine Science and Technology Center. Our objective is to measure real time 3-dimensional temperature and current velocity in an area of 10⁶ km²(10³ km × 10³ km) using low frequency sound techniques. We developed 200 Hz low frequency sound source for this system in 1992. Due to the mechanical limitation of pressure compensator of the source, falling speed should not exceed 2.0 m/s. Completion of the mooring techniques is essential for the ocean acoustic tomography system. For this purpose, using acoustic navigation system, we measured transponder's position on the mooring system when we deploy the mooring system.
In this paper, we present 200 Hz sound source mooring system and measurement method, and using measurement and numerical simulation, we analyzed falling motion for ocean acoustic tomography mooring system.

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An example of off-shore three-dimensional seismic survey is reported. The survey was provided on the Barbados accretionary prism where the Atlantic plate subducts beneath the eastern margin of the Caribbean plate. R/V Maurice Ewing of LDGO was used for the survey. Study area locates far form the Barbados Island, approximately 300 km. To take the real-time absolute location of the ship, differential GPS technique was successfully adopted for this survey and completed. Other quality control techniques are also provided for this survey. Three-dimensional seismic survey has great advantages for the earth sciences, and should be commonly provided in the near future.

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ADCP (acoustic Doppler current profiler) measurement was carried out in Irago Suido, an entrance of Ise bay, south coast of Japan, to determine the horizontal and vertical variation of tidal phase and amplitude in the inlet. R/V Tenyo had passed through Irago Suido repeatedly for 25 hours to measure the currents. As a result, the followings are noted;
1) The lower-layer velocity leads surface velocity due to the effect of bottom friction on the oscillating current.
2) The velocity at the center leads the velocities on both sides, bay-side and ocean-side. This agrees with the result of a numerical model.
3) The directions of residual currents are northwestward on the bay-side and southeastward on the ocean-side.

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A long-term temperature monitoring experiment for about one year was carried out at a biological community east of the Hatsushima Island, western Sagami Bay.Three temperature probes, which are 60 cm to 90 cm in length, had been penetrated into the sediment within 20 m of the colony, and one sensor was installed for monitoring the bottom water temperature. Temperatures were measured once a day. The obtained temperature records showed fluctuation that is clearly attributed to variation of the bottom water temperature. The fluctuation was successfully eliminated by subtracting a theoretical temperature variation derived from the Fourier transform of the water temperature data. During this procedure, two unknown parameters, geothermal gradient and thermal diffusivity, could be precisely determined, using the non-linear least squares method. Thus, long-term monitoring of the water and subbottom temperature is very useful for determination of geothermal gradient and heat flow, in cases where the effect of the bottom water temperature variation is significant.

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To elucidate the changes of the ocean current in connection with global warming and to prevent marine disaster and pollution problems in such a distant area, the development of systems that can precisely identify the fluctuations of the sea such as ocean current in the entire area of the North Pacific Ocean has been called for.
Under the present circumstances, however, there are many difficulties to attain such development because the establishment of complete observation networks in a vast ocean requires enormous time and expenses.
Various kinds of numerical simulations of the ocean are presently under way in order to understand the condition of the ocean which we find difficult to indentify because of the lack of measured values and the incompetency of present measured values. On the other hand, marine numerical models contain various uncertain elements such as basic equations and boundary conditions, thereby making it difficult to analyze the ocean circulation with a sufficient accuracy. Insofar as numerical models are incomplete, therefore, a possible method to obtain more realistic results is to make adjustments by replacing predicted values with observed values.
In this research, numerical simulations using a data assimilation system were conducted in the Pacific Ocean areas to examine the effectiveness of data assimilations on marine fluctuation prediction systems.
The results of the research which has been conducted in relation to the data assimilation system may be summarized as follows:
(1)The objective of flow analysis by the data assimilation system is to determine a flow field by giving a realistic density distribution field based on the measured results while maintaining such a field.
The results of analysis obtained by this system indicate a very high reproducibility as compared with the existing data on surface current which we are currently aware of. The results of analysis on water temperature and salinity distribution fields in the middle layer also agree well with the observed values.
(2)Moreover, in the calculations conducted by tracking sea water particles based on such a system, the results obtained could approximately reproduce the tracking of Algos-buoys, which was carried out in the past, thus supporting the aforementioned reproducibility of flow.

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