We made a magnetic anomaly map in the eastern margin of the Japan Sea using seaborne magnetic data collected during five cruises of R/V Hakurei-maru. The contours of this map are at 25 nT intervals. They are much smaller than those of the seaborne magnetic maps published previously by the Geological Survey of Japan. On a map using a procedure without cross-point control, unnatural contours are usually showed, which results from relatively large discrepancies at cross-over points of the survey lines compared with the small contour-interval. To solve this problem, we applied a cross-point control method which had been used commonly for processing airborne magnetic data. An offset of each track line was determined to minimize the sum of the square of difference at each cross-over point. The causes of the discrepancies at cross-over points include diurnal variation, secular variation, measurement error, positioning error, and ship's magnetization. It is difficult to separate these components from observed data. The crosspoint control method is hence useful for processing seaborne magnetic data as well as airborne data.
A trial construction of a drag-type current meter is introduced. Though this current meter has not been applied in real ocean, the technique used there would give good suggestions for future developments of new types of current meters. Besides, a curious directional characteristics is found for the current meter with a four bolts cage. The output increases with increase of the relative angle of the meter to the current direction up to 20°, and the maximum output occurs at about 20°. A simple model is proposed to understand this directional characteristics. Then, the current meter with a 12 bolts cage is recommended as its directional characteristics can be well represented by a cosine curve.
The time series of the vertical profile of the coastal current at every 2 m were observed at the Sendai-Wan for 15 days using the Acoustic Doppler Current Profiler set on the sea bottom (B-ADCP). The currents in layers from surface to 8 m corresponds well with the wind, but their speeds become weaker rapidly as going to the deeper layers. Notably, in the mid depth and bottom layers, distinctive periodic fluctuations of the current were found in the diurnal band. Applying the simple fourier analysis to the time series of each layer, these fluctuations are recognized as the combination of the two internal oscillations having the near inertial periods, 21.2 and 18.0 hours, which are located in both sides of the inertial oscillation (19.4 hours). These oscillations are concentrated under the thermocline. These results could not be obtained without the B-ADCP. Then, we intend to utilize the B-ADCP to make clear the characteristics of the coastal current in the various seas.
The process of surveys and researches about a submarine volcano and its hydrothermal ore deposit on the Island Arc is arranged as follows: 1) The detailed submarine topographic survey around the top of many seamounts with a narrow multi-beam echo sounder, 2) The selection of some active submarine volcanoes accompanied by a volcanic crater below 1,000min depth, 3) The geochemical inquiry of the hydrothermal plume, 4) The preliminary survey with a ROV (Remotely Operated Vehicle) and a deep sea camera, 5) The submersible observations and the gathering the volcanic rocks, ores and hydrothermal fluid by the submersible vessel, 6) The mapping the distribution of the volcanic geology and the hydrothermal activity on the sea-floor, 7) The analysis of the gathered volcanic rocks, ores and hydrothermal fluid. The research about the hydrothermal ore deposits formed on the submarine volcanoes is one of the researches realized as the compilation of the technology developed recently for the sea-floor survey. As a example of the recent result of some sea-floor surveys by the JAMSTEC 'Shinkai 2000', the discovery of the sea-floor hydrothermal ore deposits at the top of Suiyo Seamount in Sitiyo Seamounts is cited. This hydrothermal gushing is very active, which the highest temperature is 310°C. The sulfides of the vent chimneys from this hydrothermal field are rich in Cu. The highest Au content of sulfide samples is 71 ppm.
ERS-1 altimeter data taken from April 1992 to February 1993 have been processed to study mean sea surface height and its variation. The global mean sea surface height (MSSH) was obtained by averaging altimeter data of ocean products (OPR-02) of nine repeat cycles. The sea level variability was estimated from the 10-month MSSH data. It clearly shows the regions of large variability which correspond well to the strong ocean currents such as the Kuroshio, its extension, the Gulf Stream and the Agulhas current etc. Sea level anomalies derived from ERS-1 altimeter data in the Kuroshio region are compared with the sea truth data obtained by the Hydrographic Department of Japan. Warm and cold eddies are well expressed by sea level anomalies. Ocean current of the Kuroshio extension can also be estimated. Mean altimeter profiles in the Northwest Pacific region were fitted based on crossover differences, resulting in a 7.9 cm RMS crossover difference.