Heat Flow and Electromagnetic Studies in the Continental Shelf Area off Eastern China

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1. We carried out a Japan-China cooperative research program to investigate the thermal structure of the continental shelf area off eastern China. Three research cruises were conducted in the East China Sea in 1990 and 1991.2. The effect of bottom water temperature variation must be removed to determine heat flow in shallow sea areas, such as the East China Sea.(1) We tried to measure sediment temperature profiles repeatedly at the same stations using long probes, but could not make measurements on one cruise due to an accident. The data from the other two cruises were analyzed assuming annual bottom water temperature variation. Heat flow was estimated to be around 90 mW/m^2 in the northernmost Okinawa Trough and lower than 50 mW/m^2 off Shanghai. More measurements are necessary for accurate determination.(2)We deployed a long-term temperature monitoring system on the seafloor to record the bottom water temperature variation, but could not retrieve it due to some fault in the pop-up mechanism.3. We deployed ocean bottom electromagnetometers off Kyushu to determine the electrical conductivity structure. The instruments did not work properly, but we could obtain some magnetic field data, which indicate that some part of the northern Okinawa Trough has relatively low resistivity.4. We also tried to determine heat flow using drill holes.(l) In the East China Sea, we made temperature measurements in shallow holes in one hour after completion of drilling, but the records were too short to be extrapolated to the equilibrium.(2) At the northern margin of the South China Sea, heat flow could be calculated in deep oil/gas wells from temperature logs and thermal conductivity of core samples.5. We could identity problems in various methods for heat flow measurements in shallow sea areas, which can be solved in future cooperative studies.