| Summary: | 博士 === 國立臺灣大學 === 海洋研究所 === 98 === We have collected magnetic, heat flow, temperature data in water wells and monitored the temperature in a well offshore of northeastern Taiwan. After processing and integrating of the previous studies, we completed magnetic anomaly and geothermal data set. The magnetic data show that the high anomaly areas are located in the East Sea, the south of Kueishantao, the south of Ilan Plain and the Okinawa Trough. The elongated magnetic anomaly in southern Ilan Plain implied possibly the existence of volcanic having extended to the Ilan shelf. Geothermal data show the maximum heat flow was located in the south of Kueishantao. Also the significant variation of bottom water temperature was caused by the frequent hot fluid erupted from the sea floor near Kueishantao. Both the high heat flow and the variation of bottom water temperature indicate the strong post volcanic activity in the vicinity of Kueishantao.
A 2-D geothermal model across Ilan Plain in NS direction was established from the interpretation of magnetic inversion and seismic profiles. A geothermal model including sedimentary formation, basement, fault zone and intrusion body of which the general quantities for the thermal parameters were derived. The simulated temperature distribution could fit the observation of Litsu well while the age of the intrusion body is estimated to be of 23-25 thousand years or 190-220 thousand years. The age of the intrusion body is in the same order of that dated from Kueishantao rocks. Therefore, the occurrence of the two igneous rocks could be correlated. On the other hands, a 3-D geothermal model in northeastern Taiwan was established from the Curie point depth inversion and the topography. The simulation result shows the equilibrium heat flow is 68- 236 mW/m2. Two maximum heat flows located in the middle of Okinawa Trough may be correlated to the surrounding extrusion volcanoes. However, further geothermal investigation is necessary to confirm their relationship. The heat flow value derived from 3-D model is 17.9% higher than that averaged from 53 field measurements. The heat flow difference of the two data sets could be resulted from the sedimentation effects. Adapting sedimentation correction for the measured data could decrease the difference effectively.
A temperature observation well in Kueishantao has been monitored since 2006. The results show the annual temperature variations at different depths of the well are from 0.015℃ to 0.215℃. No significant period related to the seasonal variation was found from the spectrum analysis. However, a temperature declined rate of 0.048-0.116 ℃/yr has been found from the 4-year continuously monitored data. Five heat pulses with -0.022~0.150℃ in amplitude surging in tens of hours coincided with the time of typhoon visited Taiwan are of great interesting. However, it is not clear for the mechanism of borehole temperature variation triggerring by a typhoon.
The thermal gradients in the 30 water wells are low or even negative. However, there are six wells the thermal gradients are greater than 3℃/100 m (about an average value of the world). We estimated the area with thermal gradient greater than 6℃/100 m is about 36 km2. Assuming a layer of 400 m in thickness with the same thermal gradient as that measured in shallow water wells, the area of equal temperature of 35℃, 40℃,45℃are 95 km2, 53 km2, 21 km2, respectively. If the porosity of rock is roughly estimated 20-55%, the reserve volume of 40℃hot water will be 3400-9260×106 m3. According to the data issued by the Chiaohsi Hot Spring Center in northern Ialn Plain, the reserved quantity of the hot water could provide 16000 m3 consumption daily for several hunred years.
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