In order to determine the fracture toughness of rocks at high temperature, splitting tests were conducted on KIMACHI sandstone and INADA granite in the electric furnace. The rock specimens (200mmx150mmx20mm) that have a slot are prepared in this test. The slot induces stress concentration around the tip of slot when splitting load is applied to the slot, and fracture initiation occurs from the tip of slot into the specimen. Crack initiation and process of crack propagation are investigated by the measurement of splitting load, crack opening displacement and crack length. Stress intensity factors during crack propagation are calculated with the compliance calibration, and the fracture toughness KIC of rock at heigh temperature are determined. From the study of the effect of temperature on the fracture toughess, it becomes clear that two tendencies are seen in increase and decrease of the fracture toughness by heating. The fracture toughness of clastic rock such as KIMACHI sandstone increases with rising temperature, though once it decreases at 100°C. For instance, the fracture toughness at 300°C is about 1.4 times larger than that at the room temperature. On the other hand, in the case of crystaline rock such as INADA granite, the fracture toughness decreases with rising temperature. For instance, compared with the fracture toughness at 300°C and that at the room temperature, decrease of the fracture toughness is about 35%.
A self-potential survey was carried out in the Onikobe geothermal area, Miyagi prefecture, in 1983. It has already been reported that the data obtained is well interpreted by using an inclined plate model or Corwin model, and that SP anomalies seen in the Onikobe geothermal area may be related to faults. Although these techniques are useful, they provide little information on the source mechanism. Therefore a new method for the investigation of self-potential based on induced current sources is discussed in this paper. The potential field based on induced current sources due to primary flow proposed by Sill is calculated with the FEM method. In that case, it is assumed that locations of thermal sources are roughly consistent with known faults, and temperatures at arbitrary depths can be estimated from well-logging data. The potential field generated by point sources of the primary flow is compared with actual profiles. The result of D3 survey line can be interpreted in terms of the distribution of the induced current sources caused by the primary flow. On the other hand, the effect of a streaming potential may be included for B survey line, because the data shows the sinusoidal variation. It is therefore difficult that SP anomalies are correctly explained only by the thermal source model. However the fitness between both curves can be fairly well improved, if the effect of sources added newly near the surface between each fault are considered for the B survey line. This means that a source, possibly related to a streaming potential, may be considered in the Onikobe geothermal area accompaning the flow of hot water.
In order to investigate the mechanical behaviours of heated rocks, Uniaxial compression tests are carried out on 4 kinds of typical rock types as AKIYOSHI marble, INADA granite, OGINO tuff and EMOCHI welded tuff under room temperature (20°C), 150°C, 450°C and 600°C. From these tests, the following results are obtained. The mechanical behaviours of heated rocks are very different with each rock type. On AKIYOSHI marble and INADA granite, compressive strength and secant Youngs modulus take larger values as temperature rises. The former rock indicates ductility over 300°C, and the latter rock yields at 600°C by only temperature before the axial load is applied. On OGINO tuff, the values of compressive strength and secant Young's modulus are slightly smaller to 300°C as temperature goes up, and these values are larger over 300°C. On EMOCHI welded tuff, these values are larger with rising temperature. Also, the value of strain at failure point becomes larger as temperature is higher, except EMOCHI welded tuff.