HIGH SPATIAL-RESOLUTION OF THE RESISTIVITY STRUCTURE REVEALED BY A DENSE NETWORK MT MEASUREMENT - A CASE-STUDY IN THE MINAMIKAYABE AREA, HOKKAIDO, JAPAN

In 1988 we carried out a case study survey at Minamikayabe in the southern part of Hokkaido, Japan. This area is particularly interesting because of its geothermal potential. Seven control wells had been drilled prior to our operation, and also some other information has been available from other survey methods such as gravity, geology, geochemistry, DC resistivity and heat flow. Hence the area is an ideal site for a study of effectiveness of the MT method. The study area is close to the sea and a special care for the coast effect was necessary in our interpretation. The resistivity structure which we derived was found to agree with the well logging data. In this area, the geothermal reservoir is related to a series of fracture existing in an intrusive rock body in the southeast flank of Mt. Nakitsura. The relatively conducting area in our model can be interpreted as corresponding to a fractured zone in the intrusive rock body. Analyses of the data derived from network MT measurements, which we undertook in the present case study, clearly show that dense network measurements could resolve a spatially fine resistivity structure. In order to attain a high spatial resolution of subsurface resistivity structure in a geothermal or a volcanic area, where the subsurface structure is extremely complicated, we show that a dense network MT measurement is indispensable.