Abstract
MT (magnetotelluric) method has been widely used to clarify resistivity distribution for exploring deep geological structures and reservoir locations of geothermal and petroleum resources. This method was applied to structural analysis of fracture zones in deep parts of the Futagawa fault zone, an active fault striking east-northeast and west-southwest along the southern edge of Kumamoto plain, central Kyushu, southwest Japan. Three lines were set perpendicular to the fault and the MT measurements were carried out at 32 points. Average distance between adjacent two points is about 1km. To reduce the artificial noises that have effect on electric and magnetic fields at the measurement points, a remote reference technique was adopted. The original measurement data were coupled with the MT data that were acquired concurrently with this study at the two sites located 100-150km apart from the fault. Resultant apparent resistivity and phase data were applied to a smoothness-constrained 2D inversion analysis under ABIC minimization criterion for DC resistivity data. The resistivity distributions estimated by this inversion analysis show clearly the low resistivity zones smaller than 10Ω·m at all the lines, which imply that the Futagawa fault zone is accompanied with largely fractured zones by repetitive tectonic events. These zones are found to be located from the 1- to 7-km depths with 200- to 1000-m widths and deepened toward the southwest at the angle of 35°. By tracing the fracture zones, the fault can be considered to dip steeply toward the north. This deep structure corresponds to the characteristics detected by the electric sounding and radon prospecting data on the MT lines.
