The Sumikawa geothermal field is located in the Hachimantai volcanic region of northeastern Japan, where the Sumikawa geothermal power station has been operated since 1995. An advanced geophysical technique has been applied to image major fracture distributions at the Sumikawa area. The 4-D technique named as the FFT method (Fluid Flow Tomography) has been developed by Engineering Geophysics Laboratory of Kyushu University for imaging transient phenomena as a function of time. Subsurface resistivity structures could be determined from 3-D inversion of the mise-a-la-masse data, while fracture distributions could be estimated from 3-D inversion of SP (Self Potential) data using time series data obtained by the FFT method. It is concluded that distribution of potential fractures in the Sumikawa geothermal field were delineated and drilling programs were proposed for further developments of production and reinjection wells based on the joint interpretation of apparent resistivity and SP data obtained from the 4-D geoelectrical technique

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