Characterization of Temporal Change of Deep Resistivity around Geothermal Reservoir Using Magnetotelluric Survey

Abstract

MT (magnetotelluric) survey is an effective geophysical prospecting method for imaging regional geologic structures by measuring resistivity from shallow to deep parts. To clarify the effect of temporal change in physical conditions of geothermal reservoir on the MT resistivity, we applied a periodic MT survey at a fixed point. This point is situated in the western side of Mt. Aso, southwest Japan in Figure 1. The reason for selecting this area is based on our previous results by Asaue et al., (2006) that detected existence of a geothermal reservoir in the depth range 2-4 km and its configuration by MT resistivity. Measurement frequency is one or two times in a one month, from June, 2008 to January, 2009. We selected suitable days for the measurement, whose electromagnetic signals were strong so that we could obtain high-quality electric and magnetic field data. The apparent resistivity data were converted to true resistivity by an inversion method based on simulated annealing. Resultant MT resistivity from the surface to the 5 km depth highlighted that relatively large changes of resistivity occurred at the upper part and lower parts of the cap rocks. The resistivity changes in the shallow and deep parts may be caused by disturbance of the groundwater flow and volcano activity, respectively. An interesting feature is a time delay phenomena of the resistivity in the deep parts, which changed one month after the swarm of A-type volcanic earthquakes: this type earthquakes were found to occur generally in the depth range at which the resistivity tend to change. Resistivity is sensitive to water content of rocks. It is plausible accordingly that the earthquakes affect paths of the geothermal fluid flows by generating new fractures.