Three Dimensional Modeling of Geoelectrical Structure Based on MT and TDEM Data in Mori Geothermal Field, Hokkaido, Japan

Abstract

We applied a 3D modeling scheme for MT and TDEM data to the Mori geothermal field at Hokkaido, northern Japan to clarify the resistivity structure up to 3km depth. The three dimensional modeling scheme of MT and TDEM data based on the staggered grid approximation of the secondary electrical field (Fomenko et al., 1998) was extended to a model including topography by Fomenko et al.,(1999). We had made 3D resistivity model based on MT data. However, because of distributing low resistivity layer (about 5 ohm-m) at shallower depth, we were not able to identify the detailed resistivity structure at deeper part (more than 1km depth), even though using low frequency data, up to 0.001Hz. We expected TDEM survey and 3D modeling would overcome this problem, because the TDEM response seems to be more sensitive to a local structure at depth than MT. We obtained a 3D resistivity model up to 3km depth including topography, using 84 transient vertical magnetic responses obtained in the Mori. The resistivity structure of the Mori generally corresponds to the distribution of rock. A resistivity low of 5 to 10 ohm-m is distributed at the south-western part of the caldera located in the center of the Mori. This area was suggested as one of the up flow zones of geothermal fluid to Mori geothermal reservoir.