Geoelectrical Mapping on the Noya Geothermal Area by ELF-MT Method

Abstract

ELF-MT survey was carried out in a study of geothermal structure of the Noya geothermal area, Oita Prefecture. ELF-MT method uses natural electomagnetic field in Schumann resonant frequency band from 8 to 20 Hz (3 frequencies). Resistivity maps were compiled from 37 sites at approximately 0.5 km spacing. Major low resistivity zones extended NE-SW direction along the Noya-Mizuwake fault. The lowest apparent resistivity zone, less than 25 ohm-m, is associated with the Noya hot springs and alteration zones. This zone (A-zone) is the most active geothermal zone in this studied area. Temperature log in the test hole 51-WT-2, drilled near the lowest resistivity zone, shows 110°C at 60m depth and 112°C at 700m depth. This indicates that hot water was stored within very shallow formation in this low resistivity zone. The extensive alteration zone (B-zone) is located in 1 or 2 kilo-meters distant from the Noya hot springs, in the direction of Kodairadani. A high resistivity zone of more than 250 ohm-m is found in the center of B zone, and is surrounded by low resistivity zones. The temperature log in the test hole 51-WT-1, drilled near this high resistivity zone, shows maximum temperature of 177°C at 700m depth (the bottom of the hole). High resistivity in the B-zone may be caused by presence of dike rock or silicification. The apparent resistivity mapping by ELF-MT technique, thus, indicated clearly the two different typ of the geothermal zone in this area.