Fig. 6. North-south cross-section ofthe final inverted model(Fig. 5a) at east-west distances of(a) −300 m,(b) 300 m and (c) 900 m. The dashed line is the estimated orientation of the Mae Chan Fault (MCF). Locations of the MT stations are also mapped and shown at the surface along with the hot spring locations.
Another feature that can be observed in our inverted model is the resistivity contrast between the conductive features C1 and C2 in the north and the resistive zone R in the south which is oriented in the northwest-southeast direction. The location of the abrupt change of the resistivity occurs at the Mae Chan Fault mapped by Kosuwan and Lumjuan (1998). The contrast is therefore interpreted as the fault line cutting through the granite in the middle of the survey area. The contrast vanishes at depth as shown in the cross-sections of Fig. 6a–c. By following the resistivity contrastfrom the fault trace at the surface (Fig. 2) to greater depths (Fig. 6a–c), we can infer that the Mae Chan Fault dips at 40–60◦ which is not as steep as expected (Fenton et al., 2003; Kosuwan and Lumjuan, 1998). Its hanging wall is in the north and consists of a sedimentary deposit and weathered and fractured granite on top of the unweathered granite. The foot wall is in the south and consists of mostly batholith granite. Below 2 km, both sides of the fault have the same resistivity suggesting granite forms both sides ofthe fault. Further detailed surveys with MT stations closer than the current spacing of 1 km are needed to better define the fault location at depth. This study helps confirm the study of Owens (2012) that the heat source for the Mae Chan hot springs is the hot granite. The reservoir is located within the first few hundred meters from the surface in the sedimentary deposits and weathered and fractured granite. Based on our study, the hot fluid in the C1 conductor can be considered as a potential area to develop a small-scale binary geothermal power plant similar to the 300 kW plant in the Fang district or a novel low enthalpy binary geothermal power plant (Gabbrielli, 2012). However, the reservoir appears relatively small. A lack of balance between usage and re-injection could distort the shallow hydrology of the area (Heise et al., 2008). The C2 conductor should be drilled to determine if the temperature of the hot fluid is high enough for development. A larger scale geothermal power plant using the hot granite at a great depth is also possible. However, little is known about the granite at depth. This would require further measurements as our first phase of the MT survey covered only 10 km2 which is a small part of the whole batholith exposed at the surface. Expanding the MT site coverage to a larger scale may show the existence of permeable fractured granite at great depth which could be used for geothermal power. A combination with thermal gradient drilling would require assessing the granite potential.