A pilot magnetotelluric survey for geothermal exploration in Mae Chan region, northern Thailand

There are many potential sites for geothermal power plants in Thailand. After many years of geological and geophysical surveys, a pilot magnetotelluric (MT) survey was made to assess the reservoir of the Mae Chan geothermal area, northern Thailand, which is one of the key areas for geothermal development. Seven MT sites were deployed in a 3 km × 4 km area around the Mae Chan district covering the Mae Chan hot springs. The MT data were acquired at low and high frequency ranges and were inverted using a 3-D MT inversion to yield the 3-D resistivity structure of the area. The results show that there are two conductive zones near the surface associated with the hot fluid of the Mae Chan hydrothermal system. The hot fluid reservoir mostly resides at less than 500 m below the surface in weathered and fractured granite and in the overlying sedimentary deposits. Its source rock is imaged as a resistive zone corresponding to the hot granite batholith below it. The hot fluid rises up along the Mae Chan fault. The fault is clearly observed as a resistivity contrast extending from the surface to depth. It dips at a moderate angle. From the measured temperature of the fluid from a drill hole and the estimated temperature of the granite rock from the resistivity structure we conclude that the Mae Chan geothermal area is likely to be suitable for immediate development of a small-scale geothermal power plant.