An assessment of a shallow geothermal reservoir of Mae Chan hot spring, northern Thailand via magnetotelluric surveys

Abstract In a non-volcanic geothermal system, like Mae Chan hot spring and many other hot springs in Thailand, hot water is heated deep underground and seeps to the surface through fractures and faults. Some of the hot water may aggregate in a hydrothermal alteration zone along the fracture zones of granite rocks to form shallow “hot water reservoirs”. These networks lower the bulk resistivity of the granitic rock to form a low resistivity zone associated with the hot water reservoir, which can then be imaged via a magnetotelluric (MT) survey. A series of magnetotelluric surveys from 2013 to 2018 was conducted in order to assess the location, size and depth of the shallow geothermal reservoir of the Mae Chan hot spring. All data were combined to produce the final 3-D resistivity structure. The final MT survey had a high density of MT sites across the zone of interest which allowed us to precisely image the shallow reservoir for drilling purposes. Using the final MT results, five new boreholes with a maximum depth of 200 m were drilled. Hot water was found at various depths from each borehole with perfect agreement with the final resistivity structure derived from the MT data. This 3-D resistivity outline will be useful in developing the field with future production and re-injection wells.