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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. [ABSTRACT FROM AUTHOR]

After early exploration during the 1980s and 1990s, the 0.3 MW Fang geothermal power plant was built as a demonstration to supply electricity to the local community. The shallow well (100 m) drilling program produced about 22 l/s of 125 °C water, and two wells to 500 m produced about 10 l/s. Due to the lack of detailed information on the geothermal system, the plan to expand to a larger power plant was halted to avoid the drilling missing the hot fluid. The plan was resumed in the last ten years starting with the magnetotelluric (MT) survey. Thirty three MT sites were deployed on the southern part of the Fang geothermal area. A remote site was installed about 600 km south of the study area for better data quality. After data processing, the data was inverted with WSINV3DMT to produce the 3-D resistivity model which clearly matches the near-surface geology and is also in agreement with the conceptual geology of the Fang geothermal system. The high resistivity zone is interpreted as the crystalline granitic rock, while the intermediate resistivity zone is associated with the Fang sedimentary basin. The resistivity contrast between the higher and lower resistivities helps reveal the orientations of the major Mae Chan Fault (MCF) and the two local faults of the area. The two main conductors of interests, C1 and C2, are directly linked to the hot fluid found at the surface. C1 is shallow (<50 m), and found only beneath the Fang hot spring, and so it is interpreted as the fracture reservoir. C2, which was not discovered in previous studies, extends from near the surface to a depth of 500 m, and at a depth of 200 m, it is about 1 km wide. It is about 1 km south of the Fang hot spring where the warm water was found to have seeped to the surface through the MCF. Two possible interpretations are proposed for the C2 conductor. The first is that there is an impermeable clay zone trapping a relatively high resistivity geothermal fluid reservoir beneath, like the caprock of a magmatic geothermal play type. This would require a deeper well to extract the hot fluid. As with the C1 conductor, the other interpretation is that the C2 conductor is a fracture geothermal reservoir where hot fluid from the deep resides within the pores of the sedimentary rock and fractures of the altered granite. This would require a shallower well. Both interpretations suggest that the C2 anomaly is of value. Since it has never been explored, a drilling over the C2 anomaly is recommended to probe its characteristic and also to extract more hot fluid for the future expansion of the geothermal power plant. [ABSTRACT FROM AUTHOR]

• Shallow reservoirs can be precisely explored with a dense MT survey. • Hot fluid reservoir in fracture zones is imaged as low resistivity structure. • New drilling results match the resistivity structure perfectly. • Confirmation that Mae Chan hot spring has a high potential for a power plant. 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. [ABSTRACT FROM AUTHOR]

Seismicity in Thailand had been relatively low for decades prior to the Mw 6.5 earthquake of 5 May 2014 which came as a surprise and was followed by thousands of aftershocks. Most of the epicenters were located in the transition region between the Mae Lao Segment (MLS) and the Pan Segment (PS) of the Phayao Fault Zone (PFZ). We conducted a 3-D magnetotelluric (MT) survey (31 sites) to image the deep PFZ structure. The shallow 3-D resistivity structure matches very well with the surface geology, while the deeper structures disclose many interesting resistive and conductive anomalies. However, the most interesting feature of this study is the large conductive anomaly (ML) located at a depth of 4 km to the mid-crust beneath the MLS near the seismogenic zone. Our current hypothesis is that the ML conductor has a highly interconnected aqueous fluid content and also plays crucial role in the earthquake sequence of the 5 May 2014 event. As our previous seismic waveform study revealed that the MLS has a relatively high fault plane instability, the fluid within the fractured fault would further reduce the fault strength. The accumulated pre-existing tectonic stress from the north can therefore overcome the maximum frictional strength of the MLS, and hence cause it to slip and produce the main shock. With the local structural heterogeneities and fluid in the fractured fault zones, the aftershocks then occurred on both the PS and MLS. This is in contrast to the Mae Chan Fault Zone (MCFZ) in the north which many scientists expected to generate a larger magnitude earthquake than any other faults. Since instrumental record, it has only generated a few Mw 4 earthquakes. Some of our MT stations were located within the MCFZ. However, there is no deep conductor as the conductor lies beneath the MLS. A lack of interconnected fluid within the deep fault beneath the MCFZ might be one of the reasons for the lower seismic activity from the MCFZ. Other geophysical methods, such as seismic tomography, are necessary in order to confirm the presence of the fluid beneath the MLS and also the lack of a deep conductor beneath the MCFZ. [ABSTRACT FROM AUTHOR]