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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]

We use 2-D magnetotelluric (MT) problems as a feasibility study to demonstrate that 3-D MT problems can be solved with a direct solver, even on a standard single processor PC. The scheme used is a hierarchical domain decomposition (HDD) method in which a global computational domain is uniformly split into many smaller non-overlapping subdomains. To make it more efficient, two modifications are made to the standard HDD method. Instead of three levels as in the standard HDD method, we classify the unknowns into four classes: the interiors, the horizontal interfaces, the vertical interfaces and the intersections. Four sets of smaller systems of equations are successively solved with a direct method (an LU factorization). The separation significantly reduces the large memory requirements of a direct solver. It also reduces the CPU time to almost half that of the standard HDD method although it is still slower than the conventional finite difference (FD) method. To further enhance the speed of the code, a red-black ordering is applied to solve the horizontal and vertical interface reduced systems. Numerical experiments on a 2-D MT problem of a given size running on a single processor machine shows that CPU time and memory used are almost constant for any resistivity models, frequencies and modes. This is a clear advantage of our algorithm and is of particular importance if the method is applied to 3-D problems. We show that our new method results in reductions in both memory usage and CPU time for large enough domains when compared to the standard FD and HDD schemes. In addition, we also introduce a 'memory minimization map', a graphical tool we can use instead of trial-and-error to pre-select the optimal size of subdomains, which yield the best performance in both CPU time and memory even running on a serial machine. [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]