104 GEOPHYSICAL INVESTIGATION OF HOT SPRING SITES IN KANCHANABURI WITH REFERENCE TO LOCAL TOURISM DEVELOPMENT Pham Huy Giao 1 , Prinya Putthapiban 2 , and Chanarop Vichalai 1 1 GEPG Program, Asian Institute of Technology (AIT) Pathumthani, Thailand 2 Department of Physics, Mahidol University, Bangkok 10400, Thailand ABSTRACT Hot spring sites and their underneath geothermal groundwater system are valuable natural resources for geothermal utilization and tourism industry. An investigation of hot spring site is always required in the first phase for development of the concerned area, especially for location of drilling boreholes. In this paper, results of a joint geophysical survey by the AIT and Mahidol research teams at two hot spring sites in Kanchanaburi province of Thailand using electric imaging technique are presented. To account for the effect of an uneven topography, an inverse analysis using smoothness- constrained, least-squares method was employed for inversion of resistivity data. The geophysical survey results obtained could help indicate the potential areas to be drilled for hot water extraction. Developments of two investigated hot spring sites, e.g., Wangkanai and Hindat, can be helpful to promote the local tourism industry. Keywords: hot spring, electrical imaging, tourism INTRODUCTION Thermal springs in Thailand are commonly associated with the granitic terrains, in vicinity of lineaments that can potentially be active faults (Charusiri et al., 2000). Many such as those in Kanchanaburi province (Hindat), have become well-known touristic spots and consequently play an important role in the local tourist industry. Besides the mentioned economic significance, nowadays the investigation of the hot springs is also of interest to those studying the tectonic sensitivity of this western area of Thailand. As a matter of fact, at the time of the earthquake and the followed tsunami in the Indian Ocean on 26 December 2004, it was observed that the temperatures of hot springs along Three Pagodas Faults abruptly increased. For example, the temperature at the Hindat hot spring increased and water became milky at approximately 8 a.m. in that morning. Furthermore, a surface water well located at Paktho district, the southeastern end of the over 350 kms long, Three Pagoda Fault zone was experienced the same temperature change and was noticed increasing to 48°C on 28 December 2004. It is unfortunate that there was no record of such water temperature prior to 28 December. The temperature had then dropped at the rate of 2°C per day and took approximately nine days to bring the water temperature back to room temperature of that period, 29°C. Similar changes in temperature and other physical parameters have frequently been observed at many hot spring locations. One of the main objectives when investigating a hot spring site is to drill additional boreholes to trap more hot water from a deeper depth. This task can be done successfully based on the field experiences of the geologist in charge. However, the geologist frequently faces the following difficulties in making his or her decisions as follows, (i) how far the new location will be from the existing spring?; (ii) on which side of the stream along that a few springs have surfaced, on the same side or on the other side?; (iii) in which direction the new borehole can be placed, more on the upward direction toward the hill or more downward toward the valley?. The chance for a borehole to miss the required target, either in term of the flow rate or higher temperature, is not low at all as the active fault that brings the hot water to the surface is quite steep in a hard granitic terrain. On one hand, it is quite possible to drill the borehole on the other side of that fault and consequently the borehole will miss the main flowing hot water body, having a small flow rate. On the other hand, even the borehole could be placed on the correct dipping side of the fault if it is placed too far from the fault, it can only trap the water in the ruptured zone surrounding the main fault that does not have high temperature as expected. Geophysical methods can certainly assist the geologist in locating the new borehole location, either directly or indirectly. From our long-accumulated experiences, it is better to use the indirect approach of geophysical exploration, that the geophysical methods are used to locate the active fault first and to identify its extension and dipping directions. The geologist then can integrate the geophysical information with other to make the final decision. In this study electric imaging (EI) was proposed and applied by a joint team of researchers and graduate students from the Asian Institute of Technology and Mahidol University, Kanchanburi Campus. A review of GEOTHAI’07 International Conference on Geology of Thailand: Towards Sustainable Development and Sufficiency Economy