Academic Conference on Natural Science for Master and Ph. D students From Cambodia Laos Vietnam
Groundwater Mapping with Electrical Resistivity Sounding Method in Champhon District, Savannakhet Province, Lao PDR
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Groundwater Mapping with Electrical Resistivity Sounding Method in Champhon District, Savannakhet Province, Lao PDRXayavong V., Keophilavong S., Sounsuandao T., and Volachack N.Geophysics Laboratory, Department of Physics, Faculty of Science, National University of Laos, Dongdok Campus 7322, Vientiane Abstract An electrical resistivity sounding measurement was conducted at 41 sounding points in Champhone District of Savannaket Province, Lao PDR. The aim of this research work is to delineate saline water and freshwater in this study area. The Schlumberger sounding technique was carried out in the measurement with the maximum half current electrode spacing of 350 to 500 meters. Present study shows low resistivity zones of less than 8 ohm-m at shallow depth extend to downward to 75 meters depth in middle part of the study area, low resistivity zones are considered to be thick layer of clay or high salt contents in ground layers. However, moderate resistivities of approximately 25 to 100 ohm-m appear in deeper layers at the western and eastern parts of the study area, indicating suitable areas for good quality groundwater. Keywords: Electrical resistivity measurements, salinity, groundwater mapping. INTRODUCTION The study area is located at some 40 km to the southeast of the city of Savannakhet in the Central Region of the Lao PDR, covering an area of approximately 875 square km, between latitude 16.39° and 16.58° N and longitude 104.99° and 105.28° E (Fig. 1). The Mekong River separates Savannakhet province from the Mukdahan province, Thailand to the west. This area forms part of the Khorat Plateau which is the geomorphic expression of the Khorat Basin and includes the Sakon Nakhon and Savannakhet sub-basins. The study area is located within the Savannakhet sub-basin and geology is similar to the Sakon Nakhon sub-basin most of which is in northeast, Thailand. Therefore, the hydrogeology and occurrence of saline groundwater in the study area is considered as a northwest extension of Sakon Nakhon basin, northeast of Thailand 1999 [1]. The study area is presently economic active and population is increasing. Consequently, people need work and goods, technology is going on, etc and associated with trend of increasing demand water has grown in this area but water supply can not provide their need. Although, people are near surface water but they can not use surface water for drinking and household because surface water is pollution. Usually, water resources in this area have been exploited in agriculture, industry, transportation, tourism and education. However, people who use water have no understanding about hydrogeological structure and water management, which results of drilling have some problems such as water can not provide completely. It should be interesting to conduct electrical resistivity measurements in the area in order to delineate locations of fresh and saline water and mapping of potential groundwater. The information obtained from this research work will be useful for future development in the study area. Measuring electrical resistivity of ground is a conventional method for determination of groundwater horizons at depths in sedimentary area. This is because different rock types possess different electrical resistivity; the resistivity of sedimentary rock depends on porosity, degree of water saturation and the concentration of dissolved salts (Reynolds, 1997) [2]. There is great number of success in using electrical method for groundwater exploration such as Emenike (2001) [3], Shaaban (2001) [4], Lashkaripour (2003) [5], Gowd (2004) [6], Sainato et al (2003) [7], Lashkaripour (2005) [8], Abd Alla et al. (2005) [9], etc. 
Fig. 1. Show study area and location of sounding stations in Champhon District of Savannakhet Province, Lao PDR Geology and Hydrogeology The stratigraphy of the Savannakhet sub-basin is similar to that of the Khorat Plateau in Northeast Thailand. Savannakhet sub-basin is considered as a northwest extension of Sakon Nakhon basin of the Khorat Plateau, Thailand. This plateau is bounded by latitudes  N to  N and longitudes  E to  E, covering an area of about 170,000 square kilometers in northeastern of Thailand and central Laos (Figure 2). The potash deposits in the Maha Sarakham Formation of the Khorat basin in Cretaceous age. The Phu Phan range separates Khorat Plateau into two basins, the Khorat basin in south covering an area of about 36,000 square kilometers and Sakon Nakhon basin in north covering area of about 21,000 square kilometers (Hite et al., 1979 [10]; El Tabakh et al., 1999, 2003 [11, 12]; and Keith et al., 2005) [12, 13]. The Maha Sarakham Formation comprises of claystone, shale, siltstone, sandstone, anhydrite, gypsum, potash, and rock salt. A complete sequence from the bottom to top of this formation consists of a basal anhydrite, lower salt, potash zone, color-banded salt, lower anhydrite, lower clastic rocks, middle salt, middle anhydrite, middle clastic rocks, upper salt and upper anhydrite (Satarugsa et al., 2004, 2005 [14]; Cotanont, 2005 [15]; and Jenkunawat, 2005) [16].
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