Academic Conference on Natural Science for Master and Ph. D students From Cambodia Laos Vietnam



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Fig. 2. Location of the Khorat and the Sakon Nakon basins in the Khorat Plateau region (modified from El Tabakh et al., 1999)
Wiszniewski and Co-workers (2005) [17] reported that the Mahasarakham Formation occur in the subsurface just west of the study area. The maximum thickness of the rock salt is 100 m depth and the top of the rock salt is 83 m depth from ground surface at Phailom village in the deep borehole drilled by the Australian Centre for International Agricultural Research (ACIAR) project. The top of the rock salt was found in the Phailom borehole is marked by a thin layer of anhydrite overlain by saline, plastic, blue-grey clay beneath non-saline claystone with red-brown claystone forming the topmost layer. In additional, the rock salt has also been penetrated by a brine production bore at the Lak 4 salt factory, which this borehole found the top of the rock salt at 90 m depth from ground surface and was completed at 120 m depth by ACIAR project.

  1. Methodology

The purpose of electrical survey is to determine the variations in ground resistivity with depth by measuring on the ground surface. The resistivity varies greatly due to different geological materials (Table 1). Most rocks and minerals in nature usually contain water with some dissolved salts within their pores and cracks. The main factor affecting the resistivity is quantity of dissolved salts in pores of rocks (Reynolds, 1997) [2]. Rock resistivity of clean sand, saturated aquifer was explained by Archie's law:

(1)

Where: , are resistivities of rock and water respectively

a is the saturation coefficient

m is the cementation factor

 is fractional porosity


Table 1: Resistivity for different earth materials (modified from Reynolds, 1997) [2]

Materials

Resistivity (ohm-m)

Top soil

250-1700

Dry sandy soil

80-1050

Sandy clay/ clayey sand

30-215

Alluvium

10-800

Clay

10-800

Sandstone

8- 4000

Dry-Gravel

1400

Saturated-Gravel

100

Groundwater

10-800

Groundwater(fresh)

10-100

Sea water

0.2


Electrical resistivity measurements are normally made by injecting current into the ground through two current electrodes (C1 and C2), and measure resulting voltage difference at two potential electrodes (P1 and P2). The current (I) and voltage (V) values, an apparent resistivity values is calculated by:

(2)

Where: k is the geometric factor, depends on the electrodes configuration



The centre point of the electrode configuration remains fixed whereas the spacing of electrodes is increased and information deeper sections of the subsurface can be retrieved. Different electrode configurations are used for investigation, which depends on the type of investigation and field condition. In this work, electrical resistivity sounding with Schlumberger electrode configuration (Figure 3) was carried out in Champhon District in order to delineate freshwater and saline water in this area. The apparent resistivity of the Schlumberger configuration is calculated from the equation (3):

(3)

Where 2l is spacing between potential electrodes, is a spacing between current electrodes and (V/I) is reading taking from resistivity meter.

Figure 3 Schlumberger electrodes configuration

An electrical resistivity measurement was conducted at 41 sounding points in Champhon District, using a DC Terrameter, ABEM SAS 1000 (ABEM Instrument AB, Sundbyberg, Sweden) and stainless steel electrodes, including other materials that related to field work. For the Schlumberger electrode configuration was conducted in the measurement with the maximum half current electrode spacing of 350 to 500 meters, the resistivity data were interpreted by a computer program RESIST 87 (Velpen, 1988) [18] and resistivity contour maps were made by Surfer software, version 8.02.



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