Background
Soap hole phenomenon has occurred among of the paddy field at Nong Kung Noi village, Khok Krabueang Sub-district, Ban Lueam District Nakhon Ratchasima Provinece at February 2019. However, this pbhenomenon had happened at the same location a year before. Due to news reports on the event, many visitors were attracted to visit this place and some of them have behaviors that causes health risks such as drink water around soap hole location, masked body by taking mud of soap hole etc. furthermore, some people understand that this phenomenon may be supernaturally occurrence. However, quality of soil and surface water were official test. To support actual information in terms of subsurface geology and hydrology, 2D resistivity measurement were applied for illustrate subsurface geology feature at soap hole occurrence for manag
Objective
The aims of application of 2D resistivity measurement at soap hole phenomenon area to understand subsurface geological aspect and soap hole process
Location
Study area is located at the paddy field of Kung Noi Village, Khok Krabueang Subdistrict, Ban Lueam District Nakhon Ratchasima Provinece. Study area can be found in the Royal Thai Army topographic map 1:50,000 series L7018 sheet 5440 III. The boundary of study area is in coordinate range of 195000E to 197000E and 1726500N to 1727100N covering the area dimension about 1000m x 500m. (Figure 1-2)
Accessibility
Travel to the study area by car from Bankok through to pass Road No.2 (with distance of about 70 km), then turn to North along Chaiyaphoom-Sikhio road (with distance 40 km) and turn to North along Road No. 2418 (about 4 km). Turn to East into road 4008 (distance 50 km) and turn to North to Road No. 2160 (with 5 km) and the last driving about 10 km to reach the study area.
Topography
Topography of study area is slightly flat terrain with elevation about 195-205m. The area around soap hole phenomenon zone is generally found as paddy field.
Figure 1-2 Location of study area, Ban lueam District, Nakhon Ratchasima province.
Darcy’s law
Darcy's law is an equation that describes the flow of a fluid through a porous medium. The law was formulated base on results of experiments on the flow of water through beds of sand, forming the basis of hydrogeology, a branch of earth sciences. Darcy's law in the absence of gravitational forces, is a simple proportional relationship between the instantaneous flow rate through a porous medium of permeability the dynamic viscosity (K) of the fluid and the pressure drop over a given distance in a homogeneously permeable medium
Soap hole occurrence
A soap hole is a part of the land surface characterized by a local weakness of limited extent underlain by a viscous admixture of sand, silt, clay, and water. Toth (1970) described soap holes as water-saturated pockets of sand, silt, and clay in generic surface deposits of low permeability situated in an area of upward-moving groundwater was the same phenomenon as quicksand with a difference only in the local geological conditions There are 3 factors that caused soap hole as follows; 1). high pressure of ground water. 2). There are a lot of fracture that ground water can upward- move to surface. 3). The characteristic of subsurface geology consisted of clay layer that can be swell after interaction with water (Techavan, 2019)
Model of soap hole occurrence.
Principle of resistivity
The electrical resistivity method is an active geophysical method. It employs an artificial
source which is introduced into the ground though a pair of electrodes. The procedure involves
measurement of potential difference between other two electrodes in the vicinity of current
flow. Apparent resistivity is calculated by using the potential difference for the interpretation.
The electrodes which current is introduced into the ground are called Current electrodes and
electrodes between which the potential difference is measured are called Potential electrodes.
Electrode configuration
1. Schlumberger array
Schlumberger is investigation the variation of resistivity with the outer 2 electrode(A,B)are current electrode and the inner 2 electrodes (M,N) are the Potential electrode the potential electrodes are installed at the centre of the electrode array with a small separation.(Figure 2-3)
Figure 2-5 Schlumberger configuration
2. Dipole-dipole array
Dipole-dipole array used to detect resistivity changes, the spacing of current electrodes is fixed and spacing between current electrode and potential electrode isn’t fix, moving the current electrods a point forward until ending linesurvey. (U.S. Environmental Protection agency, 2016)(Figure2-4)
Figure 2-6 Dipole-dipole configuration
3. Wenner array
The Wenner array consists of four collinear, equally spaced electrodes. The outer two electrodes are typically the current (source) electrodes and the inner two electrodes are the potential (receiver) electrodes. The array spacing expands about the array midpoint while maintaining an equivalent spacing between each electrode. . (U.S. Environmental Protection agency, 2016) (Figure2-5)
Figure 2-7 Wenner configuration.
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