Utilizing 2-D electrical resistivity imaging (ERI) to investigate groundwater potential

Fresh water exists not only on the surface of the earth as streams or lakes, but also subsurface as underground water, and the water in them is called groundwater ; groundwater is simply the subsurface water that fully saturates pores or cracks in soils and rocks (USGS, 2017; Garvelmann and Weiler, 2012). Table 1 shows the resistivity values of some of the typical rocks and soil materials and the resistivity values of some type of water as shown in table 2. Wells can be drilled into the aquifers and water can be pumped out. Precipitation eventually adds water (recharge) into the porous rock of the aquifer. The pace of recharge is not the same for all aquifers, though, and that must be considered when pumping water from a well. Pumping too much water too fast draws down the water in the aquifer and eventually causes a well to yield less and less water and even run dry. In fact, pumping well too much can even lead another neighbor's well to run dry if both are pumping from the same source (aquifer). Sometimes the porous rock layers become tilted in the earth. A link does not certainly exist between the water-bearing capacity of rocks and the depth at which they are found. Much dense granite that will yield little or no water to a well may be exposed at the land surface (USGS, 2017; Adeeko and Muhammad, 2015; Abidin et al., 2017). Since there is a rapidly growing population which placed greater demand on limited aquifer resources in the study area, the aim of the survey was to investigate groundwater potential of Kedah area using 2-D electrical resistivity imaging, results indicate that borehole drilling in the study area is achievable but to a depth of 10.5 meters to allow reservoir within the aquifers.

Table 1. Resistivity values of common rocks and soil materials in survey area (Keller and Frischknecht, 1996)

Table 2. Resistivity values of some type of water (Keller and Frischknecht, 1996)

1.1. Basic concept of electrical resistivity methods

The principle that surface electrical resistivity survey is based on the circulation of electrical potential in the ground around a current-carrying electrode relies upon the electrical resistivities and circulation of the surrounding soils and rocks (EPA, 2016; Sirhan and Hamidi, 2016).  In the field, the usual procedure is to apply an electrical direct current (DC) between two electrodes inserted in the ground and to measure the difference of potential between two additional electrodes that do not transport current.  Generally, the potential electrodes are in line between the current electrodes, but in basis, they can be situated anywhere.  The current used is either direct current, changed direct current (i.e., a square-wave alternating current), or AC of low frequency typically about 20 Hz (Hajizadeh and Akhondi, 2016; Zimmermann et al., 1966; Horton and Hawkins, 1965). All analysis and interpretation are done based on direct currents.  The circulation of potential can be connected theoretically to ground resistivity’s and their circulation for some simple cases, notably, the case of a horizontally stratified ground and the case of homogeneous masses divided by vertical planes e.g., a vertical fault with a large throw or a vertical dike; for other kinds of resistivity circulations, interpretation is commonly done by qualitative comparison of observed reaction with that of idealized hypothetical models or on the basis of empirical methods (McConville et al., 2001; Stewart and McDonnell, 1991; Rosli et al., 2014; Asano et al., 2002). Mineral grains comprised of soils and rocks are fundamentally nonconductive, except in some exotic materials such as metallic ores, so the resistivity of soils and rocks is controlled generally by the volume of pore water, its resistivity, and the package of the pores; to the range that differences of lithology are followed by differences of resistivity, resistivity surveys can be useful in finding bodies of anomalous materials or in evaluating the depths of bedrock surfaces (Abidin et al., 2014; Azhar et al., 2016; Habibah et al., 2004). The groundwater surface is usually marked by an abrupt change in water saturation and thus by a change of resistivity in coarse, granular soils, however, there may be no such resistivity change coinciding with a piezometric surface in fine-grained soils; there are wide ranges in resistivity for any particular soil or rock type, and resistivity values cannot be directly interpreted in terms of soil type or lithology, since the resistivity of a soil or rock is controlled primarily by the pore water conditions. Therefore, zones of distinctive resistivity can be indicated with specific soil or rock units based on local field or drill-hole information, and resistivity surveys can be used profitably to expand field investigations into areas with very limited or nonexistent data. Also, resistivity surveys may be used as an inspection method, to detect anomalies that can be further examined by complementary geophysical methods or drill hole (Ghafar et al., 2016; Tan et al., 2017).

The 2-D resistivity inversion model of line 1-4 were shown in fig 2-5, generally, the resistivity value of the study area ranges from 1Ωm-500Ωm with total depth of 10.5m, the ERI results shown that the study area consists of two main zones, clay with resistivity value of 0.1Ωm-100Ωm, and shale materials with resistivity value of 20Ωm-500Ωm with the depth 10.5m and the saturated zone with resistivity value of <100Ωm with the depth 5m and hard layer with resistivity value of 100Ωm-500Ωm which indicated fractured zone.

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Figure 2. 2-D resistivity inversion model for line 1

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Figure 3. 2-D resistivity inversion model for line 2

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Figure 4. 2-D resistivity inversion model for line 3

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Figure 5. 2-D resistivity inversion model for line 4

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Figure 6. 2-D resistivity inversion model for all survey lines

The combination of all the survey lines were arranged according to increased elevation starting from line 1 at the top until line 4 at the bottom; therefore, the vast saturated zones arrangement was situated at shallow of the survey as shown in fig 6. Baharuddin et al., (2017); Nordiana et al., (2017) in their report investigate a shallow alluvial landscape was investigated using 2-D resistivity with EHR technique to determine the thickness of alluvium at the Lembah Bujang, Kedah (Malaysia) which show the saturated zone are the aquifer, the aquifer in this study area is therefore defined by the saturated zone which agrees with their earlier studies. Generally, the study area was an alluvium with resistivity value of 0.1Ωm-500Ωm. The first layer was divided into two groups, sandy clay is top layer with resistivity value 20Ωm - 100Ωm and clay is the bottom layer with resistivity value 0.1Ωm -20Ωm. The resistivity characteristic shows that the study area was saturated with depth < 5m, the ERI result was related with a borehole (BH10) in survey line one to identify the distinguishing subsurface and the BH10 shows overburden was alluvium (sand gravel) and depth to penetration of 14.30m which exhibit moderate yield of groundwater and that the aquifer was shallow in the study area. Interrelationship between the geotechnical data and geophysical result revealed that the area consists of two main zones. The first zone composed of sandy clay, clay; the clay has resistivity value 0.1Ωm to 10Ωm at depth of 5m with N-value 7-20, sand clay has resistivity value 7Ωm to 100Ωm at depth of 7m with N-value of 9-32 which indicated saturated zone. The second zone composed of mottle dry, content little weathered and gravel deposit with resistivity value 20Ωm to 500Ωm at depth of 10.5m have N-value of 32-70 which indicate shale materials; the spiral auger penetration is very slow as shown in table 3.

Table 3. Soil classifications of resistivity and N-value

A soil’s electrical resistivity value mostly varies inversely proportional to the water content and dissolved ion concentration as clayey soil display high dissolved ion concentration, wet clayey soils have lowest resistivity of all soil material while coarse, dry sand and gravel deposits and massive bedded and hard bedrock have the highest ERV as reported by Azhar et al., 2016; Ghafar et al., 2016; Nordiana et al., 2013.  The interactions between 2D resistivity and borehole record in the study area shows that the area consists of dark brownish sandy clay, reddish or yellowish clay and shale material which are very firm dry condition. The collection of all the survey lines, line 01 at the top and line 04 at the last were put in an order according to the increased elevation; and the considerable saturated zones arrangement were situated at top of all the lines shown in figure 6.

In summary, the sand gravel aquifer in this area tend to be thin and discontinuous, borehole completion in this area can meet the needs of most domestic users; boreholes producing from shallow aquifers are moderately to highly susceptible to contamination and there are no registered significant groundwater withdrawal facilities utilizing in this borehole BH10. As reported by Azhar et al., 2016 the electrical resistivity value is largely control by the changes of sedimentary earth materials beneath the ground surface the result gather from their study area exists of two zones, which are residual soil zone with a resistivity of 10Ωm - 600Ωm and shale zone with a resistivity of 20Ωm - 2000Ωm. According to geology result and stratigraphic information coming from existing borehole in line one of the surveys, we suggested that the resistive layer in the study area is interpreted as unconsolidated and saturated. However, the ERI have provided useful information about alluvium within the study area which has moderate yield of groundwater potential.