INDONESIAN JOURNAL ON GEOSCIENCE Vertical Electrical Sounding Exploration of Groundwater in Kertajati, Majalengka, West Java, Indonesia

- Continuously increasing population and progressive infrastructural development in the region of Kertajati International Airport, Indonesia, emphasize the need to develop a sustainable water supply network. Airport facilities require sufficient water resources, which can be obtained from surface water and groundwater. Groundwater exploration can provide necessary information for assessing water resources. The purpose of this study is to analyze the configura tion of aquifers in the studied area. A Schlumberger array was used to carry out twelve vertical electrical soundings (VES) with AB/2 electrode spacing ranging from 1.5 m to 150 m. IPI2win software was used to qualitatively interpret the VES results and it suggested the presence of three distinct lithological units interpreted as clay, alluvial sand, and a Lower Quaternary formation. In general, resistivity values in the studied area can be divided into five resistivity categories: very low resistivity with values ranging from 1 Ωm to 10 Ωm, low resistivity with values ranging from 10 Ωm to 50 Ωm, medium resistivity with values ranging from 50 Ωm to 100 Ωm, high resistivity with values ranging from 100 Ωm to 200 Ωm, and very high resistivity with values > 200 Ωm. The geo-electric interpretation revealed three geo-electric layers: topsoil (1 - 144 Ω m), sand (1 - 298 Ω m), and clay (1 - 82 Ω m). Aquifers in the studied area are lithologically composed of sand. Clay is the dominant lithology in the studied area, so the presence of aquifers in this area is very limited, and thus the supply of groundwater is also limited. The exploitation of groundwater must be limited and controlled to maintain the sustainability of groundwater in the studied area.


Background
Groundwater is a highly valuable natural resource (Song et al., 2012) and an essential geological agent in the transport of mass and energy within the earth (Llamas, 1987), providing a wide variety of ecological and social services (Houben and Weihe, 2010). The advantages of groundwater over other sources of water have been emphasized in literature (Bayewu et al., 2018). A high percentage of water users worldwide rely substantially on groundwater (Reilly et al., 2008). Demand for this resource has increased significantly throughout the world due to population growth, socio-economic development, technological and climatic changes (Alcamo, 2007). Despite its advantage of easy accessibility, surface water is often polluted by anthropogenic activities, making groundwater a desirable option to satisfy our demand for quality water (Anomohanran, 2015). The continuous increase in population and the progressive infrastructural development in Kertajati area emphasize the need to develop a sustainable water supply network. Airport facilities require sufficient water resources, which surface water and groundwater are often used to fulfill. Groundwater exploration can provide necessary information for assessing water resources. The purpose of this study is to analyze the configuration of aquifers in the area of Kertajati Airport.
Kertajati Airport is the second largest airport in Indonesia, located in the Majalengka Regency, northeastern part of West Java Province, approximately 68 km east of Bandung City.
The urgent need for groundwater has driven the application of appropriate geophysical and hydrogeologic exploration techniques (Anudu et al., 2011) to locate areas of high and reliable groundwater potential or to characterize seasonal changes in near-surface aquifers (Webb et al., 2011). A geophysical investigation is a powerful tool for exploring subsurface geology and collecting information about subsurface layers and structures (El-Sayed, 2010). Various geophysical techniques or applications have been employed in groundwater exploration in many parts of the world, including magnetic resonance sounding (MRS), remote sensing, geographic information systems, seismic refraction, and electrical resistivity, among others (Kamble et al., 2012).
The electrical resistivity method has been extensively used for groundwater aquifers mapping (Massoud et al., 2015), investigating aquifer vulnerability (Sørensen et al., 2005), and freshwater/saline water studies (Khalil, 2010). The electrical resistivity survey method is one of the oldest geophysical exploration techniques and has been extensively employed in environmental, engineering, hydrological, archaeological, and mineral exploration surveys (Reynolds, 2011). Vertical electrical sounding (VES) has been the most frequently used electrical resistivity tool in groundwater studies, as it can give information about subsurface rocks and structures at depths useful for water exploration (Araffa et al., 2015). It is also comparatively less expensive than other methods, and its methodology is simple. The VES technique is based on the fact that the subsurface layer can only transmit current because of the presence of water, since the rock itself is considered an insulator (Anomohanran, 2015). The VES technique is thus widely used to explore groundwater resources (Hafeez et al., 2018;Mohamaden et al., 2009), and has therefore been chosen to analyze aquifer configurations in this study.

Geological and Hydrogeological Settings
The geology of the studied area is composed of Lower Quaternary sedimentary rocks (Qos) and Holocene alluvium deposits (Qa) (Figure 1). The Lower Quaternary sedimentary rocks extend across almost all of the studied area and consist of tuffaceous sandstone, sand, tuffaceous silt, clay, conglomerate, and tuffaceous breccia containing pumice, which crop out in Kertajati Village in the form of conglomerates, and in Pasiripis Village in the form of coarse sand. The weathering of these rocks produces residual soils which include clays subject to swelling (Hasibuan et al., 2009) . Holocene-aged alluvium (Qa) deposits occur in the southeastern part of the studied area, as a result of flood deposition from the Cimanuk River. This alluvium consists of clays, silts, sands, and gravels which has been mainly deposited by Holocene streams (Hasibuan et al., 2009). The studied area has a widespread medium aquifer consisting of undifferentiated sandstones and tuffs, with groundwater flowing through the pore spaces in the media (IWACO- WASECO, 1990).
A geoelectrical survey using VES was conducted on November 2015. The electrical resistivity of the studied area was measured using a GL-4100 Earth Resistivity meter. A Schlumberger array was used to carry out 12 VES with AB/2 electrode spacing ranging from 1.5 m to 150 m ( Figure 2). These stations are referred to as MJL-01 -MJL-12. The geo-electrical method was adopted in this study, because it is a useful tool for ascertaining the subsurface geology of an area (Tizro et al., 2012). VES data interpretation aims to determine the true resistivities and thicknesses of the successive strata below the different stations, utilizing measured field curves (El-Gawad et al., 2018). The apparent resistivity (ρa) values were obtained from the voltage (mV) and current (mA) read from the resistivity meter and its calculated corresponding geometric factor (Zohdy, 1975).
A VES station, MJL-05, was located beside a borehole with known lithologic logs to serve as parametric measurements, which were helpful in interpreting the VES data (El-Gawad et al., 2017). The lithology data obtained from well TW-88, which were drilled by the Groundwater Development Project (P2AT), Ministry of Public Works, Republic of Indonesia, were used to calibrate the geo-electrical models obtained from the apparent resistivity curves. Figure 3 shows the correlation between geo-electrical parameters of well TW-88 and the geology obtained from station MJL-05. Figure 3 shows the correlation between geo-electrical parameters measured at VES station MJL-05 and the geological data obtained from well TW-88. The measured vertical electrical soundings were interpreted qualitatively and quantitatively to build a geo-electrical model, which was initiated using all available data about the geologic and hydrogeologic settings. The data were calculated to obtain the apparent resistivity and thickness values, which were again used in the computerized interpretation to obtain the true resistivity and thicknesses of the various layers encountered. The interpretation of geo-electrical resistivity data from the twelve VES curves was conducted by converting the values of AB/2 and ρa into a multilayer model. The quantitative interpretation has been applied to determine the correlation between the geo-electrical parameters obtained from station MJL-05 and geological information from drilled hole TW-88 ( Figure 3). The initial models have been constructed using the available geologic data from the existing boreholes. IPI2 win is a programme provided by Moscow State University, Russia, to produce quantitative interpretations of the geo-electrical sounding curves. It is an inverse modeling programme for interpreting resistivity sounding   electric survey were used to establish the depth of the aquifer layer and to construct a geo-electric section for the studied area. These results were then used to describe the geological framework for the studied area (Tizro et al., 2012;Anomohanran, 2015). The pseudosections and geoelectrical resistivity sections were obtained from the quantitative interpretation of the VES data.

Results
Twelve VES measurements were conducted at locations around the area of West Java International Airport, Kertajati Subregency, Majalengka Regency, West Java Province (Table 1).
Resistivity values in the studied area can be divided into five resistivity categories: very low resistivity with values ranging from 1 Ωm to 10 Ωm, low resistivity with values varying from 10 Ωm to 50 Ωm, medium resistivity with values ranging from 50 Ωm to 100 Ωm, high resistivity with values between 100 Ωm and 200 Ωm, and very high resistivity categories with values > 200 Ωm. Table 2 shows the details of resistivity and the thickness for each layer as inferred from resistivity inversion using IPI2win software.

Groundwater
At MJL-01, the aquifer layer was interpreted lithologically as a sand layer at a depth range of 5 -14 m, with an aquifer resistivity value of 31 Ωm. At MJL-02, the aquifer layer was interpreted lithologically as a sand layer at a depth range of 13 -40 m, with an aquifer resistivity value of 133 Ωm. At MJL-03, the aquifer layer was estimated to lie at a depth range of 2 -71 m, with an aquifer re-

MJL1
Layer  Table 3. VES Interpretation and Their Inferred Lithologies sistivity value of 247 Ωm. At MJL-04, the aquifer layer was estimated to lie at a depth range of 5 -9 m, with an aquifer resistivity value of 298 Ωm. At MJL-05, the aquifer layer was estimated to occur at a depth range of 13 -150 m, with an aquifer resistivity value of 7 Ωm. At MJL-06, the aquifer layer was estimated to exist at a depth range of 2 -20 m, with an aquifer resistivity value of 6 Ωm. At MJL-07, the aquifer layer was estimated to occur at a depth range of 5 -23 m, with an aquifer resistivity value of 7 Ωm. At MJL-09, the aquifer layer was estimated to appear at a depth range of 5 -17 m, with an aquifer resistivity value of 1 Ωm. At MJL-10, the aquifer layer was estimated to lie at a depth range of 4 -12 m, with an aquifer resistivity value of 258 Ωm. At MJL-11, the aquifer layer was estimated to occur at a depth range of 4 -12 m, with an aquifer resistivity value of 33

I J O G
Vertical Electrical Sounding Exploration of Groundwater in Kertajati, Majalengka, West Java, Indonesia (G.U. Nugraha et al.) 365 Ω. At MJL-12, the aquifer layer was estimated to exist at a depth range of 5 -15 m, with an aquifer resistivity value of 33 Ωm. The detailed summary of aquifer resistivity is described in Figure 4.
The values of aquifer resistivity in the studied area are very diverse ( Figure 5). This is consistent with previous studies that measured the resistivity values of sand layer aquifers (Maiti et al., 2011;Obiora and Ibuot, 2020;Reynolds, 1997;Telford et al., 1990). Differences in the degree of compaction and also the physical properties of the sand within the aquifer layer can cause very significant differences in the resistivity values of the aquifer in the studied area (Dobrin and Savit, 1988;Kearey and Brooks, 1991;Reynolds, 1997;Telford et al., 1990;Tiab and Donaldson, 2012). Figure 6a shows a sand layer thickening towards the north. The existence of this continuous layer shows that this layer is part of the hydrogeological system in the studied area. The impermeable zone of the groundwater system in the studied area was dominated by the presence of a clay layer which acts as an aquiclude layer in the studied area. Aquiclude layers may store water easily, but do not transmit the groundwater easily (Fetter, 2001;Freeze and Cherry, 1979;Harter, Figure 2).

2003
). The clay layer can be classified as an aquiclude layer (Freeze and Cherry, 1979;Fetter, 2001;Lopez-Gunn et al., 2011;Singhal and Gupta, 2010), while the sand layer can both store and transmit groundwater, acting as an aquifer (Freeze and Cherry, 1979;Fetter, 2001;Wal, 2010). The pattern distribution of sand in cross-section 1 tends to follow the elevation of the studied area. A layer of sand that is thick enough to serve as an aquifer is found at the MJL-10 VES station, but nevertheless does not have the potential to be an aquifer, because it is present only locally and noncontinuously. Groundwater may be present in this layer but is not sustainable, because it does not have a continuous water supply. Figure 6b shows a layer of sand that is thick enough to act as an aquifer, with a small lens from the clay layer. In this cross-section, the sand layer functions as an aquifer layer and the clay layer functions as an aquiclude. Figure 6c shows that the sand layer is continuous towards the northwest. The clay layer dominates in almost all cross-sections, and the aquifer potential is found in the sand layer. Figures 6a and 6b show that the clay layer predominates in all studied areas. Due to the dominance of the clay layer compared to the sand layer, the presence of aquifers in the studied area is very limited. With limited aquifer layers, the groundwater supply in the studied area is also limited (Fetter, 2001;Lopez-Gunn et al., 2011;Ramsar, 2006). Figure 6 shows the geometry of the aquifer, which suggests that groundwater cannot be exploited excessively if the sustainability of groundwater in the studied area is not to be maintained. IWACO-WASECO (1990) states that in the studied area, there is an aquifer with medium potential. Resistivity values indicate that clay layers dominate the studied area.

I J O G
Vertical Electrical Sounding Exploration of Groundwater in Kertajati, Majalengka, West Java, Indonesia (G.U. Nugraha et al.) 367

Conclusions
Twelve VES measurement points were conducted around the area of West Java International Airport, Kertajati Subregency, Majalengka Regency, West Java Province. In general, resistivities in the studied area can be divided into five categories from very low resistivity to very high resistivity. Aquifer resistivities falling into all of these categories are found in the studied area. The aquifer resistivity values for each VES station are 31 Ωm for MJL1, 133 Ωm for MJL2, 247 Ωm for MJL3, 298 Ωm for MJL4, 7 Ωm for MJL5, 6 Ωm for MJL6, 7 Ωm for MJL7, 41 Ωm for MJL8, 1 Ωm for MJL9, 258 Ωm for MJL10, 33 Ωm for MJL11, and 33 Ωm for MJL12. In general, aquifers in the studied area are lithologically composed of sand. The sandy aquifer layer was typically found at relatively shallow depths of less than 20 m, although at a few sites the aquifer extended deeper. The dominance of the clay layer compared to the sand layer means that the presence of aquifers in the studied site is very limited, and that the groundwater supply at the studied site is thus also limited. Groundwater exploitation, therefore, cannot be excessive but must be moderate and controlled to maintain the sustainability of groundwater in the studied area.