Determination of groundwater potential using geoelectricity in Tanah Bumbu, South Kalimantan

This survey was conducted to determine the distribution of groundwater in the Tanabumbu area of South Kalimantan. In this study, we aim to obtain the resistivity of soil layers using the Wenner geoelectric method and his one-pass, two-point dipole-dipole configuration. Field survey data were processed using RES2DINV software. Measurement data results are available in the form of 2D resistivity cross visualizations that are analyzed to determine groundwater distribution. In addition, we performed a comparative analysis of the accuracy of geoelectric measurement results using the results of data with different configurations. Based on the results of geoelectric measurements, we are able to find the aquifer location related to a high probability of groundwater.


Introduction
Water is one of the main needs of humans.With the increase in population, this resource's utilization has greatly grown [1].Water that is present underground in saturated zones below the surface of the soil is known as groundwater [2].The potential existence of groundwater is an important thing to consider in mining activities, especially for a long-term sustainability in a open pit coal mining [3].Groundwater is also very necessary in the agricultural and social sectors [4].One of processes to detect the presence of a water-bearing layer at a certain depth is geoelectrical method.
Geoelectricity is one of the geophysical methods that aims to determine the rock layer below the ground surface by injecting electric current into the soil to obtain an overview of the layers below the ground surface, and the possibility of groundwater at a certain depth [5][6][7].The main principle of geoelectrical method is to inject electrical current into the earth and calculate the resulted potential difference by using the two current electrodes of C1 and C2, and two potential electrodes of P1 and P2.The objective of geoelectricity survey is to obtain an subsurface image of an electrical properties of rock.One of which is pertained to a possibility of groundwater at a certain depth.
The presence of groundwater is influenced by several properties: anomalies of the area, geological conditions, geomorphological conditions, and geohydrological conditions [5,6].Geoelectrical measurements using Wenner and Dipole-Dipole configuration can produce variations in potential difference that will result in the variation of earth's resistance [8].This electrical property can be utilized to understand the subsurface structures and materials which are propagated by the electrical current.
Geoelectricity plays a vital role in understanding and managing groundwater resources, impacting various aspects of hydrogeology and environmental science.Geoelectric surveys involve measuring the electrical resistivity of subsurface materials, allowing researchers to infer the distribution and properties The impact of geoelectricity on groundwater is manifold.It aids in locating potential areas for groundwater exploration, providing valuable information for well siting and drilling.Geoelectric surveys help characterize aquifer properties such as porosity, permeability, and water saturation, which are crucial for understanding the potential yield and sustainability of groundwater sources [9].Additionally, these surveys can identify zones of groundwater contamination or intrusion, assisting in pollution assessment and remediation efforts.Continuous monitoring of geoelectric parameters can offer insights into changes in subsurface water content, assisting in drought and flood prediction and management.
Geoelectricity serves as a non-invasive tool that enhances our understanding of groundwater systems.By facilitating accurate mapping and characterization of subsurface conditions, it aids in sustainable groundwater management, resource planning, and safeguarding water quality for both human consumption and ecological balance.
In this study, we apply the Wenner and Dipole-Dipole configurations in order to show the vertical and horizontal layer of the soil layer.The geoelectric survey were implemented to observe Tanah Bumbu area, South Kalimantan, in order to obtaining the exact position of groundwater distribution.This study result is expected as a consideration of serious environmental problems in the future in carrying out mining activities.

Methods
Tanah Bumbu Regency is geographically located between 2°52' -3°47' South latitude and 115°15' -116°04' East longitude.Tanah Bumbu Regency is one of the 13 (thirteen) regencies in South Kalimantan Province which is located right at the southeastern tip of Kalimantan Island.Tanah Bumbu has an area of 5,066.96km2 (506,696 Ha) or 13.50 percent of the total area of South Kalimantan Province.Several studies stated that the rocks found in the Tanah Bumbu area mostly consist of clastic sedimentary rocks with coal inserts, carbonate sediments, ultrabasic igneous rock, metasediments, and metamorphic rock [8].Stratigraphically, Tanah Bumbu Regency is included in the Barito Basin.The Barito Basin is a deltaic coal depositional environment.Regional geology in the Tanah Bumbu is complex as evidenced by the presence of anticlines, synclines, thrust faults, horizontal faults, and descending faults [10].Tanah Bumbu also has the potential for groundwater aquifers, where the Pagatan Basin is one of the sources of groundwater that residents use as their daily source of water.) is a map of Tanah Bumbu Regency, South Kalimantan province.Tanah Bumbu Regency is bordered by Kotabaru Regency and Hulu Sungai Tengah Regency to the north, Kotabaru Regency to the east, Java Sea to the south, and Tanah Laut Regency and Banjar Regency to the west.
In this chance, we aim to do geoelectrical survey to assess the groundwater potential of Tanah Bumbu Regency.Geoelectricity is one of the geophysical methods that aims to determine the rock layer below the ground surface by injecting electric current into the soil to obtain an overview of the layers below the ground surface, and the possibility of groundwater at a certain depth [5].The main principle of geoelectrical method is to inject electrical current into the earth and calculate the resulted potential difference by using the two current electrodes of C1 and C2, and two potential electrodes of P1 and P2.The objective of geoelectricity survey is to obtain an subsurface imaging of an electrical properties of rock.One of which is pertained to a possibility of groundwater at a certain depth.Data from resistivity surveys are interpreted in the form of values of apparent resistivity ρa.Apparent resistivity is defined as the resistivity of an electrically homogeneous and isotropic half-space.Equation (1) shows the formula to determine the ρa, with ΔV represents the measured potentials, the I shows the current used in the measurement, and K is the eccentricity factor that depends on the electrode configuration we used.The K parameter can be obtained from Equation (2), where the r parameters shows electrode distance depicted in Figure 3.

𝜌𝑎 = 𝐾 ∆𝑉 𝐼
(1) Geoelectrical field survey activities were carried out in the Tanah Bumbu area using ARES geoelectric equipment.The resulted data were processed using RES2DINV software.The basis of the subsurface estimation method with geoelectricity is carried out on the basic physical properties of soil or rock to electric current, in which each of the soil or rock type has a certain specific resistance value when given an electrical current (Direct Current).
The methods used in this investigation are the Wenner and Dipole-Dipole configuration methods.The Wenner configuration is one of the configurations in geophysical exploration with the electrode arrangement located in a line that is symmetrical to the center point.The Wenner electrode configuration has a good vertical resolution and a high sensitivity to lateral changes but weak to deep current penetration.An illustration of the Wenner configuration used in the study can be seen in Figure 2.  The dipole-dipole configuration resistivity geo-electrical method can be applied for the purpose of obtaining subsurface images on objects with relatively deeper penetration.This method is often used in resistivity surveys because of the low electromagnetic effect generated between current and potential circuits [9].The electrode array of the dipole-dipole configuration can be seen in Figure 3.

Figure 3. Dipole-Dipole configuration setup
The distance between the two current electrodes and the potential electrode has the same na.This configuration has another factor, na, which is the ratio of the distance between electrodes C1 and P1 to C2 -C1 or P1 -P2 with a separation distance a.The number of electrodes connected to each stretch has 24 electrodes on the left and right sides.So that the total electrodes installed in each stretch is 48 electrodes.The flowchart of the study can be seen in Figure 4.

Results and Discussion
Geoelectrical measurements in this study were conducted at two points in the Tanah Bumbu area.After analyzing the data and interpreting the data along with considerations of geological and hydrogeological factors, the results of subsurface estimation are obtained which include aspects estimation of depth positin of a lithology, and estimation the presence of groundwater aquifers.
Measurements are made on one track that has several points or centers as the center of the tool (ARES) to start measurement activities.In this measurement, the cable stretch uses a spacing of 10 m (meters) and each point or center is the same, using a different configuration.The measurement data inversion results were processed using RES2DINV software.5 and 6, we can see that the distribution of groundwater is probably not too deep from the ground surface (<the depth 30 km).The high resistivity in the middle of the track is probably caused by the condition of igneous rock bedrock which is a resistive rock.We suspect that the groundwater aquifer layer is in the sandstone layer above the igneous bedrock, in accordance with the characteristics of the Pitap formation.On the surface, there is a resistive layer which may indicate impermeable rock that is not drained by groundwater.It is suspected that this layer closes the aquifer and the aquifer is stored between the sandstone layers beneath the impermeable layer.The results of data processing in Figure 6 show that the distribution of groundwater at that point is quite large.However this is less reliable considering the nature of the wenner configuration which is not good for reaching deep areas.According to the data results using the Dipole-Dipole configuration, there is one large groundwater source that is quite shallow.This low resistivity zone extends from 100 to 500 meters at a depth of about 12 kilometers.Same as Track 1 point A, we suspect this condition is due to groundwater association in the sandstone area of the Pitap Formation.In the Wenner configuration results, it can be seen that the low resistivity zone continues to a depth of 60 km, but we cannot consider this condition, considering that the Wenner configuration has a poor vertical quality.

Conclusions
The geoelectrical survey has been successfully implemented to observe the distribution of groundwater in Tanah Bumbu Area.It can be concluded that Point B is more potential to groundwater prone area than Point A. Geoelectric measurements using the Wenner and Dipole-Dipole configurations have their own advantages and disadvantages.The Wenner configuration has the advantage of being able to produce more accurate data.However, on the other hand, the Wenner configuration has the disadvantage of not being able to reach deeper into the subsurface area.The Dipole-Dipole configuration is able to image the deeper layer than the Wenner configuration.But, the limitation of this configuration is that the results of the groundwater distribution data covered will look small so it needs comparative data before deciding the results.

Figure 2 .
Figure 2. The running current pattern of the Wenner configuration

Figure 4 .
Figure 4. Flowchart of this study

Figure 5 -
Figure 5-8 depict the result of inversion of field data using the Dipole-Dipole and Wenner configuration in Track 1 point A and Track 1 point B. Low resistivity values are indicated by cool colored bars, and high resistivity values are indicated by warm colored bars.In general, groundwater aquifers have low resistivity.The results at Track 1 point A have significant soil layer resistivity.Based on the results shown in Figure5 and 6, we can see that the distribution of groundwater is probably not too deep from the ground surface (<the depth 30 km).The high resistivity in the middle of the track is probably caused by the condition of igneous rock bedrock which is a resistive rock.We suspect that the groundwater aquifer layer is in the sandstone layer above the igneous bedrock, in accordance with the characteristics of the Pitap formation.On the surface, there is a resistive layer which may indicate impermeable rock that is not drained by groundwater.It is suspected that this layer closes the aquifer and the aquifer is stored between the sandstone layers beneath the impermeable layer.The results of data processing in Figure6show that the distribution of groundwater at that point is quite large.However this is less reliable considering the nature of the wenner configuration which is not good for reaching deep areas.

Figure 5 .Figure 6 .
Figure 5. Measurement results of Track 1 at point A using Dipole-Dipole configuration

Figure 7 .
Figure 7. Measurement results of Track 1 at point B using Dipole-Dipole configuration

Figure 8 .
Figure 8. Measurement results of Track 1 at point B using the Wenner configuration