GEOELECTRIC INVESTIGATION USING VERTICAL ELECTRIC SOUNDING METHOD TO DETERMINE UNDERGROUND WATER AQUIFERS IN THE CASE OF PUDAK AND ITS SURROUNDING AREAS

Resistivity is a determinant of the type of rock and the electrical properties of the rock or water-carrying layer. Primary data is the main source of data for the investigation of the subsurface with the Vertical Electric Sounding (VES) method. The purpose of the study is to determine the resistivity of subsurface geological conditions to detect aquifer and reservoir lithology types designed as planned wells. The data of geoelectric investigation point was carried out on as many as 10 (ten) points and produced in the form of (1) Claystone: resistivity of 1.4-6.4  m and thickness of 4-4.6 meters, (2) Sandstone: resistivity of 7.5-40  m and thickness of 0.2-55 meters, (3) Breccia: resistivity of 45-1088  m and thickness of 0.5-33.2 meters. For a depth aquifer of 5.8-68 meters and resistivity of 12.40-23.60  m with a shallow well condition around reaching a depth of 15-19 meters and freshwater results are obtained.


Introduction
The Pudak area and its surroundings became the location of the research area by using geoelectricity to reach the formation depth. The formation of the geoelectric investigation area is carried out on the notopuro formation or known as the kaligetas formation. The Kaligetas formation was deposited incongruously (unconformity) on top of the Damar formation. Kaligetas formations are composed of volcanic materials such as lava and lava flows, volcanic breccia, tuff, or volcanic sand, and tuff sandstone. In general, Kaligetas Formation forms a highland landform unit that extends from the eastern part to the western part of Semarang City.
The geoelectric method or electrical resistance method is one of the geophysical methods used in the investigation of estimating subsurface states and rocks based on measurements of the electrical properties of rocks. This method can also be used to estimate groundwater position.
Operational techniques in geoelectric surveys. There are two types of resistance-type investigations, namely Horizontal Profiling (HP) and Vertical Electrical Sounding (VES) or depth investigations, with the distinction of an anisotropic cross-section in the horizontal ORIGINAL ARTICLE Uniska PPJ-JST direction and the distinction of anisotropic estimation in the vertical direction.
According to Afifah (2007), lithological characteristics in the Simongan Manyaran and Pudak Payung Banyumanik areas, Semarang indicate the presence of water catchment areas in the sandstone layer of the Kaligetas Formation. A water catchment area is an underground water aquifer area (groundwater). Afifah (2022b) found that the availability of groundwater and productivity of local productive groundwater basins shows the graduation of water and discharge of the surrounding area wellbore. The groundwater formed will flow laterally due to the gravity of the recharge area to the discharge area through the productive layer.
According to Kodoatie (1996), saturated water being under groundwater is the basic definition of groundwater. The regional geological formation is the main influencing factor of groundwater as a reservoir, globally the volume of groundwater has the greatest capacity compared to the flow on the ground surface, so it can be said to be an important water resource.
The study aims to determine the value of electrical resistance and detect water catchment areas as well as the type of reservoir lithology that will be designed for making planning wells. Based on the explanation above, researchers conducted a study with the title geoelectric investigation with the vertical electric sounding method to determine underground water aquifers in the case of Pudak and surrounding areas.

Data collection
The study used primary and secondary data, as follows: 1. Primary data: resistivity value (resistance type) on the site plan to be drilled, well data of nearby residents, and water quality data.

data:
geological maps, characteristics of the territory All primary and secondary data are analyzed to get a comprehensive picture of the condition of underground water in the study area, regarding its potential and utilization. The final results of the evaluation of primary and secondary data are presented thoroughly in the form of reports equipped with the necessary drawings, tables, and maps. In the study, primary data as the main data for subsurface rock investigations in the Vertical Electric Sounding (VES) method used.

Data presentation
Layer Order resistivity measurement results can be interpreted with the physical properties of layers that can be used as a basis for estimating the condition of the soil and rock layer order of the study area. Overall, the order of soil and rock layers based on the electrical resistance value is as in Table 1. Breccia or Lapili *m (Ohm-meter) = amount of electrical resistance (Afifah, 2007) 1. Alluvial An alluvial is a layer of soil derived from marine deposition environments that have an electrical resistance of 0.24 to 1 ohm meters. This very small type of resistance value is due to alluvial containing salty seawater which is very easy to conduct electricity.

Claystone
Claystone This is a rock layer that has an electrical resistance of 1,2 to 6,8-ohm meters. The value of the small type of resistance is because clay stones easily conduct electricity.

Tuff
The Tuff layer is located above the breccia and claystone. This layer functions as an aquifer (water-carrying layer) and has an electrical resistance that varies from 9,45-10,80 ohm meters.

Sand and Tuff Stone
A layer of sand and tuff stone is located on top of the breccia and sand of the faucet. This layer of sand and rock serves as an aquifer (water-carrying layer). This layer has an electrical resistance value that varies from 11-35,50 ohms meters.

Breccia or Lapili
The breccia layer is a very hard layer characterized by a very large electrical resistance value of between 36 -75,7 ohms meters.

Research Techniques
Geoelectric measurement methods aimed at determining the location of underground water aquifers are carried out on measurements with the Vertical Electric Sounding (VES) method with Schlumberger configuration to determine vertical variations, fixed stations, electrode distance changes, and measurement span lengths as far as an average of 400 m. From the field data generated, processed, and then interpreted by equalizing the curve (curve matching) to the standard curve that has been issued by Schlumberger. This curved equalization is done to determine the electrical resistance parameter mathematically in a rock layer model.
The principle of the Schlumberger method is one of the methods of prospecting rock electrical resistance that places its four electrodes, namely 2 current electrodes and 2 potential electrodes at a certain distance. This method is used for measurement rules with variations in resistivity values to depth. The electrodes for the Schlumberger method are between the current and potential electrodes as Figure 2.
The Schlumberger configuration ideally keeps the MN distance as small as possible, so that the MN distance theoretically does not change. But because of the limited sensitivity of the measuring instrument, when the AB distance is relatively large, the MN distance is changed. The change in MN distance is not greater than 1/5 of the AB distance as in Figure 3. The disadvantage of the Schlumberger configuration is that the voltage reading on the MN electrode is smaller, especially when the AB distance is relatively far, so a multimeter measuring instrument that has high impedance characteristics is needed by adjusting the voltage to at least 4 digits or 2 digits behind the comma or using current equipment that has a very high DC voltage.
The advantage of the Schlumberger configuration is the ability to detect the inhomogeneous nature of rock layers on the surface, namely by comparing the value of apparent resistivity when there is a change in the distance of the MN/2 electrode (Afifah, 2007).
According to Patra and Nath (1999), Schlumberger's interpretation of the VES curve uses conventional curves both on the inversion of standard data of electrical resistance, as well as connecting between types of VES curves from field survey results.
Schlumberger's interpretation method with a three-layer curve is by the equalization curve method (matching curve). The three kinds of curves that need to be considered in Schlumberger interpretation with the curve equalization method are standard curves, auxiliary curves, consisting of types H, A, K and Q, and field curves.
To find out the type of auxiliary curve used, it is necessary to know the general shape of each field curve as Figure 4  The measurement curve is used as a basis for the interpretation of the type of lithology that composes the subsurface conditions in the area of the measurement location point. The water content in the pores of rocks will affect the results of measuring the resistance of rock types because of the electrolyte properties of water that can function as a good conductor of electricity.
According to Sunaryo et al. (2003), Apparent electrical resistance (a) in resistivity measurement, in general, is by injecting current into the soil through 2 current electrodes (C1 and C2), and measuring the results of the potential difference it causes on 2 potential electrodes (P1 and P2). Where K is the geometry factor that depends on the arrangement of the 4 electrodes mentioned earlier. The calculated resistivity value is not the actual subsurface resistivity value but is an apparent value which is the resistivity of the earth that is considered homogeneous that gives the same resistance value for the same electrode arrangement. Determining the true value of subsurface resistivity requires an inversion or forward calculation process using computer assistance, although manual calculations are still widely done with a very large degree of subjectivity, as in the cumulative ways of curve matching. According to Sharma (1997), The curve matching technique is part of the vertical electric sounding (VES) interpretation process obtained by horizontal data. This method involves a comparison of curve a  measurements with several parent curves and can be used if: 1. Field data is obtained and determined from the number of layers determined using the parent curve 2. The master curve is for the shape of the electrode configuration system used to obtain field data, 3. The logarithmic modulus on a sheet of paper is used to plot or equalize the standard curve Based on the electrical resistance value for each layer, the interpretation of lithological types and possible water-bearing layers is carried out by considering geological data. Each geoelectric measurement point will produce an electrical resistance value that will be used as a basis for interpretation of the type of soil/ rock that makes up the subsurface conditions. The results of estimating rock layers illustrate the condition of the location of the study area.
The resistivity value as a measure in the presence of groundwater catchment areas. The availability of groundwater in an area is influenced by geological conditions which include: rock lithology, structure, and porosity of rocks according to geological conditions in Indonesia. One of the influences is the graduation of water, namely nesting and liquid properties, as stated in the classification of the graduation value of water (K) from various kinds of rocks. The water pass coefficient is the velocity of water in an aquifer expressed in centimeters/second (cm / s) and is commonly abbreviated as K (Afifah, 2022). Classification of Water Graduation Values (K) of various rocks is listed in Table 2. Where: 10 5 , 10 4 , 10 3 , 10 2 , 10 1 , 1, 10 -1 , 10 -2 , 10 -3 , 10 -4 , 10 -5 is the fixed value of the factor times the pass value on the rock to the groundwater flow (Afifah, 2022a).

Location of Investigation
Areas of study based on regional geology are included in the kaligetas formation. The distribution of geoelectric points in as many as 10 (ten) location points were carried out in the Pudak RT 04 / RW 03 Village area of Banyumanik District, Semarang. The presence of rock layers is summed up as follows : 1. The GL-1, GL-2, GL-3, GL-4, GL-6, GL-6, GL-7, GL-8 point in the form of Claystone to very fine sandstone 2. GL-9 in Sandstone 3. GL-10 is Breccia

Vertical Electric Sounding (VES) -Schlumberger Configuration
Vertical Electric Sounding (VES) with Schlumberger configuration to determine vertical variations, fixed stations, and changing electrode distances. Working Principle of 10 (ten) points of investigation, carried out as follows: 1. Electrode configuration by injecting direct electric current or low-frequency alternating current through a current (AB) electrode. The potential difference produced at the potential electrode (MN) is 1/5 of the distance AB, which is 40 meters. 2. The configuration of the Schlumberger electrode used is the potential difference between the measured M and N electrodes 3. Matching field data processing (the auxiliary point method). The matching method applies empirical master curves consisting of two parts, namely the twolayer standard curve and the auxiliary curve. The value of the electrical resistance in the study area is more than 1 (one), so use master curves as Figure 5

Geoelectric Investigation
The subsurface investigation by conducting geoelectric measurements aims to determine the location of the water-carrying layer (aquifer) in the well plan. The types of lithology that compose subsurface conditions in the area of the measurement location point are claystone, sandstone, and breccia. Sandstone interpretation indicates the presence of water content in the pores of rocks will affect the results of measuring the resistance of rock types because of the electrolyte properties of water that can function as a good conductor of electricity.
Results of geoelectric investigations from GL-1 to GL-10 soil layers containing tuff sandstone rocks. The GL-9 point is the depth at which the aquifer layer is present. The depth of the GL-9 aquifer layer at 5,8-68 meters with a resistivity value of 12,40-23,60 m (Ohmmeters) with a shallow well condition around reaching a depth of 15-19 meters and obtained freshwater results.

Conclusion
The conclusion of the study is as follows: the value of the electrical resistance for each layer, an interpretation of the type of lithology is a water-carrying layer. Rock types based on the value of electrical resistance consist of Claystone, Sandstone, and Breccia. Layer The water-carrying layer (aquifer) is in sandstone with a shallow well condition in the form of fresh water.