Groundwater Investigation Using Schlumberger Vertical Electrical Sounding (VES) in Baturraden Geothermal Working Area (GWA), Central Java, Indonesia

Goundwater investigations are increasingly becoming important for Indonesia. It is natural resources for sustainable development of a region. In this study, PT Sejahtera Alam Energy conducted an investigation of subsurface conditions, especially related to groundwater potential, carried out by a resistivity investigation method by means of Vertical Electrical Sounding (VES) or Geoelectricity in Embung Area, Baturaden Geothermal Project, Pandansari Village, Paguyangan Subdistrict, Brebes Regency. This method is one of the subsurface estimation methods that is considered suitable for water investigations as well as in terms of accuracy as well as in terms of low cost and faster implementation time compared to other geophysical methods.


Introduction Abstract
Goundwater investigation is becoming increasingly important for Indonesia.It is natural resources for sustainable development of a region.In this study, PT Sejahtera Alam Energy conducted an investigation of subsurface conditions, especially related to groundwater potential, carried out by a resistivity investigation method by means of Vertical Electrical Sounding (VES) or Geoelectricity in Embung Area, Baturaden Geothermal Project, Pandansari Village, Paguyangan Subdistrict, Brebes Regency.This method is one of the subsurface estimation methods that is considered suitable for water investigations as well as in terms of accuracy as well as in terms of low cost and faster implementation time compared to other geophysical methods.
Geological condition based on the geological map of the Purwokerto sheet.Location of the investigation included in the Mount Slamet Lava Formation which is composed of volcanic lava deposits, tuffs and lava.

Methods
Electric current (direct current) is flowed through two current electrodes A and B, there will be a potential difference between the two currents.The potential difference is measured through 2 potential electrodes M and N which are then recorded by the receiver (receiver).The measurement results of the electrical values will be converted into rock types, the position of depth and thickness.

Figure 3. Schlumberger Electrodes Arrangement and Electrical Flow Patterns
In the Schlumberger configuration method the arrangement of electrodes is set with certain distance values.This configuration method of potential electrodes M and N is always placed between the current electrodes A and B. Ohm's law that is used as the basis for calculating the geoelectric probe in obtaining the resistance of the apparent type must be multiplied by the distance factor (K factor).

Results
The results of geoelectric measurements at the Embung Area location, Baturaden Geothermal Project, found the range of resistivity values between 20.97 and 6315.50 ohmmeters.The range is divided into certain intervals and can be interpreted as lithological layers.

Table 1. The intervals resistivity and lithology of geoelectric measurements
The results of the interpretation of the spread of the resistivity value in a vertical type with a geoelectric cross-section at the study site are divided into 9 cross sections.

Conclusions
After doing data processing and interpretation of layer types from geoelectric data are supported by geological, geohydrological and field observations related to soil water aspects, so it can be concluded: In general, geoelectric measurements in the Embung area, Baturaden Geothermal Project (GL-01 to GL-25) obtained below-ground image are as follows: 1.The distribution of aquifers is evenly distributed around the Embung site.2. The Embung Area is a groundwater basin area.3. The lithology condition of the Embung location area consists of soil with a thickness of about 5 meters, laying of mudflow deposits and tuffaceous-sandstone with varying depth and thickness.

Introduction
Geological condition based on the geological map of the Purwokerto sheet (Figure 1).Location of the investigation included in the unraveled Mount Slamet Lava Formation (Qvs) which is composed of volcanic lava deposits, tuffs and lava.Vertical Electrical Sounding (VES) to find locations that have potential groundwater content and the position of depth of groundwater which will be used to determine locations that have deep groundwater potential and can be used economically by using deep drill well, besides that it is also used as a basis for the preparation of a budget plan that is needed for the manufacture of boreholes.The geoelectric survey work is carried out in Embung Area, Baturaden Geothermal Project, Pandansari Village, Paguyangan District, Brebes Regency with twenty five sites spread (Figure 2).

Data and Methods
Electric current (direct current) is flowed through two current electrodes A and B (Figure 3), there will be a potential difference between the two currents.The potential difference is measured through 2 potential electrodes M and N which are then recorded by the receiver (receiver).
The measurement results of the electrical values will be converted into rock types, the position of depth and thickness.In the Schlumberger configuration method the arrangement of electrodes is set with certain distance values.This configuration method of potential electrodes M and N is always placed between the current electrodes A and B. Ohm's law that is used as the basis for calculating the geoelectric probe in obtaining the resistance of the apparent type must be multiplied by the distance factor (K factor).The apparent resistivity obtained from the calculation according to the above equation is then plotted on a transparent double logarithmic paper to half the distance of half the current electrode (AB / 2) where the distance of the AB stretch to the measurements carried out is 160-250 meters.After plotting it will be obtained forms of apparent resistivity curves to be interpreted quantitatively.
In processing the data carried out the method of interpretation is used 3 (three) layers, ie each apparent resistivity curve is placed above the standard curve for matching, so that the numbers of true resistivity ρ 1, ρ 2, ρ 3 with depths d 1, d 2 , d 3 .The third layer (d 3 ) is based on numbers.This apparent will have infinite depth (d 3 =∞)

Resistivity Log
The results of geoelectric measurements at the Embung Area location, Baturaden Geothermal Project, found the range of resistivity values between 20.97 and 6315.50 ohmmeters.The range is divided into certain intervals and can be interpreted as lithological layers.

Isoresistivity Map
The Isoresistivity map provides an overview of the distribution of aquifers in the study location.The Isoresistivity map presented is the value of type resistances at depths of 20, 50, 70 and a depth of 100 meters which illustrates the potential of deep aquifers.
From the isoresistivity map can be seen the distribution of the type of resistivity values that are interpreted as aquifers which are between the value of 11-100 ohm meters which are colored blue.The orange color shows the distribution of lava sediment or lava deposits with a type of resistivity value between 100-1000 ohmmeters.

Conclusions
After doing data processing and interpretation of layer types from geoelectric data are supported by geological, geohydrological and field observations related to soil water aspects, so it can be concluded: In general, geoelectric measurements in the Embung area, Baturaden Geothermal Project (GL-01 to GL-25) obtained below-ground image are as follows: 1.The distribution of aquifers is evenly distributed around the Embung site.2. The Embung Area is a groundwater basin area.
3. The lithology condition of the Embung location area consists of soil with a thickness of about 5 meters, laying of mudflow deposits and tuffaceous-sandstone with varying depth and thickness.

Figure 3 .
Figure 3. Schlumberger Electrodes Arrangement and Electric Flow Patterns AB current electrode separation (meter) l = potential MN electrodes spacing (meter) Figure 4. Geoelectrical Sections

Figure 5 .
Figure 5. Isoresistivity Map of 20 metersAt 50 meters depth the distribution of aquifers accumulates in the plains around the retention basin site at points GL-1, GL-3 GL-4, GL-5,

Table 1 .
The intervals resistivity and lithology of geoelectric measurements