Groundwater exploration using Vertical Electrical Sounding ( VES ) Method at Toro Jaya , Langgam , Riau

Vertical Electrical Sounding (VES) method is one of many methods in geophysics that was conducted in this research at Toro Jaya, Langgam, Riau. The aim of this research is to investigate and locate the depth of groundwater layer (aquifer). Data acquisition of VES was carried out using GEOCIST with Schlumburger configuration of electrode. There are four VES point in this research with various length of cable ranging from 135 m up to 200 m. From four VES point shown two different of resistivity value range. Very low resistivity value with range 0,34 divided into two types which are unconfined aquifer and confined aquifer. Meanwhile, another resistivity value ranging from 1,69


Introduction
In agriculture, water resources to fulfill the need of plant is important in large amount.Groundwater is the best alternative water resources to supply the water needed for agriculture, industrial and domestic purpose (Chandra et al., 2010;Putra and Yuskar, 2017).Groundwater is defined as water that fill the porosity of soil or rock that reserve the water in subsurface.Generally, groundwater classified into two types which are confined aquifer and unconfined aquifer.Both of them only differentiated based on appearance of impermeable layer above the aquifer.
Geophysics survey especially geo-electrical survey is widely used in order to locate the groundwater resources (Asry et al., 2012;Azhar et al., 2016;Hamzah et al., 2008Hamzah et al., , 2007;;Jumary et al., 2002).One of geoelectrical methods is Vertical Electrical Sounding also known as VES.VES is a method to generate one dimensional of resistivity distribution in vertical section.This method very popular used to determine the subsurface resistivity anomaly that very helpful to investigate water saturated layer.A survey of subsurface resistivity value using one dimensional method was conducted at agriculture area in Toro Jaya village, Ukui district, Riau Province.The aim of study is to locate and identify the water saturated layer (aquifer) that can be water resources for agriculture purpose and domestic purpose (Kausarian et al., 2018). -(Fig.1).

Setting of The Study Area
The study area characterized by gentle hill with elevation from 45 m to 70 m.Geological setting of study area is included in Central Sumatra Basin (CSB) that consist of Minas Formation, Old Alluvium and Young Alluvium.Young Alluvium deposited during Holoceneaged with materials gravels, sands and clay.The old Alluvium generated from aged Pleistocene to Holocene consists of gravels, sands, clays, vegetation rafts and peat swamps.Whilst, the oldest formation of study area is Minas Formation that consists of gravels, distribution of pebbles, sands and clays (Clarke et al., 1982).rivers that considered as fluvial meandering system.Meandering river lead to high erosional process at outer bank of channel and high depositional process at inner bank of channel (Yuskar et al., 2018;Yuskar and Choanji, 2017, 2016a, 2016b).
Based on the study area consist of two types of aquifer , Productive aquifer with wide spreading and intermediate productive aquifer with wide spreading.The maps of hydrogeological can be seen at Fig 2.

Methodology
Vertical Electrical Sounding (VES) is a method that involve in electrical resistivity survey.Basically electrical resistivity survey is measuring potential between two electrodes while transmitting a direct current between another two electrodes.The penetration of current is parallel with the distance between electrodes.The various of spacing between electrode will lead to information of resistivity stratification (Hamzah et al., 2007).VES survey was carried out using GEOCIS that consists of Geocis machine, electrodes, cable, connector, cable clip, battery and laptop (Fig. 3).Four electrodes arranged in a straight line with various electrode spacing.The maximum length of wire to connect four electrodes is 200 m while the minimum is 2 m.Four electrodes arrange using Schlumberger array where two outer electrodes as current electrode and two inner electrodes as potential electrode (Koefoed).The current electrode spacing (AB) ranging from 2 m to 200 m (AB/2 = 1 m to 100 m).The distance used for potential electrodes (MN) ranged from 0,5 m sampai 10 m. the data acquisition at field was carried out by changing electrode spacing between each measurement (Hamzah et al., 2007).

Discussion
Water saturated or aquifer layer is the target of this study.The resistivity value of aquifer layer is associated with low resistivity value.From the result of VES1, VES2, VES3 and VES4 shown the resistivity value of study area ranging from 0 resistivity value (Table 1).Furthermore, water saturated layer of study area divided into two types they are confined aquifer and unconfined aquifer.Unconfined aquifer layer detected at VES2 and VES3 with depth reach 3 m while confined aquifer layer found at all VES point with various depth starting at 3 m to 7 m.Correlation between VES2 and VES3 showing the distribution of unconfined aquifer at 2,3 m to 3 m of depth.Both of VES located at high elevation around 56 m to 60 m. the unconfined aquifer proved by there are some wells around study area.The groundwater level of the well is very near with surface just around 5 cm.meanwhile, confined aquifer of study area has various of depth.Based on those depth a map of distribution confined aquifer was generated (Fig. 7).The confined aquifer of study area is getting shallower to western part of study area with the deepest aquifer found at 7 m.Exploration of groundwater as alternative water resources using Vertical Electrical Sounding (VES) at Rumbio Jaya, Ukui, Riau has been conducted.The resistivity of study area ranging from 0,34 7 . the groundwater represented by very low resistivity value ranging from 0,34 -1 . Aquifer layer at study area divided into two types which are confined aquifer and unconfined aquifer.Confined aquifer is the target of this study can be found at depth 3 7 m. the deepest aquifer is 7 m that found at eastern part of study area (VES3) and gradually getting shallow toward western part of study area.

Figure 1 .
Figure 1.Map of study area

Figure 3 .
Figure 3. Equipment of Vertical Electrical Sounding (VES)The data collected from the field are current (I) and Voltage (V).Resistance (R) value can be calculate with equation between current (I) and voltage (V): = /To calculate the apparent resistivity (ρa), geometry factor (k) is needed.Geometry factor (k) for schlumberger can be calculate with formula: = ( 2 −  2 /4) From resistance (R) value and geometry factor (k), apparent resistivity (ρa) calculate with formula:  = .

Figure 5 .
Figure 5. Field data acquisition using GEOCIS equipment Unconfined aquifer layer defined by low resistivity surface.Below unconfined aquifer layer interpreted as clay layer with thickness approximate 4,6 m bounded from depth 2,3 at upperpart and 6,9 at bottom part.detected from VES3 is very low resistivity value (0,51 the layer covered by clay layer above.The thickness of confined aquifer layer cannot be determined because the layer below not detected.4.4 VES4maximum depth of VES4 is more than 5 m with resistivity value ranging from 0,9 divided into two layers which are clay layer and aquifer layer.The clay layer represented by low resistivity value with range 1,4 3,4 m starting from surface up to 3,4 m of depth.Clay layer act as cap layer for below layer because clay is impermeable that prevent water to infiltrate to subsurface.Beneath the clay layer is the layer with low water saturated layer with unknown the thickness of layer because the layer below is not detected.This water saturated layer classified into confined aquifer because the layer above is clay layer.

Table 1 .
Resistivity value interpretation of study area