Sismanto Geophysics Laboratory, Department of Physics, Faculty of Mathematics and Natural Science University of Gadjah Mada, Indonesia

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 6, June 2018, pp , Article ID: IJCIET_09_06_129 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed LANDSLIDE POTENTIAL MAPPING IN PENGGUNG PURWOSARI VILLAGE, DISTRICT GIRIMULYO, KULONPROGO, YOGYAKARTA PROVINCE, INDONESIA USING DIPOLE- DIPOLE RESISTIVITY METHOD Sismanto Geophysics Laboratory, Department of Physics, Faculty of Mathematics and Natural Science University of Gadjah Mada, Indonesia Nasharuddin Department of Physics, Faculty of Mathematics and natural Science University of Gadjah Mada, Indonesia ABSTRACT A Geophysical survey using resistivity method with Schlumberger and Dipoledipole configuration had been used to determine subsurface landslide area at Penggung Purwosari Village area of Girimulyo, Kulonprogo Yogyakarta in Indonesia. This study aims to determine the structure of the subsurface layer using 1D and 2D resistivity methods, as well as to know the characteristics of slip-surface and type of ground movement that would occur. The result of resistivity data processing shows that the variation of subsurface resistivity value consists of three layers. The first layer is characterized by a resistivity value of <9Ωm interpreted as soil saturated with water in the form of surface soil, the second layer is characterized by a resistivity value of 9-36Ωm is interpreted as a water-unsaturated soil layer, and the third layer is characterized by a resistivity value of 36-70Ωm interpreted as a base layer (bedrock) of andesitic-breccia. The soil and weathering layer can be accumulated above a curved bedrock interpreted as a layer that has a possibility to create slippage plane and produced a rotational slide and has the volume about 790,612m 3. Areas with high landslide vulnerability could be at north and north-west of the study area. Keyword: Dipole-dipole, Resistivity, Landslide, Yogyakarta. Cite this Article: Sismanto and Nasharuddin, Landslide Potential Mapping in Penggung Purwosari Village, District Girimulyo, Kulonprogo, Yogyakarta Province, Indonesia Using Dipole-Dipole Resistivity Method, International Journal of Civil Engineering and Technology, 9(6), 2018, pp editor@iaeme.com

2 Landslide Potential Mapping in Penggung Purwosari Village, District Girimulyo, Kulonprogo, Yogyakarta Province, Indonesia Using Dipole-Dipole Resistivity Method 1. INTRODUCTION The study area is in Penggung, Purwosari Village area of Girimulyo, Kulonprogo District in Yogyakarta Province (Figure 1), Indonesia. Yogyakarta Province is located in south side of central Java. The study area is a hilly and densely enough populated area. They live on a steep hillside slope with a very limited area. If there is enough flat area used, it will use as a plantation or rice field. Their income comes largely from those farms and plantations, but high risk of seismic hazard and land mass movement disaster. Therefore, this area is very potential for a landslide, mass movement and avalanches, especially in rainy session or earthquake occurs [1]. Landslide is a process of natural degradation of the earth's surface material, occurs when there is a mass movement of rocks, debris, or soil material that slide along a certain slope. In the rainy season, changes in surface stress in the soil pore and the increase in soil mass weight due to water seeping into the soil can trigger mass transfer (gravitational instability). Physically, the factors that influence landslide events are the additional force of soil or rock mass, the decreasing of the binding force between the soil and the rock, and the increasing slope. The most influential causes of landslides are the slip surface or the shear plane. Knowledge of subsurface structures is required in predicting the level of landslides in an area. The study of physical characteristics of subsurface structures can be done by utilizing geophysical methods. The geophysical method applies the principles of physics in studying the subsurface shape based on the physical properties of rocks. The geoelectric method is one of the geophysical methods that can be applied in the mapping of the subsurface structure; this method utilizes the nature of rock resistivity to study the condition of the subsurface. This method has been applied to landslide investigations since the late 1970s [2]. The resistivity method can illustrate the condition of the structure, the sliding plane, and the type of landslide that will occur via resistivity contrast which indicates a difference in texture and porosity. The one-dimensional resistivity method (1D) is used to describe the distribution of vertical resistivity values (sounding) and the two-dimensional method (2D) is used to describe the distribution of subsurface resistivity values laterally (mapping) [3]. Thus, the shape of the structure, as well as the thickness of the soil, is more easily illustrated by contrasting measured resistivity values [4]. The use of the 2D resistivity method is appropriate for landslide studies as it may produce sub-lateral spatial images laterally with better resolution than the 1D resistivity method. However, the 2D resistivity method in general needs to be supported by the 1D resistivity method to see deeper subsurface conditions. To mapped the bedrock elevation can be done by using microtremor survey [5], Georadar survey [6, 7] and 2D geoelectrical survey [4,7,8] as the main parameter to determine the potential sliding plane. For geostrategic landslide hazard mitigation from airborne mapping to ground monitoring had been formulated by Supper [9]. To identify properties and Geology Age of Landslides Hazards of Volcanic Soil Characteristics is the important as well [10]. A landslide study was conducted in the Kulonprogo Mountains located in an ancient volcano complex located above Paleogenic rocks covered by Neogen-age carbonate rocks. Rahardjo [11] stated that the Kulonprogo Mountains have experienced several tectonic and volcanic phases editor@iaeme.com

3 Sismanto and Nasharuddin Figure 1 The study area is in Penggung, Purwosari Village area of Girimulyo, Kulonprogo District (insert), Yogyakarta Province that is located in south side of central Java. First, it occurs after the formation of Nanggulan Formation i.e. Oligocene-Miocene. Tectonic activity reached its peak, marked by the elevation of the Old Andesite Formation. The existence of North-South-to-Northeast-southwest direction of the time resulted in the formation of horizontal fault, folding and fracture formation on the Old Andesite Formation which was later filled by andesitic, dacites, and quartz veins. As a result, it formed Idjo volcano in the south, Mount Gadjah in the middle, and Mount Menoreh on the north side which is the core of the Kulonprogo Mountain dome. The three volcanoes produce fragmental fractional volcanic rocks, outer igneous rocks of lava and shallow intrusive rocks that make up the Old Andesite Formation. Second, at the beginning of the final Miocene, there is a decrease resulting in inundation. At that time, the Jonggrangan Formation and Sentolo Formation were intercepted, lying unconformity above the Old Andesite Formation. Third, occurs at the beginning of the Pleistocene, the Kulonprogo areas undergoing uplift to form a morphology of hills and a few folds forming a dome extending towards the Northeast- Southwest as far as 32km and extending towards the West-Southeast Sea as far as km (Figure 2). This appointment process also causes the tilting of layer and radial pattern fault [12]. Figure 2 The landscape view of Oblong Dome as the remains of the ancient volcanoes of Idjo, Gadjah, and Menoreh [12]. This study aims to determine the structure of the subsurface layer using 1D and 2D resistivity methods, as well as to know the characteristics of slip-surface and type of ground movement that would occur. The-Slip surface identification in vulnerable zones uses geoelectricity or resistivity method. These methods produce subsurface lithology based on resistivity value to identify slip surface position and possible mass movement type. By using Rockwork software in 3D, it can be determined the volume of mass that is potentially sliding if the conditions of the landslide are completed editor@iaeme.com

4 Landslide Potential Mapping in Penggung Purwosari Village, District Girimulyo, Kulonprogo, Yogyakarta Province, Indonesia Using Dipole-Dipole Resistivity Method 2. EXPERIMENTAL PROCEDURES [FIELD WORK] The study was conducted on March 2017 in Penggung Village Purwosari Village geographically located at '14,88" '27,64 '' BT and 7 43'45,12" -7 43'58, 21 "LS. The research location maps and measurement designs are equipped with information on landslide events are shown in Figure 3. The study was conducted on paddy fields with lines and points of measurement centered on one location. The 1D measurement points are symbolized by a blue circle of 3 points (1, 2, and 3), the total length (electrode spacing A-B) is 500 m. This extended will reach depth is about 80 m below the surface. The position and direction of the 2D lines are 6 lines symbolized by a black line. The line of the A-A ', B-B', C- C ', and D-D ' are spread out of South to Northward along 220m, while the line E-E 'and F-F' are extended from west to east as long as 300 m. Figure 3 Map of the Schlumberger configuration survey in blue dot (3 points) and dipole-dipole configuration lines (A-A ; B-B ; C-C ; D-D ; E-E ; and F-F ), and landslide area information that usually were occurred (red star). The 1D resistivity method is carried out using the Schlumberger configuration, as shown in Figure 3. A middle point becomes the center of the layout of two potential electrodes and two current electrodes. The potential electrode distance b is 0.2 m up to 12 m and current electrode distance a is 1 m up to 250 m. The 2D resistivity method is carried out using the Dipole-dipole configuration. Two current AB electrodes and two potential MN electrodes have the same spacing (a) of 10m, then both pairs of current and potential electrodes moving step by step are separated as far as 1 n 6 [13]. The horizontal position of the apparent resistivity value of the measurement is defined at the midpoint of the electrode arrangement used in the measurement. While the vertical positions show as a distance proportional to the distance of the electrode spacing. The measured depth determination is based on the maximum electrode spacing (a), the spacing factor between the electrode pair n, the maximum length of the expanse (x), and the depth factor [14]. The target of depth to be achieved in dipole-dipole configuration can be known based on depth determination. For example, the value of a = 10 m, n = 6, x = 35 m, the depth is i.e., about 7.56 m. The processing 2D data are used Res2Dinv software by Loke [15]. 3. RESULT AND DISCUSSION 3.1. Analysis line A-A Figure 4 is the 2D resistivity cross-sectional model line A-A' showing a material having a resistivity range of less than 9 Ωm marked in light blue to marine blue as a soil saturated layer of water with a thickness of more than m. The runoff water pass through the pores of the soil is easily fulfilled when there is sufficient surface water. The saturated soil layer has a editor@iaeme.com

5 Sismanto and Nasharuddin depth of ±9 m at distance between m and m that are located between unsaturated soil of water. At a distance of m, there is a saturated soil of water over unsaturated soil, so the water will accumulate in large quantities When the water breaks through the boundaries between layers, the soil that is not unsaturated has been being the slippage potential area. Soils that are not saturated with a concave form can be a landslide rotation. There are a number of andesitic rock on the surface of the line A-A at a distance of m, the chunk will be a part of the subsurface andesitics-breccia as a bedrock. The existence of andesites has two possibilities. First, it is estimated by lifting, when there is a greater separation of the rock body. Second, the result of volcanic eruptions carrying igneous rock (andesitic-breccia) to the present position. Layers of materials that have a resistivity range of 9 Ωm to 36 Ωm are marked light green to yellow in the form of weathered andesitic (unsaturated breccia). The material spreads out along the line from a distance of m and fills the gap between bedrocks. A layer of material having a resistivity value of 36.0 Ωm to 70.0 Ωm is characterized by light brown to deep purple as interpreted as bedrock (andesitic-breccia) with a large body 100 m long. The influence of tectonic and volcanic forces that ever occurred resulted in the rocks being separated into smaller than before, the separate rock has an empty space and then inserted material between separate rocks. The third layer at a distance of m has the same resistivity as the separated andesitic layer, where this layer has two possibilities. First, the decomposing of the base layer then transported to the place at this time. Second, the layer is the smoothness of the body of the base layer with a large dimension Analysis line B-B The 2D resistivity cross-sectional model (Figure 5), shows that the line B-B' material has the same layer as the line A-A. The outcrop material, tuff is located about 2 m from the line, indicate a saturated soil of water. At a distance of m, there are a number of rocks andesitic and a number of ground fractures with the width of cm at a distance of m. The fracture on the line subsides at some point, an open fracture point speeding the process of water into the deeper layers. The water-unsaturated layer located at a depth of up to 15 m can be a plane of slippage when water comes to the surface between two layers. A landslide can occur in blocks of distance from m above a layer of soil that is not saturated in a concave shape with a type of landslide in the form of a rotational slide Analysis line C-C The 2D resistivity cross-sectional model in Figure 6, shows that the line C-C ' material has the same covering materials as line B-B. The water-unsaturated layer in the second layer has the potential to become a plane when the water is able to penetrate the border between the two layers. Soil unsaturated water-shaped concave in the second layer allows the occurrence of an avalanche in the form of a rotational slide. There are a number of igneous rocks on the surface at a distance of m, ground cracking and subsidence at a distance of m. The fracture and soil subsidence have a considerable avalanche potential, if soil saturated is located just below it undergoing movement following the soil unsaturated surface model which it becomes a slippage. The distance of 160 m there is subsidence about 7 m, the amount of water entering the soil layer saturated water will result a material movement Analysis line D-D The 2D resistivity cross-sectional model of Figure 7 shows that the line D-D material has three main materials. First layers of materials that have a resistivity of <9 Ωm are marked in light blue to marine blue is a soil saturated layer of water with a thickness of about 10 m. The editor@iaeme.com

6 Landslide Potential Mapping in Penggung Purwosari Village, District Girimulyo, Kulonprogo, Yogyakarta Province, Indonesia Using Dipole-Dipole Resistivity Method second layers of materials that have a resistivity of 9 Ωm to 36 Ωm are marked light green to yellow is an unsaturated layer at distance of m as the bedrock. The soil will move with the increase of water content in the subsurface layer and it is a potential avalanche block. The third layers having a resistivity e of 36 Ωm to 70 Ωm are marked in light brown to purple in the form of bedrock (andesitic-breccia), located at a distance of m at a depth of 9.25 m below the surface until 18.4 m, the high resistivity layer is a stable block. Subsidence that occurs at m along 20 m and at 160 m is perpendicular to the direction of the line. A material avalanche occurs 2 m in the line as along 10 m Analysis line E-E The 2D resistivity cross-sectional model of Figure 8 line E-E' have three main materials. The first material has resistivity range of <9 Ωm that is marked in light blue to dark blue as a soil saturated with water. The second material that has a resistivity range of 9 Ωm to 36 Ωm are marked light green to yellow as bedrocks, this bedrock material has changed in shape and position caused by various forces acting on the material. The third material layer with a resistivity value of 36 Ωm to70 Ωm is marked a light brown to dark purple is andesiticbreccia. The andesitic material moves from the origin position to fill in the empty space. The cross lines of A-A 'to D-D' and E-E ' shows the close correlation of resistivity values Analysis line F-F The 2D resistivity cross-sectional model (Figure 9) of line F-F consists of three materials. A First material having a resistivity range of <9 Ωm and the second material that has a resistivity range of 9 Ωm-36 Ωm is a bedrock and unsaturated soil. The last is the material with a resistivity value of 36 Ωm-70 Ωm is andesitic-breccia as bedrock. The third layer lies at a depth of more than m. Analysis of covering soil on the surface and physical properties of materials such as the ratio between the resistivity value and porosity indicates the material has a high porosity produce a low resistivity. In contrast, a high resistivity indicates that the porosity is low. So if the porosity is high it is then able to precipitate water in a large scale resulted, in addition increasing the mass the possibility of landslides occurs is high. Based on the inversion model, the curved in structure shows the characteristics type is a rotational slide. There are two types of materials that can be potentially the rotational slide type in the field. Figure 4 Resistivity cross-section model line A-A' with topography, interlayer boundaries, and surface information. a) lumps at a distance of 60 m, and b) of m. Figure 5 2D cross-sectional resistivity model of line B-B, the outcrop at a distance of 20 m is located ± 2 m from the line, the block is spread from m, and fractures at some point along the m editor@iaeme.com

7 Sismanto and Nasharuddin Figure 6 2D cross-sectional resistivity model of the line C-C, a number of bits at a distance of m, and fractures at several points along the m range, and subsidence ± 5 m on a track of 160 m. Figure 7 2D of line D-D resistivity model, the subsidence at a distance of m, landslide ± 2 m westward of the line at a distance of m, subsidence at a distance of 160 m, and landslide about 3 m eastern ward from the end of the line. Figure 8 The 2D resistivity cross-sectional model with E-E' Figure 9 The 2D resistivity cross-sectional model with F-F' 3.7. VES (Vertical Electrical Sounding) analysis The structure of the line A-A, the surface material is clarified by Schlumberger 1D configuration resistivity method. Figure 10 shows the resistivity results for three points (1, 2, and 3) in depth function, and the positions are shown in Figure 3. At point 1, the top layer has a resistivity value of Ωm is a road material with thickness of <1 m, then a second layer Ωm and a third layer Ωm are the surface soil (unsaturated soil) and the depth is > 1-19 m. The fourth layer has low resistivity of 6.30 Ωm as saturated water at m depth. The last layer has resistivity of Ωm is andesitic as a bedrock with depth more than 56 m. At point 2, the top layer has a resistivity of Ωm is a surface soil with a thickness of about 3m, then the second layer is andesitic has a higher resistivity than the first layer of Ωm until 31m below the surface. The resistivity value decreases in the third layer with a significant difference is about 9.99 Ωm that is a water-saturated layer and maybe as the aquifer till more than 31m. At point 3, the top layer has a resistivity of Ωm is a road material with a thickness of less than 1m.The second layer with 5.13 Ωm and third layer with 3.12 Ωm are as saturated soil by various in depth of 0.5 to 41 m. The fourth layer has a resistivity of Ωm is interpreted as bedrock until 41 m editor@iaeme.com

8 Landslide Potential Mapping in Penggung Purwosari Village, District Girimulyo, Kulonprogo, Yogyakarta Province, Indonesia Using Dipole-Dipole Resistivity Method Point 1 Point 2 Point 3 Figure 10 The subsurface resistivity of 1D at point 1, 2 and 3 resistivity measurements to clarify the lithology of line A-A ', B-B', and D-D ' (the unit; depth in m and the resistivity in Ωm) D Visualization The result of measurement and distribution of resistivity value on each line are combined to build a resistivity volume using Rockwork Software. Using topography data in the area the total volume covering base layer (bedrock), water-unsaturated layer, and a water-saturated layer is 1,905,836 m 3 (Figure 11). The distribution of resistivity values of 36Ωm-70Ωm marked light green to red has a volume of 425,264 m 3 of the base layer (the bedrock of andesitic-breccia) spread out East-West along the line E-E ' (Figure 12). The distribution of resistivity values of 9 Ωm-36 Ωm marked dark green to light blue has a volume of 689,960 m 3 is an unsaturated water layer (Figure 13). The distribution of resistivity values less than 9 Ωm marked dark blue to purple has a volume of 790,612 m 3 of the water-saturated layer or soil saturated with water. These soil in Figure 14 has a high potential to move as a rotational slide as landslide especially in rainy season and if any trigger such as an earthquake. Figure 11 The total 3D visualization volume of the combined line A-A to F-F. Figure 12 The bedrock of 3D visualization of the stable layer consists of andesitic-breccia editor@iaeme.com

9 Sismanto and Nasharuddin Figure 13 The material over bedrock is a soil unsaturated the water layer, the volume is about 689,960 m 3. Figure 14 The surface material is a soil saturated with the water layer, which has high landslide vulnerability especially in rainy season; the volume is about 790,612 m CONCLUSIONS 1. The result of the measurement of 1D and 2D resistivity methods is shown of three types of material structures. The first material layer is characterized by resistivity values <9 Ωm interpreted as water-saturated soil on the surface that easy to slide, the second layer is characterized by 9-36 Ωm resistivity value interpreted as an unsaturated soil of water that is not able to slide, and the third layer is characterized by Ωm resistivity values are interpreted as bedrock layer of andesitic-breccia. 2. Based on the subsurface structure of the material layering, material sliding and curved in andesitic-breccias are identified as potentially sliding plane with different depths will produce a rotational-land slide type. 3. North and northwest areas have the highest landslide vulnerability potential compared to other areas. ACKNOWLEDGMENTS Thanks to Department of Physics and Geophysics Laboratory Gadjah Mada University for the financial and Resistivity-meter support that had been used in this project with contract number: 0197/J /PL.06.02/ editor@iaeme.com

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