Fakultas Teknik, Universitas Negeri Padang 4 th International Conference on Technical and Vocation Education and Training Padang: November 9-11, 2017 Optimize of Least-Square Inverse Constrain Method of Geoelectrical Resistivity Wenner-Schlumberger for Investigation Rock Structures in Malalak Districts of Agam West Sumatra Akmam 1 , Amir Harman 2 , Putra, Amali 3 Jurusan Fisika FMIPA, Universitas Negeri Padang, Indonesia Fakultas Teknik, Universitas Negeri Padang, Padang, Indonesia ABSTRACK: Numerous studies have been conducted on an inversion method, focus on constraining factor, singuler value, speed of convergence. However, the result of inversion is not unique and bivalent. In this research, we optimize of Least-Square constrain by using damping factor. This method used for interpretation of the volumes and rock structure in Malalak Distric of Agam West Sumatra. This is undertaken because Malalak districts of Agam West Sumatra that passed by highway Padang and Bukittingi is a frequent area of landslide. Furthermore, the frequency of the landslide depends on the type of rock and the angle of the slope. The depth of the slide surface can be predicted by using the least squares inversion constrain method of Geoelectric Resistivity. Landslides resulted in disruption of transportation between the city of Padang and another district in Sumatra. Based on the above, to determine the rock's structure, the depth and tilt angle of the slide surface in Malalak districts Agam West Sumatra has to take place. Data obtained through Geoelectrical exploration using with automatic resistivity meter equipment. Constrains were obtained using the Marquat inversion method. The result of the research is first, the damping factor for structures which have wide range resistivity is 0.02 and the smallest damping factor is 0.015. Second, the rock structure in Malalak of Agam consists of clay, sandstone, andesite, and limestone and dolomite.. Implementation this research can be used to develop mitigation of landslide deserter. Keywords: Investigation, Slide surface, Geoelectrical Resistivity, Least-Square Inverse, Constraint 1. INTRODUCTION The Geolectrical resistivity method is one of the oldest geophysical techniques which is intensively used for the investigation of the deep and shallow structure of the subsurface. By introducing the electrical current directly into the ground through a pair of current electrodes, the difference of the resulting voltage can be measured between the other pair of potential electrodes. The apparent resistivity of the subsurface can be calculated in this way in order to get the resistivity variation with depth. The depth of the penetration depends on the distance between the current electrodes. Increasing the depth of the penetration can be carried out by enlarging the distance between the current electrodes from a small distance in the beginning to larger distances at the end of the array. This problem needs to get attention and scientific studies to avoid landslide recurrence. If repeated how the distribution of rock point locations that have the potential of landslides. The research can be used for landslide mitigation study in West Sumatera. The boundary between an avalanche material and the hard rock beneath which acts as a base is called the slip plane. The soft layer acts as a landslide material. Avalanche material is characterized by low resistivity value and landslide fields characterized by high resistivity material [1]. Electrical slip is characterized by the presence of two soil layers of highly contrasting resistance values [2], [8],[4],[5]). The slip field usually consists of low permeability and solid rock. Assistance in the field of type resilient slip (200- structures can be known based on the resistivity of these rocks. In general, the slip surface has the following characteristics: first, the existence of the plating of the rock such as the surface of contact between the ground cover and the bedrock. Second, the presence of contact fields between rocks cracked with strong rocks. Third, the existence of contact fields between rocks that can pass water with rocks that can not pass water (impermeable). The depth of the slide surface which is the boundary between the moving and the fixed mass of the soil surface is essential for the description of avalanches [6]. The depth of a plane is useful to know how big the risk of landslide that occurred. Thus, the active landslide always moves on the plane at all times or throughout the season, while the old landslide can re-activate as long as there are trigger factors for landslides. The sliding surface is formed by the saturation of the water that accumulates and moves laterally above the surface of the soil layer or the rock that is difficult to penetrate with water called the waterproof layer [7]. If water penetrates to a waterproof layer, then the waterproof surface of the waterproof layer will decay, thus becoming slippery. This slippery layer is called the slide surface. Layers that located over the plane of the slip will move along the slope and out the slope. As a result, excessive volume of water will cause soil or rock instability on the slope. The study used geoelectrical method to map the landslide potential areas that many researchers akmam_db@fmipa.unp.ac.id