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. Th is 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 resistivitymeter 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.

West Sumatra has wavy and steep topography. Beside, West Sumatra has topographic slope more than 30, it also has high rainfall. This condition makes this area has a high potential of landslide risk. This problem needs to get a scientific study of how the surface characteristics of the landslide field as a preliminary study of landslide mitigation in West Sumatera. Information on the characteristics of landslide surfaces in landslide prone areas in West Sumatra has not been widely available. Based on the above reason, research has been done to find out the depth and slope of the landslide surface in Padang and Agam Regency of West Sumatra. This explorative research uses resistivity method using Ares equipment set (Cheko production). The apparent resistivity data is interpreted using the optimized optimized least-squares smoothnessconstrain inversion method in order to obtain true resistivity 2D cross-section. The layer thickness and the slope of landslide surface the targeted area was calculated base on true resistivity 2D cross-section that synchronized with topographic data. The results showed that the average both depth and angle of the landslide surfaces in Agam regency of West Sumatra were 41degrees and 16.5 meters respectively. The average both depth and angle of the landslide surfaces in Padang were 38 degree and 12.5 meters respectively. The rock that acts as a landslide surfaces is Clay