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ABSTRACT

Land movement occurs due to reduced soil strength which is affected by water pressure. The Self Potential (SP) method is a geoelectric method that can detect well the flow of groundwater. The Self Potential method can also be used to detect the presence of debris flow  in the form of weathered soil, tuff sand, cobble gravel and other rock fragments that have mixed with water. Based on the results of the SP measurement, debris flow has a medium SP with a value (3 - 12) mV, rock that has shifted (sandstone associated with andesite fragment) has a SP value <  2mV, while water seepage and water flow has an SP value (13 - 36) mV.

Landslide is a geological event that occurs because the movement of slope-forming material in the form of soil, rock or combination of material type to a lower place due to the influence of gravity. The trigger factor of landslide in Pasanggrahan, South Sumedang, Indonesia is the increase of water content in the slope and the slip plane. The slip plane began to actively to turn on when the rainy season arrives. The infiltration of rainwater into slopes as an avalanche trigger can be detected by Self Potential (SP) method. SP measurements were performed to determine changes in subsurface water flow. SP data acquisition using fixed base technique and leap frog technique. The rocks in the research area that serve as the slip plane are clays associated with Volcanic Breccia. One of the geophysical methods that can identify the slip plane is the Electrical Resistivity Tomography Method. The ERT method can detect slip plane based on rock resistivity. Based on the local geology that has been correlated with the value of Self Potential and Electrical Resistivity Tomography, the following results are obtained: water table : (1 - 13) Ohm.m and SP value : (16 - 50) mV, clay : (72 - 100) Ohm.m and volcanic breccia: (171 - 550) Ohm.m.

Abstrak. Penelitian gerakan tanah dan akuifer telah dilakukan disekitar lereng Bandung Giri Gahana (BGG), Jatinangor, pada bulan Juni -November 2017. Faktor pemicu terjadinya gerakan tanah yaitu adanya batuan yang dapat berfungsi sebagai bidang gelincir serta lapisan pembawa air (akuifer). Bidang gelincir adalah bidang yang berada pada lapisan yang stabil dan bersifat kedap air (impermeable). Akuifer adalah lapisan batuan yang permeable baik yang terkonsolidasi atau yang tidak terkonsolidasi dengan kondisi jenuh air. Metode Geofisika yang dapat mengidentifikasi bidang gelincir dan akuifer yaitu Metode Geolistrik Electrical Resistivity Tomography (ERT). Berdasarkan hasil pengukuran ERT, bidang gelincir terdapat pada lempung pasiran dengan nilai resistivitas : (50 -78) Ohm.m, sedangkan bidang geser yang menyebabkan pergerakan tanah terdapat pada bahan rombakan dengan nilai resistivitas : (79 -660) Ohm.m. Berdasarkan hasil pengukuran ERT, gerakan tanah yang terdapat dilokasi penelitian pergerakannya lambat sehingga dikategorikan ke dalam tipe rayapan. Lapisan pasir (akuifer) yang terdapat dibawah bidang geser memiliki resistivitas rendah dengan nilai : (9 -25) Ohm.m. Kata kunci : akuifer, bidang gelincir, bidang geser, ERTAbstract. Land movement and aquifer studies have been conducted around the slopes of Bandung Giri Gahana (BGG), Jatinangor, from June to November 2017. The triggering factor of the land movement is the presence of rocks that can serve as a slip plane and the water carrier layer (aquifer). The slip plane is a plane that is in a stable layer and is impermeable. Aquifers are permeable rock layers either consolidated or unconsolidated with water-saturated conditions. Geophysical methods that can identify the slip plane and aquifer that is Electrical Resistivity Tomography (ERT) Method. Based on ERT measurement results, the slip plane area is found on the sandy clay with resistivity value: (50 -78) Ohm.m, whereas the shear plane causing the movement of the soil is present in the revamp material with resistivity value: (79 -660) Ohm.m. Based on the ERT measurement results, the movement of the soil in the location of the study is moving slowly so it is categorized into the type of creep. The sand layer (aquifer) contained below the shear plane has a low resistivity with values: (9 -25) Ohm.m.