INTERPRETASI GEOLISTRIK DI AKIFER PANTAI PADA ZONA TAK TERKEKANG DI DESA USO, KECAMATAN LUWUK, KABUPATEN, BANGGAI, SUMATERA SELATAN

• Main Author: O S Doloksaribu, Togar
• Format: Theses
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/71481
• Institution: Institut Teknologi Bandung

Each rock has different resistivity value, depending on the mineralogy, porosity and fluid content (Telford, 1994). Knowledge on mineralogy, porosity and fluid composition is very useful in rock resistivity interpretation, such as data obtained from geoelectric measurement. In Uso Village, District of Luwuk, Banggai Residence, Province of Sulawesi Tengah, a resistivity measurement, vertical electrical survey (VES) with Wenner method, has been conducted at 31 nodes with maximum span of 900 m. From the interpretation of apparent resistivity, near the shore line (AB line) there is low resistivity rock (0 – 10 ?.m) which, based on regional geology, was interpreted as sandstone or conglomerate with saltwater or marl. The low resistivity rock was checked with 100 m drilling of Exploration Well near GL-4. From cutting analysis and groundwater chemistry data, it was found that the low resistivity rocks (0 – 10 ?.m) is neither sandstone or conglomerate with salt water nor marl, but intercalation of limestone and conglomeratic limestone with fresh water. Two sequences of limestone and conglomeratic limestone intercalation from the rock cutting analysis of Exploration Well show different resistivity responses. Limestone and conglomeratic limestone intercalation at 6,5 – 27 m (Sequence – A) has a rather high resistivity, according to a calculation using Barnes method (average of 38,51 ?.m); while limestone and conglomeratic limestone intercalation at 57,5 – 85,5 m (Sequence – B) has a low resistivity (average of 1,92 ?.m). To understand the factors which affect resistivity response on the two sequences, an analysis was conducted on 10 thin sections and salinity. Resistivity function of Sequence – A is: 2 3 4 12 1 y =1897,02 + 0x ? 3,49×10? x + 0x ?14,74x and of Sequence – B is: 3 4 13 1 2 y = ?67,06 + 0x + 0x +1,00×10? x + 0,53x . In both equations, the most dominant factor is the salinity of fluid within the rock (x4). Calcite content (x1) contribute none. Content of other minerals (x2) and porosity (x3) contribute none to negligible. From the Barnes interpretation, lithology of the research area is dominated by nine sequences of limestone. Furthermore, stratigraphic relations of each sequence is not simple. Based on the lithologic log from both Pilot Well and Exploration Well, as well as the Barnes lithologic interpretation, aquifer type (confined or unconfined) in the research area is not clear. To obtain the aquifer type, an analysis on pumping test result has been conducted and the result shows that the aquifer type in the research area is unconfined aquifer. iv Because the study area is located on a coastal region, sea water intrusion analysis was also conducted. Sea water intrusion interface in the study area shifts toward the sea as a result of groundwater flow (hydrodynamic condition) in northwest – southeast direction.