In Mt. Salak, there are six volcanic facies divided by eruption time seen from geomorphology data analysis and to identified the subsurface layer DC Resistivity method is applied. Beside resistivity, geostatistical parameters also influence the result model interpretation, so for obtain best model correlation parameters such as tilting, surfacing, variogram, grid method, and logarithmic distribution is applied. Using 18 points of acquisition data subsurface model is produce and then section model made to describe vertical resistivity distribution then correlated with facies lithology model. Based on that, produce three facies resistivity type namely: 0 – 100 Ohm.m (Low Resistivity Value) Interpreted as pyroclastic material composed as tuff and breccia that lies under lava. 100 – 300 Ohm.m (Medium Resistivity Value) Interpreted as breccia lithology type. Harder that pyroclastic material due to by this product is avalanches of lava. And >300 Ohm.m (High Resistivity Value) Interpreted as lava lithology that lies at high elevation and the hardest lithology in this area. From the model, pyroclastic layer that is modeled found at low elevation and based on the direction it described as oldest facies layer, but at the bottom of this layer lies high resistivity value that unknown product. It can be Mt. Pangrango product due to at low elevation predicted as combine area product from product of Mt. Salak and Pangrango. High resistivity value show lava lithology and lava facies located in high elevation and medium resistivity describe breccia lithology as avalanche product of lava (youngest pyroclastic facies) and found at 500 – 100 meters msl.

ABSTRAK Airtanah sangat diperlukan dan meningkatnya jumlah penduduk akan menyebabkan pengurangan cadangan airtanah yang tersedia. Gunung Tangkubanparahu merupakan daerah endapan vulkanik yang memiliki potensi sistem akuifer airtanah yang baik. Tujuan penelitian ini adalah untuk mengetahui potensi airtanah dari sistem akuifer vulkanik berdasarkan pengamatan geologi dan pengukuran geolistrik 1D sebanyak 100 titik. Hasil penelitian menunjukkan empat kelompok nilai resistivitas yang merepresentasikan kondisi geologi bawah permukaan. Kelompok pertama mempunyai nilai 0-100 ohm.m, berupa kelompok batuan piroklastik yang tercampur tanah. Kelompok kedua mempunyai nilai resitivitas 101-250 ohm.m, berupa kelompok batuan perselingan antara pasir tufan dan tuf kasar. Kelompok ketiga mempunyai nilai 251-600 ohm.m, berupa kelompok batuan breksi. Kelompok keempat mempunyai nilai resistivitas >600 ohm.m, berupa kelompok batuan aliran lava. Sistem airtanah yang dapat diasumsikan sebagai akifer tersebar merata pada kedalaman 50 m, 75 m, dan 100 m berupa kelompok batuan tuf.

Administratively, the research area is located in around Cikopomayak Village, Jasinga, Bogor, West Java, Indonesia. Based on the Hydrogeological and Groundwater Basin Map, this research area was classified to non-potential groundwater basin. In addition, when the summer comes, the lack of water will be happened around the area. Consequently, the water reserve wasn't enough to fulfill the necessity of freshwater. Thus, the research about identifying aquifer location was needed. This research aims to determine the depth and thickness of aquifer for the alternative ways to overcome the problem, particularly, for the Cikopomayak Village. The research used the geophysical method by modeling the several data such as Geoelectric 1-D to know the aquifer location based on the section of medium resistivity value. Then, the geoelectric data was correlated to the borehole drilling data that is Gamma Ray (GR) Log, Spontaneous Potential (SP) Log, Resistivity Log, and the cutting data. Commonly, the most suitable lithology for an aquifer is sandstone because of its good characteristic of permeability and porosity. Logging data result will show the small value of GR log, SP Log leaning to the right, and low-value resistivity for indicating the aquifer zone. Based on geoelectric data, the research zone has 3 groups of resistivity value and lithology that are shaly claystone and siltstone with low-value resistivity (0 -50 Ω.m), sandy claystone, medium sandstone, and coarse sandstone with medium value resistivity (50 -150 Ω.m), fine sandstone, sandy claystone, and andesite with high-value resistivity (> 150Ω.m). The aquifer system in the research area was classified into 3 groups that are aquifer, aquitard, and aquiqlud.

Andesitic volcanic hydrosystems in Indonesia are mostly hydrogeologically unknown despite their socio-economic importance. The development of robust and easy-to-implement methodologies to conceptualize and quantify the water cycle components becomes a prerequisite for their sustainable management.We developed a lumped hydrological model to mimic the structure and functioning of a previously unknown hydrosystem located on the flanks of the Salak volcano (West Java). The structure of the aquifers was revealed with electrical resistivity tomography. The distinction between springs fed by the extensive artesian aquifer and others fed by shallow perched aquifers was obtained mostly using hydrochemistry. The elevation of the recharge area was identified using isotopic analysis of spring water.After designing the hydrological model structure, we carried out a probabilistic parameters exploration using the multiple-try differential evolution adaptive Metropolis algorithm to calibrate aquifer discharge. Multiple Markov chains allow a better exploration of the parameter values. The Bayesian approach provides the best water cycle simulation with a parameter uncertainty analysis, improving the accuracy and representation of the water cycle appropriate for previously unknown hydrosystems.

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