HYDROSTRATIGRAPHIC MODELING OF THE PURWOKERTO-PURBALINGGA GROUNDWATER BASIN BASED ON GRAVITY AND RESISTIVITY DATA IN KALIMANAH DISTRICT, PURBALINGGA REGENCY

Abstract

The utilization of gravimetric satellite data for mapping gravity anomalies on the Earth’s surface has been carried out in the Purwokerto-Purbalingga Groundwater Basin area. The research has produced a 3D subsurface rock density model with values ranging from 1.20 – 4.80 g/cm³. The inversion modeling results showed that the shape of the discharge area of Purwokerto-Purbalingga Groundwater Basin resembled a bowl filled with alluvial sediment with density values ranging from 2.10 – 2.55 g/cm³ and a depth of more than 4 km. The results were justified using resistivity data, one of which was carried out in Kalimanah District, Purbalingga Regency. The aim of this research was to obtain a hydrostratigraphic model based on resistivity data so the aquifer layers can be easily identified. The obtained results were subsurface rock resistivity logs which spread across five sounding points with resistivity values ranging from 1.31 – 110.88 Wm. These sounding points were located at geographical positions 7°24’24.93” and 109°19’13.58” to 7°24’23.28” and 109°21’10.29”. The interpretation of resistivity logs has produced lithological logs at each point. Correlation of all lithology logs has resulted a hydrostratigraphic model of the research area consisting of five layers, which are topsoil (1.31 – 18.38 Wm); clay, silt, and sand (9.27 – 110.88 Wm); sand with variations in grain size (13.0 5 – 64.31 Wm); tuffaceous sandstone and breccia (28.32 – 77.28 Wm); as well as sand, tuff, and conglomerate (11.72 – 20.32 Wm). The second, third, and fifth layers are interpreted as the most potential aquifer layers.