Groundwater exploration using integrated geophysics method in hard rock terrains in Mount Betung Western Bandar Lampung, Indonesia

The presence of hard rock in Mount Betung has caused the misalignment of the groundwater aquifers, and resulted in many drilling failures for groundwater. An integrated geophysics method using gravity survey and Geoelectric Vertical Electrical Soundings (VES) were conducted to study the effect of basement and hard rock on groundwater prospects. From the gravity method, 38 mapping points were carried out randomly, with a distance of 1-2 km in-between. Meanwhile, from the geoelectric method, 51 VES points were acquired at the foot of Mount Betung. The acquisition was conducted with a Schlumberger configuration with AB/2 = 1 m to 250 m. The results show the Bouguer Anomaly in the west is 50-68 mgal due to the presence of hard rock in Mount Betung. This anomaly responds to relatively shallow hard rocks near surface. Hard rocks composed of andesite and breccia normally present at the depth of 5-180 m during well construction. Resistivity isopach mapping from VES data (at AB/2 = 50 m, 100 m, and 150 m) shows the dominant constituents of hard rock. Fractures in hard rock contribute to secondary porosity, which could be a prospect zone that transmit groundwater. This finding shows that the fractures are randomly scattered, causing several well failures that have been worked. Furthermore, the fractures in the hard rock at the foot of Mount Betung acts as conduits between recharge at Mount Betung and the aquifer in the Bandar Lampung Basin.