Identification of Carbonate Freshwater in Perak Island, Kepulauan Seribu Using Ground Penetrating Radar Method

Abstract

A research has been carried out to identify freshwater and its distribution in Perak Island, Kepulauan Seribu using Ground-penetrating Radar (GPR) method. This research location is known for an isolated carbonate platform located on the equatorial belt. A freshwater well was found in the middle of Perak Island and used for daily needs for residents and tourists. Ground-penetrating radar (GPR) provided a promising approach to investigate the extent of the freshwater lens or aquifer volume, and to gain detailed information about the geological and hydrogeological features of the aquifer. It can give a high determination image of the dielectric properties of the features from a few tens of meters on the surface. This research purpose is to detect the presence of subsurface freshwater accumulation and the reservoir. By performing GPR measurements at a frequency of 100 MHz and stacking number 32 to increase signal to noise ratio, the subsurface results depict the depth of the well as far as 20 m, while the target is located at 2 - 5 m below the surface. Analyzing the results of data processing, it is clear that there is a fluid boundary contrast that indicates the presence of fresh-water. The result indicates the barrier zone for separating saline water and groundwater at depth of 2.5 – 3.75 m. The section result also describes the presence of vertical barriers formed by secondary fracture to prevent seawater intrusion to the reservoir.