UTILIZATION OF MAGNETIC ANOMALY DATA TO LOCALIZE GROUNDWATER FRACTURES IN VOLCANIC ROCK COMPLEXES IN THE SITU TIRTA MARTA AREA, PURBALINGGA REGENCY, CENTRAL JAVA, INDONESIA

Abstract

Magnetic surveying is one of the geophysical exploration techniques which can be applied to investigate subsurface conditions including groundwater by utilizing the magnetic susceptibility properties of rocks. The study purposes to localize groundwater fractures in the volcanic rock complexes of the Situ Tirta Marta Kutasari area of Purbalingga Regency based on magnetic anomaly data analysis. The research area is an area below the groundwater recharge area for the Purwokerto-Purbalingga Groundwater Basin located on the slopes of Slamet Volcano. The study began with the acquisition of total magnetic intensity data at 256 predetermined location points. The spatial distance between data points was made closer (i.e. 100 m) with a regular pattern forming a grid. The processing of total magnetic intensity data was carried out in several stages including some corrections and reductions so that local magnetic anomalies data were obtained with values ranging from -1408.16 - 775.95 nT. Furthermore, the local magnetic anomalies data were reduced to the pole so that the values obtained range from -2022.48 - 1522.89 nT. First Horizontal Derivative (FHD) analysis of local magnetic anomalies data that have been reduced to the pole shows a maximum horizontal gradient patterns interpreted as the boundaries of lithological contacts in the form of fractures within or between volcanic rock complexes. The fractures in the volcanic rocks are estimated to be groundwater flow paths originating from the recharge area to the discharge area, including the groundwater manifestation area in the Situ Marta complex, Purbalingga Regency.