SUBSURFACE INVESTIGATION OF LO AND BANGI CAVES USING THE VERY LOW FREQUENCY (VLF) METHOD FOR GEOTOURISM DEVELOPMENT

The existence of Lo Cave and Bangi Cave area, which are close to each other in the karst area of Kedungsalam Village, Donomulyo District, Malang Regency, has excellent potential to be developed as a geotourism destination that supports nature conservation and economic empowerment of the local community. However, until now, there is no adequate scientific data regarding the structure and direction of the cave system in the area. This study aims to determine the pattern and direction of the cave system as a basis for developing geotourism in the Lo Cave and Bangi Cave area. Data acquisition was conducted using the Very Low Frequency (VLF) geophysical method on ten measurement lines with 10 m intervals, which were adjusted to the topographic conditions and karst features in the field. The transmitter used the NWC station located in North West Cape, Australia, with a frequency of 19.800 Hz. In data processing, Baker and Myers' corrections were used to reduce the influence of topography on the data. The Fraser filter is used to make the anomaly position more visible and easier to interpret, and the Karous-Hjelt filter visualises the rock conductivity pattern against depth. VLF data processing results showed consistent conductive anomalies on all lines, indicating the presence of cavities and underground water flow paths. 3D modelling revealed that the cavity system extends from Lo Cave to the northwest, then branches at Line 3. The first branch heads southeast across Lines 6, 2, 1, and 9, while the second branch heads north until it reaches Bangi Cave. These findings provide information about the subsurface patterns and structures around Lo Cave and Bangi Cave as a scientific basis that can be used for the development of safe and sustainable geotourism.