Structural Control on Interpreted Subsurface River Flow Patterns in a Karst Landscape: A Case Study of the Bekah Area, Purwosari, Gunungkidul, Yogyakarta

Abstract

The Bekah karst area in Purwosari, Gunungkidul, Yogyakarta, is part of the Pegunungan Sewu karst landscape, where groundwater availability is closely related to subsurface river systems. This study investigates structural control on interpreted subsurface river-flow patterns using integrated geological, geomorphological, and Very Low Frequency Electromagnetic (VLF-EM) approaches. Field mapping was conducted to identify lithology, karst landforms, faults, joints, fractures, lineaments, and cave-related features. VLF-EM data were acquired along six west–east survey lines, with line lengths of approximately 540–800 m and a station spacing of 20 m. The data were processed into equivalent current density (ECD) sections to detect conductive subsurface anomalies. The results show that the dominant structural trend is northeast–southwest, corresponding with valley orientation, lineament patterns, and several high-conductivity anomalies. These anomalies occur mainly at approximate depths of 20–100 m, with a deeper response reaching about 140 m along Line 4. The anomalies are interpreted as potential water-bearing zones, saturated fractures, or structurally controlled subsurface drainage pathways. The findings indicate that subsurface drainage in the Bekah karst area is likely influenced by fault and fracture systems rather than lithology alone.