A Mutually Constrained Geophysical– Photogrammetric Workflow for Deep Groundwater Exploration in Karst Terrain, Tuban, Indonesia

Abstract

Groundwater availability is a major issue in Klakeh Village, Bangilan Subdistrict, Tuban Regency, East Java, where karst topography leaves shallow groundwater scarce and seasonally unreliable. This study identifies deep groundwater potential by integrating passive audio-magnetotelluric (ADMT) sounding with UAV photogrammetry. The two techniques are applied as a mutually constraining rather than parallel workflow: the UAV orthomosaic supplies the terrain used to position the sounding transects and screens resistivity anomalies against surface morphology, while the resistivity models are converted into georeferenced drilling coordinates. Sixty-five stations were recorded at 5 m spacing along five intersecting transects of 50–100 m, each modelled from 10 m to 300 m depth. Two aquifer types were identified: an unconfined shallow aquifer at 8–20 m, vulnerable to seasonal fluctuation and surface contamination, and confined deep aquifers between 95 and 272 m. The most prospective zone lies below 195 m and is confirmed at three independent crossing points of transects A–B, C–D and E–F, where it returns the lowest resistivity values in the dataset together with the greatest thickness and lateral continuity. Two drilling points are recommended, at 111.6866186° E / 6.9757246° S and 111.6866167° E / 6.9758148° S, targeting 210–270 m. As no borehole log was available, these are prioritised targets rather than verified aquifer positions. The workflow is replicable at the village and district scale, where conventional deep resistivity imaging and exploratory drilling are logistically and financially prohibitive.