Estimation of Subsurface River Flow Using the Very Low-Frequency (VLF) and 2D Resistivity Methods of the Guwoterus Karst Area, Tuban

Karst is a landscape that has the characteristics of steep hills, dolina, many caves, and subsurface rivers. One of the problems in the karst area is determining the existence of an underground river which is a source of clean water for the community. Therefore, this research aims to estimate the underground rivers at Guwoterus Karst area, Tuban. This study used a Very Low-Frequency Electromagnetic (VLF-EM) method with 6 tracks and each track length of 200 meters, and 2D resistivity using Wenner configuration of 4 tracks with the length of 150 meters. In this research, 3 filtering was performed on data processing using the VLF-EM method, including the NA-MEMD, Fraser, and Karous-Hjelt filters. Identification of subsurface river flows has been done by correlating the VLF-EM and 2D resistivity methods through the inversion of the field observation data. The processing data shows that the depth of the subsurface river ranges from 1-21 meters with resistivity values from 20-100 ohm. The flow of the river leads from North-South following the topography of the research area and consists of limestone with intensive cracks. Based on the results of 2D resistivity processing, lithology assumptions were obtained at the research area. A resistivity of <100 ohm is assumed to be topsoil, while a resistivity of <300 ohm is assumed as a cave full of water. Then the resistivity value between 300 ohm to 700 ohm is assumed to be limestone, and the resistivity value >700 ohm is assumed to be hollow limestone.