| Abstrak/Abstract |
An integrated geophysical investigation using very low frequency electromagnetic (VLF-EM) waves has been carried out to detect underground rivers in the karst area of Gunung Kidul Regency, Indonesia, with coordinates 8◦03′18.4′′S 110◦36′35.1′′E to 8◦03′01.3′′S 110◦35′56.8′′E. A total of 5 VLF-EM wavers have been carried out to identify the underground river positions in the drought area in the three villages (Timun Sari, Mojo Sari and Peyuyon village). The interpretation of VLF-EM data was carried out qualitatively using Fraser and Karous-Hjelt filters using 2LAYINV Markku’s software. The interpretation of VLF-EM data usually represents an appropriate underground river based on higher values of the relative apparent current density than kars rock (limestone). The results of the study showed the presence of underground rivers, which were prominently oriented in the three villages. The results also showed that most of the underground rivers were located at a depth range of 50-160 m within the subsurface. Based on the fifth of the VLF-EM lines, the underground river flow can be estimated to have a depth of ±50 m in the upstream area (the first line) and a final depth of ±100 and ±160 m in the downstream area (the fifth line). The use of the VLF-EM geophysical survey tool with Karous-Hjelt filter interpretation using 2LAYINV Markku’s software is able to provide 2D mapping of subsurface structures. These results prove that VLF-EM waves can be useful for detecting underground rivers based on the contrasting levels of resistivity between water and rocks. |
