Identification of underground river with microtremor method in Dersono karst area, Pacitan

Many islands in Indonesia are formed due to the lifting of the seafloor which forms limestone lithology and forms karst morphology. The karst region has a large freshwater potential that accumulates in the underground river system. The existence of an underground river system as a water reservoir is very important in fulfilling the needs of the community, especially in island countries such as Indonesia that have many karst regions. Research to map the position and distribution of underground rivers is carried out by applying geophysical methods. One of the geophysical methods applied is the microtremor method. The working principle of this method is the identification of underground rivers based on Particle Motion method and Time Frequency Analysis (TFA). That analysis resulted in the particles movement direction that perpendicular to the vibration source which is identified as the underground river. The existence of the underground river is then validated based on the TFA parameters of the underground river karst region. The results of the Particle Motion analysis at the research location show the character of the river flow pattern that has a Northeast-Southwestern orientation, and based on TFA there is amplitude continuity in the river flowed area with a frequency range of 4 - 5 Hz and an amplitude of 2,000 dB - 20,000 dB on component Z (vertical).