Detection of Divaška Jama corridors behind (to the SW) Trhlovca cave using low frequency high power ground penetrating radar

Abstract

Ground penetrating radar (GPR) named “Roteg” was used to detect known and unknown passages of Divaska Jama and Trhlovca caves in SW Slovenia. “Roteg’s” main characteristics are an extraordinary high power output (20 MW) and high voltage on antennas (up to 20 kV), which allows penetration more than ten times deeper than common GPRs. During the measurement we used 3-m long antennas (50 MHz) with a central frequency of 50 MHz and we clearly detected the reflections from the depth of 200 m after data processing. During field survey 22 profiles were completed with a total length of 4487.97 m. Pretnerjeva and Žibernova Dvorana chambers in Divaska Jama were well visible on radarograms. New big cavities, which were detected below profiles P18, P21 and P22, are almost all at the same level of 350 – 400 m a.s.l. There is another group of cavities at a level of 420 – 450 m a.s.l., which corresponds to Trhlovca. Both cave groups are separated by the gap which extends in the vertical direction, and probably presents a tectonic zone, which is as well visible on the radarograms. By the use of “Roteg” at the karst surface we were able to detect known caves, new - unknown caves and tectonic zones up to 200 m below the surface. Key words: ground penetrating radar, caves, karst, Divaska Jama, Trhlovca, Slovenia. Dolocitev rovov Divaske jame v JZ smeri za Trhlovco z uporabo zelo mocnega nizkofrekvencnega georadarja Nizkofrekvencni georadar (GPR), imenovan »Roteg«, smo uporabili pri zaznavi znanih in neznanih rovov Divaske jame in Trhlovce v JZ Sloveniji. »Rotegove« glavne znacilnosti so izjemna moc (20 MW) in visoka napetost do 20 kV na antenah, kar omogoca penetracijo, ki je vec kot desetkrat globlja, kot jo dosežejo obicajni georadarji. Med meritvami smo uporabili 3 m dolgi anteni (50 MHz) s srediscno frekvenco 50 MHz. Tako smo po obdelavi podatkov jasno zaznali odboje iz globine 200 m. Med terenskimi meritvami smo izmerili 22 profilov s skupno dolžino 4490 m. Pretnerjeva in Žibernova dvorana v Divaski jami sta bili dobro vidni na georadarskih profilih. Nove velike jame, ki smo jih zaznali pod profili P18, P21 in P22, so skoraj vse na istem nivoju, 350–400 m nad morjem. Druga skupina jam na nadmorski visini 420–450 m ustreza Trhlovci. Obe skupini jam sta loceni z vrzeljo v navpicni smeri, ki je verjetno tektonska cona in je dobro vidna na georadarskih profilih. Z uporabo »Rotega« na kraskem povrsju smo dolocili znane jame, nove neznane jame in tektonske cone do globine 200 m pod povrsjem. Kljucne besede: georadar (GPR), jame, kras, Divaska jama, Trhlovca, Slovenija.