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Ground penetrating radar (GPR) named “Roteg” was used to detect known and unknown passages of Divaška 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 Divaška 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, Divaška Jama, Trhlovca, Slovenia. Določitev rovov Divaške jame v JZ smeri za Trhlovco z uporabo zelo močnega nizkofrekvenčnega georadarja Nizkofrekvenčni georadar (GPR), imenovan »Roteg«, smo uporabili pri zaznavi znanih in neznanih rovov Divaške jame in Trhlovce v JZ Sloveniji. »Rotegove« glavne značilnosti so izjemna moč (20 MW) in visoka napetost do 20 kV na antenah, kar omogoča penetracijo, ki je več kot desetkrat globlja, kot jo dosežejo običajni georadarji. Med meritvami smo uporabili 3 m dolgi anteni (50 MHz) s središčno 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 Divaški 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 višini 420–450 m ustreza Trhlovci. Obe skupini jam sta ločeni z vrzeljo v navpični smeri, ki je verjetno tektonska cona in je dobro vidna na georadarskih profilih. Z uporabo »Rotega« na kraškem površju smo določili znane jame, nove neznane jame in tektonske cone do globine 200 m pod površjem. Ključne besede: georadar (GPR), jame, kras, Divaška jama, Trhlovca, Slovenija.

During heavy rainfalls between September 17 and 19, 2010 large part of Slovenia has suffered extensive floods that last for nearly two weeks. For the river Iška record discharge of 59.3 m3/s was measured on September 19 on the gauging station in Iška vas located at the southern rim of Ljubljansko barje. In the first hour of September 21, 2010 two weak earthquakes (ML=0.6 and ML=0.2) occurred within one minute near Iška vas. They were felt by some inhabitants who reported also a rumbling noise (brontides). During the flood recession period, the water of river Iška started to sink into the gravely stream bed or rocky left banks and the gauging profile completely dried on September 23, day and a half after the first earthquake. Water reappeared again on September 25. In the period September 21 − October 4 additional seven weak earthquakes occurred in the same area. All earthquakes from this series occurred at or near the surface and deviate in hypocentral depth from the seismicity pattern characteristic for the southern rim of Ljubljansko barje, which was analysed for comparison. The epicentres of the first two earthquakes are in good agreement with the location of the dried river bed. It is therefore probable that both phenomena are related. Analyses of seismograms have shown that it is not likely that the observed events are collapse earthquakes, but they are tectonic events. Although earthquakes were relatively weak, it seems that they could be accompanied by small near-surface tectonic movements, because they occurred at the position of a known fault. These movements are probably connected to the opening of pre-existing fissures in the karstified valley bottom, although the primary reason for sinking of the river is that high waters removed the clogged river bed that enables intensive sinking into the river bottom during the flood.

High frequency ground penetrating radar (GPR) is usually applied for cavities detection in a shallow subsurface of karst areas to prevent geotechnical hazards. For specific projects, such as tunnel construction, it is important to detect also larger voids at medium depth range. However, dimensions of classical rigid low frequency antennas seriously limit their applicability in a rough terrain with dense vegetation commonly encountered in a karst. In this study recently developed 50 MHz antennas designed in a tube form were tested to detect cave gallery at the depth between 12 m and 60 m. The Divaca cave was selected because of a wide range of depths under the surface, possibility of unknown galleries in the vicinity and a rough terrain surface typical for Slovenian karst. Seven GPR profiles were measured across the main gallery of the cave and additional four profiles NE of the cave entrance where no galleries are known. Different acquisition and processing parameters were analysed together with the data resolution issues. The main gallery of the cave was clearly imaged in the part where the roof of the gallery is located at the depth from 10 m to 30 m. The width of the open space is mainly around 10 m. Applied system was not able to detect the gallery in the part where it is located deeper than 40 m, but several shallower cavities were discovered which were unknown before. The most important result is that the profiles acquired NE of the cave entrance revealed very clearly the existence of an unknown gallery which is located at the depth between 15 m and 22 m and represents the continuation of the Divaca cave. Access to this gallery is blocked by the sediment fill in the entrance shaft of the cave. The results of the study are important also for future infrastructure projects which will involve construction of tunnels through karstified limestone and for speleological investigations to direct the research efforts. Keywords: ground penetrating radar, cavity detection, spatial resolution, limestone, Divača cave.

The tectonic setting of W Slovenia is characterised by NW-SE trending dextral strike-slip fault systems and moderate seismicity. Monitoring of tectonic movements along five presumably active faults or in their vicinity using TM 71 extensometers was set up in 2004. In five years of monitoring some clear trends of displacement were established. The morphologically most prominent fault in W Slovenia is Idrija Fault having a total length of more than 120 km. The average lateral displacement measured along a crack in its inner fault zone in Učja valley was 0.26 mm/year. Short-term rates were even greater and reached 0.54 mm/year. Raša Fault monitoring site at the foot of Vremščica Mt. established first an average uplift of SW block for 0.16 mm/year and left-lateral displacement of 0.16 mm/ year. It was followed by down-slip of the same block at the rate of 0.37 mm/year. In Postojnska Jama two instruments, 260 m apart, were installed at the fault zone, which extends about 1 km northeast from Predjama Fault. We detect small tectonic deformations, dextral horizontal movement of 0.05 mm in 5 years for Postojna 1 and extension of 0.03 mm in 5 years for Postojna 2. Both devices recorded similar reactions to some earthquakes with magnitude range 3.1-5.2 and epicentral distance of 12-95 km. The amplitude of individual peaks is in the order of 0.08 mm. The monitoring at Kneža Fault started at the end of 2006. In two years clear oblique displacement was established with left-lateral rate of 0.06 mm/year and uplift of SW block for 0.06 mm/year. Monitoring in Pološka Jama situated in vicinity of the Ravne Fault started in 2008. Preliminary results show 0.08 mm of horizontal displacement between two limestone beds. Established displacements proved the active tectonic movement of all monitored faults. Observed deformation rates can be compared with the regional deformation rate in W Slovenia established from GPS measurement, which is in the order of 2 mm/year.