Test of the maximum penetration depth of the Roteg GPR above the Hranice Abyss and in the Moravian Karst

Abstract

A new kind of Ground Penetrating Radar (GPR) ”Roteg“ was tested at generally known speleological sites in the Czech Republic. The first examined site – the Hranice Abyss located in the Hranice Karst – is the deepest underwater cave in the world. This GPR is characterised by much higher pulse power, antennas with rather high voltage (5–15 kV), and, in particular, the special design of the pulse generator. The radar survey near the Hranice Abyss has shown that it is possible to detect reflections of electromagnetic pulses coming from the speleogenic structures of the abyss itself and from lithological boundaries occurring below the water table – something which was not anticipated and was verified for the first time ever. Plausibly detectable reflections were detected from the depths of 580 m below the surface – which is approximately 515 m below the water level – using the longest available 6-metre antennas tuned to the frequency of 25 MHz. The second site tested was the quarry of Mala dohoda near the municipality of Holstejn, the Moravian Karst, the Czech Republic. The GPR used was the same as above except the power output to the transmitting antenna which produced pulses of 20 kV. The radarogram showed cavities located at the depth of up to 300 m, the layers on the boundary between Lažanky and Vilemovice members of limestone at the depth of 400 m, basement sandstones and conglomerates at the depth of 600–700 m, and granite rocks below this level. Both of the tests mentioned above confirmed the extraordinary big penetration depth of the GPR signal which exceeded 500 m when using the maximum power on transmitting antennas.