Application of Ground penetrating radar for investigating sediment-filled surface karst features (Krk Island, Croatia)

Ground penetrating radar (GPR) has been widely used in the last decade for investigating various issues in karst environments. Valley-like depressions and other sediment-filled irregular features on karst surface in the south-eastern part of Krk Island, Croatia, were noticed but their genetic mechanism could not be clarified only by geomorphological field observations and the analysis of orthophoto images. It was unclear which processes (surface and/or underground) prevailed during the formation of the investigated surface features, so the GPR method was used to study different karst features and their interconnections. To test the applicability of the GPR method for detecting such features, two different antennae, a 250 MHz shielded and a 50 MHz unshielded Rough Terrain Antenna (RTA), were applied to estimate the suitability of various frequencies in certain karst environments. On the basis of additional information obtained from GPR data, it become clear that the studied sediment-filled surface karst features are result of the denudation processes. Furthermore, linkages between surface sediment-filled features also became apparent, which prove the existence of an unroofed cave. The linkages are expressed as areas with stronger attenuation on GPR radargrams due to different electromagnetic properties of the sediments that fill up denuded structures and the mainly bare karst surface that surrounds them. These findings indicate that the investigated sediment-filled surface features of various shapes and sizes belong to one former underground cave system which is now exposed on the recent surface as a result of denudation.