Abstract
Ground penetrating radar (GPR) using a special unshielded 50 MHz Rough Terrain Antenna (RTA) in combination with a shielded 250 MHz antenna was used to study the capability of this geophysical method for detecting cave sediments. Allochthonous cave sediments found in the study area of Lanski vrh (W Slovenia) are now exposed on the karst surface in the so-called “unroofed caves” due to a general lowering of the surface (denudation of carbonate rocks) and can provide valuable evidence of the karst development. In the first phase, GPR profiles were measured at three test locations, where cave sediments are clearly evident on the surface and appear with flowstone. It turned out that cave sediments are clearly visible on GPR radargrams as areas of strong signal attenuation. Based on this finding, GPR profiling was used in several other places where direct indicators of unroofed caves or other indicators for speleogenesis are not present due to strong surface reshaping. The influence of various field conditions, especially water content, on GPR measurements was also analysed by comparing radargrams measured in various field conditions. Further mineralogical-geochemical analyses were conducted to better understand the factors that influence the attenuation in the area of cave sediments. Samples of cave sediments and soils on carbonate rocks (rendzina) were taken for X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses to compare the mineral and geochemical compositions of both sediments. Results show that cave sediments contain higher amounts of clay minerals and iron/aluminium oxides/hydroxides which, in addition to the thickness of cave sediments, can play an important role in the depth of penetration. Differences in the mineral composition also lead to water retention in cave sediments even through dry periods which additionally contribute to increased attenuation with respect to surrounding soils. The GPR method has proven to be reliable for locating areas of cave sediments at the surface and to determine their spatial extent, which is very important in delineating the geometry of unroofed cave systems. GPR thus proved to be a very valuable method in supporting geological and geomorphological mapping for a more comprehensive recognition of unroofed cave systems. These are important for understanding karstification and speleogenetic processes that influenced the formation of former underground caves and can help us reconstruct the direction of former underground water flows.
Highlights
Unroofed caves are surface karst features, the result of a general surface lowering originating from the dissolution of soluble rocks, mainly carbonates
Epikarst and subsoil karst features were revealed during the construction works and over 350 new caves were opened in the Karst region, including unroofed caves [12]
Systematic mapping of unroofed cave features such as cave sediments and flowstones can yield a better insight into the spatial distribution of cave systems
Summary
Unroofed caves are surface karst features, the result of a general surface lowering originating from the dissolution of soluble rocks (denudation), mainly carbonates. They present the former underground features that are exposed on the surface [1,2]. Apart from karst investigations incorporated, for instance, in the earthworks preceding motorway construction, studying karst environment is mostly limited to observing features that are visible on the surface. Traditional destructive methods such as drilling and trenching are time-consuming, expensive and often unfeasible due to the rough and inaccessible karst terrain. Surface and underground features are closely linked and the pre-existence of underground karstification has conditioned the formation of surface features [5,6], which is why geophysical methods in general and especially ground penetrating radar (GPR), are very useful non-invasive and relatively fast methods to provide subsurface information
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