Investigating karst cave sediments of unroofed caves with GPR, XRF and XRD

Abstract

Ground penetrating radar (GPR), a nondestructive geophysical method, was used to detect cave sediments, now exposed on the karst surface (in so-called “unroofed caves”) due to a general lowering of the surface (dissolution of carbonate rocks). It turned out that cave sediments are expressed as areas with strong signal attenuation on GPR radargrams. In order to test the GPR's capability and reliability in detecting cave sediments, further mineralogical-geochemical analyses were conducted to reveal which factors influence the attenuation the most. At the testing location, samples of cave sediments and soils on carbonate rocks 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/aluminum oxides/hydroxides which can strongly affect GPR 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. Based on these findings, GPR profiling was used on several other places where direct indicators of unroofed caves are not present due to strong surface reshaping. GPR has proven to be a reliable method for locating these areas and can be a valuable complementary method to geological mapping for a more comprehensive recognition of unroofed cave systems. These are important for understanding karst erosion processes and geomorphology. For instance, studying unroofed cave systems can help with reconstructing the direction of former underground water flows.