GPR-based detection of Pleistocene periglacial slope deposits at a shallow-depth test site

Abstract

Pleistocene periglacial slope deposits (PPSD) almost completely cover the mountainous regions of Germany and are the parent material for soil formation. It is therefore necessary to map their thickness and spatial distribution. The mapping of periglacial slope deposits can be performed more quickly than conventional probing by drilling or soil profile analysis if ground penetrating radar (GPR) is used. Because the use of GPR to map soils or soil depth depends to a large degree on the suitability of soil properties, the performance of GPR in mapping layered substrates was evaluated at a test site with varying water content and substrate properties. We focus on the efficiency of GPR in mapping loess-rich layers above layers with a high stone content within a depth range of 1 m. Different materials such as loess, metabasalt and slates, including heavily weathered slates (saprolite) were studied. The test site was equipped with several time domain reflectometry (TDR) probes to monitor soil water content changes for the period of one year. Systematic investigations of the reflection patterns were performed with 100, 200, 400, 500 and 900 MHz antennae with respect to the quality of measurement, dielectric coefficient and soil water content of different material combinations. It became clear that the 400 MHz antenna is most suitable for creating images of loess and stone layers at the depth range of 1 m and under natural climatic conditions. The results prove that the most important effect the substrates at the test site have on GPR is produced by the differences in water content of bordering layers, making the soil water capacity of the materials a decisive parameter for evaluating GPR performance. The water content dependent dielectric permittivities show higher values for saprolite (heavily weathered slate) compared to loess by a factor of 1.3 to 2, with water content increasing from 15 to 40 vol.% respectively. In doing so we discovered the dielectric permittivity of loess to be 1.2 to 1.5 times higher than that of unweathered slate at the same water content. We were able to locate horizontal layer boundaries at the test site starting at a minimum of 0.5 vol.% water content difference and a reflection coefficient r ≥ 0.02 between the layers. The results obtained at the test site show the ability of GPR to detect layering in PPSD at different water contents and help to predict the performance of GPR measurements in mapping of PPSD.