Ground-penetrating radar study of coastal landscape on Hiiumaa Island, Estonia

This case study presents the results of a ground-penetrating radar (GPR) profiling in Hiiumaa, west Estonian archipelago. Most of the coast of Hiiumaa has emerged from the Baltic Sea during Limnea Sea stage. The study sites are located on Tahkuna Peninsula, northern Hiiumaa where the coastal plain is characterized by a series of beach ridges. The beach ridges are separated by narrow wet depressions where non-calcareous sandy deposits are covered with peat. Due to tectonic land uplift, the landforms are located at different distance from the nowadays shore and at different altitudes. The morphology of landforms and the character of deposits influence the soil formation and the development of peat layer. The layer of varved clays under the sand prevents infiltration and stabilizes the water regime. The water movement also depends on the orientation, height and width of the beach ridges. The achieved results are based on cartographic analysis, fieldwork data including topographic survey and geological radar survey along transects by using SIR-3000. In order to verify the radar image interpretation, 33 boreholes were drilled on the profiles. The main objective of the study was to determine the thickness of peat layer and to characterize the topography of the surface of sub-peat mineral soil by GPR. Two ~1 km long GPR profiles perpendicular to the ridges were examined. A 100 MHz frequency was used, which reflected to approximately 10 m of depth. The study sites in Tahkuna have developed in nutrient-poor environment - the peat layer is thin (up to 1 m) and transitional bog peat makes up a small part of it. The thickness of subjacent sand is 2-4 m. After awhile, the depressions will be filled and the peat layer will cover the beach ridges entirely, which leads to surface leveling, reduction in plant species and simplification of the landscape structure, in general.