Influence of measurement conditions on depth range and resolution of GPR images: The example of lowland valley alluvial fill (the Obra River, Poland)

摘要

There are many studies referring to the analysis of alluvial deposit structures using GPR (ground-penetrating radar). However, the geophysical surveys are usually conducted at a single set of land surface and groundwater level conditions. The main question is whether an optimal set of such conditions resulting in optimal (highest possible) resolution and depth range can be determined? A field experiment, based on the GPR surveys conducted at various groundwater levels and parameter settings, was carried out to study their influence on depth range and resolution of the GPR surveys. The test was conducted in the lower course of the Obra River (western Poland). Three sets of the GPR measurements were done: at low groundwater level, at the groundwater level situated close to the land surface and when the surface of the floodplain was inundated and frozen. The results indicated significant differences in the depth range and resolution of particular surveys. The GPR images from the survey conducted on the frozen floodplain featured low depth range in comparison with the remaining experimental measurements. Despite the smooth surface of the frozen floodplain that seemed suitable for the geophysical surveys, the images were obscured by numerous reverberations originating from a layer of water underlying the ice cover. The presented results are the first step to create an atlas of georadar images illustrating various depositional environments. The research showed that such a database should consist of GPR images illustrating given sedimentary environment by at least three different sets of hydrogeologic conditions and GPR measurement settings taking into account such parameters as e.g. time window, number of stacks or sampling frequency.