GPR full-waveform inversion at different scales to image the critical zone

Abstract:

Ground penetrating radar (GPR) measurements can be performed quickly and effectively to map and characterize the critical zone. The obtained permittivity is highly correlated with soil water content and porosity, whereas the electrical conductivity depends on soil properties such as soil texture and clay content. Therefore, GPR is well suited to non- or minimal invasively characterize and improve our understanding of dynamic processes that are taking place in the critical zone. In the last decade, GPR full-waveform inversion has proved to be a powerful tool to map, characterize and monitor the critical zone with a higher resolution than standard GPR methods. Especially the application for crosshole GPR demonstrated a great potential to resolve decimeter-scale resolution images and to resolve small-scale high contrast layers that can be related to zones of high porosity, zones of preferential flow or clay lenses. This approach was successfully applied to several different aquifers and comparison to independently measured e.g. logging data proved the reliability of the method.

Here, recent developments and monitoring methods of crosshole and field-scale surface GPR are presented that yield high resolution medium properties at different scales. Thereby, we will show the potential where and how the methods can be applied and which new insights the methods can provide. As one example for the current achievements for vertical crosshole GPR full-waveform inversion, we present the results of a sand-gravel aquifer, where a domain of 25m by 50m was characterized using several crosshole planes. Thereby, the obtained high resolution images showed more details and higher resolution than standard GPR methods and ERT. The new information of small scale structures helped to explain why a previously observed plum splitting during an infiltration experiment using ERT occurred at this site. Further developments to investigate the potential to map and monitor soil water content variation at Minirhizotrone using horizontal GPR boreholes measurements are shown. The Rhizotrones allow the monitoring of soil water content at different soils, different depths and stress conditions (dry and wet) over a growing season of wheat.