Time-lapse GPR full-waveform inversion to monitor heat tracers at the Krauthausen test site

Master thesis (2023)

Authors

B.L. Mueller Civil Engineering & Geosciences

Contributors

Jan van der Kruk Forschungszentrum Jülich (supervisor 1)
Anja Klotzsche Forschungszentrum Jülich (supervisor 2)
Faculty
Civil Engineering & Geosciences
Copyright: © 2023 Brianna Mueller
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Published Date

25-08-2023

Language

English

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Faculty

Civil Engineering & Geosciences

Abstract

Understanding solute and heat transport processes in aquifers is crucial for monitoring and protecting the groundwater critical zone. Crosshole ground-penetrating radar (GPR) is a useful method for enhancing transport characterization in aquifers. Particularly, GPR full-waveform inversion (FWI) can provide subsurface images with resolution at the subwavelength scale, making it a well-suited method for monitoring tracer plumes. While previously applied to a salt tracer test, its effectiveness for other tracer types with different
conductivity and permittivity contrasts remains unexplored. Here, we apply this method to two tracer tests conducted at the Krauthausen test site in northwest Germany: (1) a natural gradient heat tracer test and (2) a forced-gradient combined heat-salt tracer test. Both tracers use hot water, which provides a contrast in both electrical conductivity and dielectric permittivity with the groundwater. This should permit improved monitoring capabilities with GPR
FWI in comparison to the salt tracer, which only provides a contrast in electrical conductivity. However, this comes with new challenges in the processing workflow, specifically, in the starting model strategy. The results illustrate that using the ray-based permittivity tomogram and
a homogeneous conductivity as FWI starting models for each time-lapse dataset is the best strategy. Additionally, we applied an amplitude analysis approach to improve the starting model in regions with low ray coverage. The effects of the heat tracer were detected over the entire depth range of the aquifer, from 3-11 m, and especially at mid-aquifer depths from 6-8.5 m. For the heat-salt tracer, we were able to detect separate effects from the salt and heat: the salt was observed at depth in the aquifer, in accordance with the salt tracer test, and the
heat was observed approximately two days after the salt effects at mid-aquifer depths (from 5.5-7.5 m). In regions where minimal effects from the tracer were observed, specifically for the heat tracer, the consistent results between independent time-lapse datasets demonstrate the repeatability of the method, indicating the suitability of GPR FWI for hydrogeophysical time-lapse studies.