Journal Article

PreJuSER-11994

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Full-waveform inversion of cross-hole ground-penetrating radar data to characterize a gravel aquifer close to the Thur River, Switzerland

Klotzsche, A.FZJ* ; van der Kruk, J.FZJ* ; Meles, G. A. ; Doetsch, J. ; Maurer, H. ; Linde, N.

2010
EAGE Houten

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Please use a persistent id in citations: doi:10.3997/1873-0604.2010054

Abstract: Cross-hole radar tomography is a useful tool for mapping shallow subsurface electrical properties viz. dielectric permittivity and electrical conductivity. Common practice is to invert cross-hole radar data with ray-based tomographic algorithms using first arrival traveltimes and first cycle amplitudes. However, the resolution of conventional standard ray-based inversion schemes for cross-hole ground-penetrating radar (GPR) is limited because only a fraction of the information contained in the radar data is used. The resolution can be improved significantly by using a full-waveform inversion that considers the entire waveform, or significant parts thereof. A recently developed 2D time-domain vectorial full-waveform crosshole radar inversion code has been modified in the present study by allowing optimized acquisition setups that reduce the acquisition time and computational costs significantly. This is achieved by minimizing the number of transmitter points and maximizing the number of receiver positions. The improved algorithm was employed to invert cross-hole GPR data acquired within a gravel aquifer (4-10 m depth) in the Thur valley, Switzerland. The simulated traces of the final model obtained by the full-waveform inversion fit the observed traces very well in the lower part of the section and reasonably well in the upper part of the section. Compared to the ray-based inversion, the results from the full-waveform inversion show significantly higher resolution images. At either side, 2.5 m distance away from the cross-hole plane, borehole logs were acquired. There is a good correspondence between the conductivity tomograms and the natural gamma logs at the boundary of the gravel layer and the underlying lacustrine clay deposits. Using existing petrophysical models, the inversion results and neutron-neutron logs are converted to porosity. Without any additional calibration, the values obtained for the converted neutron-neutron logs and permittivity results are very close and similar vertical variations can be observed. The full-waveform inversion provides in both cases additional information about the subsurface. Due to the presence of the water table and associated refracted/reflected waves, the upper traces are not well fitted and the upper 2 m in the permittivity and conductivity tomograms are not reliably reconstructed because the unsaturated zone is not incorporated into the inversion domain.

Keyword(s): J

Note: This work was initiated as an MSc project within the Joint Master programme for Applied Geophysics of the Idea League, involving the universities TU Delft, ETH Zurich and RWTH Aachen (http://www.idealeague.org/geophysics). We thank Jacques R. Ernst for helpful discussions and advice. An internal review by Stewart Greenhalgh greatly improved the manuscript. We are indebted to our collaborators within the RECORD project and also wish to thank Ludovic Baron for performing borehole deviation logging. Funding for this study was provided by the Swiss National Science Foundation (SNF) and the ETH Competence Center for Environment and Sustainability (CCES). Further, we want to acknowledge Brian Wylie and Zoltan Szebenyi from the JSC at the Research Center Julich for their help in implementing the code on the JUMP cluster. We would also like to thank the Center for Computing and Communication from the RWTH Aachen for the use of the HPC-Cluster for our calculations. One of us (A. Klotzsche) also wishes to thank Wintershall Holding AG for providing a scholarship to support her studies.

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Research Program(s):

1. Terrestrische Umwelt (P24)

Appears in the scientific report 2010

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