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PhD thesis defense

Bolette Badsberg Jensen, IGN, defends her thesis at Geology Section

PhD thesis defense — Bolette Badsberg Jensen 8 FEB 2023

Info

Date & Time:

Place:
Aud. C, Department of Geosciences and Natural Resource Management, Øster Voldgade 10, 1350 Copenhagen K

Hosted by:
Geology Section

Cost:
Free

Bolette Badberg Jensen defends her thesis,

Developing crosshole ground penetrating radar towards efficient contaminated site investigations

Supervisors:
Associate Professor Majken C. L. Zibar, IGN
Nina Tuxen, Capital Region of DK
Aikaterini Tsitonaki, WSP Denmark
Thomas Mejer Hansen, AU
Professor Lars Nielsen, IGN

Assessment Committee:
Senior Lecturer James Irving, University of Lausanne – Switzerland
Chief Consultant Flemming Jørgensen, Central Denmark Region
Professor Karsten Høgh Jensen (chair), IGN

Summary:
Heterogeneous glacial deposits dominate large parts of the Northern Hemisphere. In these landscapes, high resolution characterization of the geology is crucial for understanding contaminant transport at polluted sites. Geological characterization based on borehole logs often proves insufficient in describing heterogeneity between boreholes. This PhD project was initiated with the objective of developing a methodology to use ground penetrating radar (GPR) as part of contaminated site investigations as a minimally invasive method for mapping geological heterogeneity between boreholes.
First, a simple linear approach for geophysical inversion of recorded radar amplitude data was developed. A linear model can introduce errors in the estimated subsurface models. This modeling error was quantified and accounted for by including a Gaussian description of the error in the inversion. Accounting for the modeling error was fundamental to successfully obtain 2D subsurface models from inversion of amplitude data.
Then, since water in the boreholes seemed to affect the GPR data to a surprisingly large degree, datasets obtained in boreholes with and without water were compared to quantify the influence from water-filled boreholes in clay till environments. The results show that the water-filled boreholes cause a three-fold increase of radar amplitudes, and that the mean frequency of the waveform data is reduced by approximately one third. In order to obtain high-quality subsurface estimates from crosshole GPR recorded in clay till, removing the water from the boreholes is recommended to ensure accuracy of the data hence increase the reliability of subsequent inversion results.
Finally, crosshole GPR was used at an industrial contaminated site in a clay till setting and the developed inversion approach was applied to obtain subsurface model estimates of the radar wave velocity and attenuation, independently. The GPR results were compared to borehole logs, grain size analyses and relative permeability data from the site. The GPR data analysis provided valuable information about the geology between boreholes and the original conceptual geological model could be refined. A silt layer with a thickness of a few decimeters, likely important for flow and transport processes, was resolved by GPR data.

A digital version of the PhD thesis can be obtained from the PhD secretary Mikala Heckscher at mikala@ign.ku.dk

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