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@PHDTHESIS{Hoven:1032276,
      author       = {Hoven, Dominik},
      title        = {{M}ulti-dimensional {GPR} full-waveform inversion for
                      small-scale hydrogeophysical soil characterization},
      volume       = {643},
      school       = {RWTH Aachen University},
      type         = {Dissertation},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2024-06115},
      isbn         = {978-3-95806-781-3},
      series       = {Reihe Energie $\&$ Umwelt / Energy $\&$ Environment},
      pages        = {IX, 163},
      year         = {2024},
      note         = {Dissertation, RWTH Aachen University, 2024},
      abstract     = {A detailed understanding of the processes within the
                      critical zone, which covers the area from the earth’s
                      surface down to the aquifer, is essential for sustainable
                      resource management and environmental protection. This zone
                      exhibits complex flow and transport processes and supports
                      critical ecosystem services such as water supply,
                      agriculture, and climate regulation. However, imaging the
                      complex critical zone accurately, especially at high
                      resolutions required for a detailed analysis, presents
                      significant challenges because of the variability of soil
                      water content and complex subsurface structures. This thesis
                      introduces a novel 2.5D ground penetrating radar (GPR)
                      full-waveform inversion (FWI) method that enhances
                      subsurface imaging by accurately incorporating 3D
                      geometries, such as air and water filled boreholes, finite
                      length antenna models, and lysimeter geometries, in the
                      forward modeling of the GPR FWI. Furthermore, the 3D-to-2D
                      data transformation with its assumptions, e.g. for the
                      far-field, necessary for 2D GPR FWI, is not required with
                      this method. We show in synthetic studies with different
                      inversion methods (2D FWI, 2.5D FWI, 2.5D FWI with borehole,
                      and 2.5D FWI with borehole and antenna) an improved source
                      wavelet reconstruction with the inclusion of realistic
                      borehole and antenna geometries for the data. The inclusion
                      of these geometries in the forward model of FWI approaches
                      can significantly improve the accuracy of conductivity
                      reconstructions, with a reduction in the mean relative
                      absolute error of conductivity of more than $20\%$ compared
                      to simple 2D FWI and 2.5D FWI. The improvement is
                      particularly noticeable in high-contrast zones. Although
                      including antenna geometries significantly increases
                      computational requirements by a factor of í10, the quality
                      of reconstruction remains similar to the case with only
                      borehole inclusion. In contrast to ray-based inversion
                      (RBI), where artifacts arise when using high-angle data
                      (72.35°), FWI still provides reliable results. In a
                      following analysis, we tested if a model that includes
                      boreholes and finite length antenna models for experimental
                      data measured with transmitter and receiver positioned in
                      air and water filled boreholes can improve the effective
                      source wavelet estimation. A synthetic test shows that using
                      this approach, only one wavelet can be used for the
                      reconstruction of both the unsaturated and saturated zone.
                      However, we still observed challenges with the current
                      antenna model to account for the different coupling in air
                      filled boreholes for measured data. Using the new 2.5D FWI
                      with borehole and antenna models and a single source
                      wavelet, the results of the saturated zone reconstruction
                      were similar to those observed in previous studies where
                      four effective source wavelets were considered. To obtain
                      reliable results in the unsaturated zone, it is necessary to
                      adapt the antenna model to resolve existing discrepancies.
                      Next to an improved reconstruction of small-scale structures
                      in aquifers, small-scale processes in the
                      soil-plant-atmosphere continuum are also of interest. In
                      order to achieve a higher reconstruction resolution with the
                      FWI for these processes, higher frequencies are necessary.
                      In a first part, we indicate the constraints imposed by
                      high-frequency GPR data, which require more precise starting
                      models to fulfill the half-wavelength criterion of the GPR
                      FWI. This cannot be met by the regular starting model
                      approach of using RBI models. We show that a
                      frequency-hopping approach can be used to generate starting
                      models that meet these requirements. Furthermore, we
                      investigated the influence of first-arriva and amplitude
                      changes in the source wavelets on high-frequency GPR FWI.
                      Utilizing an adapted heterogeneous model, we were able to
                      show a more detailed reconstruction with higher frequency
                      data compared to lower frequency data. In a next step, we
                      extended the model building process of the 2.5D GPR FWI and
                      are now able to include more complex geometrical structures
                      like lysimeters in the forward model. As we faced challenges
                      to use the 2D GPR FWI on experimental high-frequency data
                      acquired on lysimeters, we first investigated the different
                      GPR waves in synthetic studies at lysimeters filled with
                      homogeneous and heterogeneous soils. We show the complexity
                      of the GPR data, that includes air, direct, and reflected
                      waves. We created a synthetic 3D GPR lysimeter dataset with
                      a center frequency of 450 MHz and applied the novel 2.5D GPR
                      FWI to this dataset. It demonstrates an exceptional good
                      reconstruction of the soil and fit of the dataset by the
                      inversion results, effectively simulating air-, real soil-,
                      and reflected waves as well as revealing intricate soil
                      properties. The newly developed 2.5D GPR FWI presented in
                      this thesis enables the modeling and reconstruction of
                      small-scale structures with high resolution. The application
                      ranges from aquifer characterization to the now possible
                      inversion of GPR data measured at lysimeter, providing a
                      foundational framework for future research in high
                      resolution subsurface imaging.},
      cin          = {IBG-3},
      cid          = {I:(DE-Juel1)IBG-3-20101118},
      pnm          = {2173 - Agro-biogeosystems: controls, feedbacks and impact
                      (POF4-217)},
      pid          = {G:(DE-HGF)POF4-2173},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      urn          = {urn:nbn:de:0001-20241120144832318-5634765-9},
      doi          = {10.34734/FZJ-2024-06115},
      url          = {https://juser.fz-juelich.de/record/1032276},
}