@article{LorekWagner,
  author    = {Lorek, Andreas and Wagner, Norman},
  title     = {Supercooled interfacial water in fine grained soils probed by dielectric spectroscopy},
  series = {Cryosphere},
  journal   = {Cryosphere},
  doi       = {10.5194/tc-7-1839-2013},
  url       = {http://nbn-resolving.de/urn:nbn:de:gbv:wim2-20170516-31840},
  pages     = {1839 -- 1855},
  abstract  = {Water substantially affects nearly all physical, chemical and biological processes on the Earth. Recent Mars observations as well as laboratory investigations suggest that water is a key factor of current physical and chemical processes on the Martian surface, e.g. rheological phenomena. Therefore it is of particular interest to get information about the liquid-like state of water on Martian analogue soils for temperatures below 0 °C. To this end, a parallel plate capacitor has been developed to obtain isothermal dielectric spectra of fine-grained soils in the frequency range from 10 Hz to 1.1 MHz at Martian-like temperatures down to -70 °C. Two Martian analogue soils have been investigated: a Ca-bentonite (specific surface of 237 m2 g-1, up to 9.4\% w / w gravimetric water content) and JSC Mars 1, a volcanic ash (specific surface of 146 m2 g-1, up to 7.4\% w / w). Three soil-specific relaxation processes are observed in the investigated frequency-temperature range: two weak high-frequency processes (bound or hydrated water as well as ice) and a strong low-frequency process due to counter-ion relaxation and the Maxwell-Wagner effect. To characterize the dielectric relaxation behaviour, a generalized fractional dielectric relaxation model was applied assuming three active relaxation processes with relaxation time of the ith process modelled with an Eyring equation. The real part of effective complex soil permittivity at 350 kHz was used to determine ice and liquid-like water content by means of the Birchak or CRIM equation. There are evidence that bentonite down to -70 °C has a liquid-like water content of 1.17 monolayers and JSC Mars 1 a liquid-like water content of 1.96 monolayers.},
  subject      = {Grundwasser},
  language  = {en}
}
@article{ScheuermannHuebnerSchlaegeretal.,
  author    = {Scheuermann, Alexander and Huebner, Christof and Schlaeger, Stefan and Wagner, Norman and Becker, Rolf and Bieberstein, Andreas},
  title     = {Spatial time domain reflectometry and its application for the measurement of water content distributions along flat ribbon cables in a full-scale levee model},
  series = {Water Resources Research},
  journal   = {Water Resources Research},
  doi       = {10.1029/2008WR007073},
  url       = {http://nbn-resolving.de/urn:nbn:de:gbv:wim2-20170425-31601},
  abstract  = {Spatial time domain reflectometry (spatial TDR) is a new measurement method for determining water content profiles along elongated probes (transmission lines). The method is based on the inverse modeling of TDR reflectograms using an optimization algorithm. By means of using flat ribbon cables it is possible to take two independent TDRmeasurements from both ends of the probe, which are used to improve the spatial information content of the optimization results and to consider effects caused by electrical conductivity. The method has been used for monitoring water content distributions on a full-scale levee model made of well-graded clean sand. Flood simulation tests, irrigation tests, and long-term observations were carried out on the model. The results show that spatial TDR is able to determine water content distributions with an accuracy of the spatial resolution of about ±3 cm compared to pore pressure measurements and an average deviation of ±2 vol \% compared to measurements made using another independent TDR measurement system.},
  subject      = {Damm},
  language  = {en}
}
@article{BumbergerMaiSchmidtetal.,
  author    = {Bumberger, Jan and Mai, Juliane and Schmidt, Felix and L{\"u}nenschloß, Peter and Wagner, Norman and T{\"o}pfer, Hannes},
  title     = {Spatial Retrieval of Broadband Dielectric Spectra},
  series = {Sensors},
  journal   = {Sensors},
  doi       = {10.3390/s18092780},
  url       = {http://nbn-resolving.de/urn:nbn:de:gbv:wim2-20180906-37831},
  pages     = {1 -- 22},
  abstract  = {A broadband soil dielectric spectra retrieval approach ( 1 MHz- 2 GHz) has been implemented for a layered half space. The inversion kernel consists of a two-port transmission line forward model in the frequency domain and a constitutive material equation based on a power law soil mixture rule (Complex Refractive Index Model - CRIM). The spatially-distributed retrieval of broadband dielectric spectra was achieved with a global optimization approach based on a Shuffled Complex Evolution (SCE) algorithm using the full set of the scattering parameters. For each layer, the broadband dielectric spectra were retrieved with the corresponding parameters thickness, porosity, water saturation and electrical conductivity of the aqueous pore solution. For the validation of the approach, a coaxial transmission line cell measured with a network analyzer was used. The possibilities and limitations of the inverse parameter estimation were numerically analyzed in four scenarios. Expected and retrieved layer thicknesses, soil properties and broadband dielectric spectra in each scenario were in reasonable agreement. Hence, the model is suitable for an estimation of in-homogeneous material parameter distributions. Moreover, the proposed frequency domain approach allows an automatic adaptation of layer number and thickness or regular grids in time and/or space.},
  subject      = {Theoretische Elektrotechnik},
  language  = {en}
}
@article{BoreWagnerLesoilleetal.,
  author    = {Bore, Thierry and Wagner, Norman and Lesoille, Sylvie Delepine and Taillade, Frederic and Six, Gonzague and Daout, Franck and Placko, Dominique},
  title     = {Error analysis of clay-rock water content estimation with broadband high-frequency electromagnetic sensors—air gap effect},
  series = {Sensors},
  journal   = {Sensors},
  doi       = {10.3390/s16040554},
  url       = {http://nbn-resolving.de/urn:nbn:de:gbv:wim2-20170418-31248},
  pages     = {1 -- 14},
  abstract  = {Broadband electromagnetic frequency or time domain sensor techniques present high potential for quantitative water content monitoring in porous media. Prior to in situ application, the impact of the relationship between the broadband electromagnetic properties of the porous material (clay-rock) and the water content on the frequency or time domain sensor response is required. For this purpose, dielectric properties of intact clay rock samples experimental determined in the frequency range from 1 MHz to 10 GHz were used as input data in 3-D numerical frequency domain finite element field calculations to model the one port broadband frequency or time domain transfer function for a three rods based sensor embedded in the clay-rock. The sensor response in terms of the reflection factor was analyzed in time domain with classical travel time analysis in combination with an empirical model according to Topp equation, as well as the theoretical Lichtenecker and Rother model (LRM) to estimate the volumetric water content. The mixture equation considering the appropriate porosity of the investigated material provide a practical and efficient approach for water content estimation based on classical travel time analysis with the onset-method. The inflection method is not recommended for water content estimation in electrical dispersive and absorptive material. Moreover, the results clearly indicate that effects due to coupling of the sensor to the material cannot be neglected. Coupling problems caused by an air gap lead to dramatic effects on water content estimation, even for submillimeter gaps. Thus, the quantitative determination of the in situ water content requires careful sensor installation in order to reach a perfect probe clay rock coupling.},
  subject      = {Wassergehalt},
  language  = {en}
}
@article{WagnerBoreRobinetetal.,
  author    = {Wagner, Norman and Bore, Thierry and Robinet, Jean-Charles and Coelho, Daniel and Taillade, Frederic and Delepine-Lesoille, Sylvie},
  title     = {Dielectric relaxation behavior of Callovo-Oxfordian clay rock: A hydraulic-mechanical-electromagnetic coupling approach},
  series = {Journal of Geophysical Research: Solid Earth},
  journal   = {Journal of Geophysical Research: Solid Earth},
  doi       = {10.1002/jgrb.50343},
  url       = {http://nbn-resolving.de/urn:nbn:de:gbv:wim2-20170428-31681},
  pages     = {4729 -- 4744},
  abstract  = {Water content is a key parameter to monitor in nuclear waste repositories such as the planed underground repository in Bure, France, in the Callovo-Oxfordian (COx) clay formation. High-frequency electromagnetic (HF-EM) measurement techniques, i.e., time or frequency domain reflectometry, offer useful tools for quantitative estimation of water content in porous media. However, despite the efficiency of HF-EM methods, the relationship between water content and dielectric material properties needs to be characterized. Moreover, the high amount of swelling clay in the COx clay leads to dielectric relaxation effects which induce strong dispersion coupled with high absorption of EM waves. Against this background, the dielectric relaxation behavior of the clay rock was studied at frequencies from 1 MHz to 10 GHz with network analyzer technique in combination with coaxial transmission line cells. For this purpose, undisturbed and disturbed clay rock samples were conditioned to achieve a water saturation range from 0.16 to nearly saturation. The relaxation behavior was quantified based on a generalized fractional relaxation model under consideration of an apparent direct current conductivity assuming three relaxation processes: a high-frequency water process and two interface processes which are related to interactions between the aqueous pore solution and mineral particles (adsorbed/hydrated water relaxation, counter ion relaxation and Maxwell-Wagner effects). The frequency-dependent HF-EM properties were further modeled based on a novel hydraulic-mechanical-electromagnetic coupling approach developed for soils. The results show the potential of HF-EM techniques for quantitative monitoring of the hydraulic state in underground repositories in clay formations.},
  subject      = {Lehm},
  language  = {en}
}
@article{ChenSchwingKarlovšeketal.,
  author    = {Chen, Zhen and Schwing, Moritz and Karlovšek, Jurij and Wagner, Norman and Scheuermann, Alexander},
  title     = {Broadband Dielectric Measurement Methods for Soft Geomaterials: Coaxial Transmission Line Cell and Open-Ended Coaxial Probe},
  series = {International Journal of Engineering and Technology},
  volume    = {2014},
  journal   = {International Journal of Engineering and Technology},
  number    = {volume 6, number 5},
  doi       = {10.7763/IJET.2014.V6.728},
  url       = {http://nbn-resolving.de/urn:nbn:de:gbv:wim2-20210408-43984},
  pages     = {373 -- 380},
  abstract  = {Broadband dielectric measurement methods based on vector network analyzer coupled with coaxial transmission line cell (CC) and open-ended coaxial probe (OC) are simply reviewed, by which the dielectric behaviors in the frequency range of 1 MHz to 3 GHz of two practical geomaterials are investigated. Kaolin after modified compaction with different water contents is measured by using CC. The results are consistent with previous study on standardized compacted kaolin and suggest that the dielectric properties at frequencies below 100 MHz are not only a function of water content but also functions of other soil state parameters including dry density. The hydration process of a commercial grout is monitored in real time by using OC. It is found that the time dependent dielectric properties can accurately reveal the different stages of the hydration process. These measurement results demonstrate the practicability of the introduced methods in determining dielectric properties of soft geomaterials.},
  subject      = {Impedanzspektroskopie},
  language  = {en}
}