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@ARTICLE{Stingaciu:5798,
      author       = {Stingaciu, L.-R. and Pohlmeier, A. and Blümler, P. and
                      Weihermüller, L. and van Dusschoten, D. and Stapf, S. and
                      Vereecken, H.},
      title        = {{C}haracterization of unsaturated porous media by
                      high-field and low-field {NMR} relaxometry},
      journal      = {Water resources research},
      volume       = {45},
      issn         = {0043-1397},
      address      = {Washington, DC},
      publisher    = {AGU},
      reportid     = {PreJuSER-5798},
      pages        = {W08412},
      year         = {2009},
      note         = {Many thanks to Y. Q. Song (Schlumberger-Doll Research,
                      Connecticut) for giving us access to his software for
                      inverse Laplace transform. The DFG (projects PO 746/2-1 and
                      Transregional Collaborative Research Centre 32) has to be
                      acknowledged for financial support and Norbert Klitzsch and
                      Oliver Mohnke (Applied Geophysics, RWTH Aachen University)
                      for providing access to the low field setup. Finally, the
                      reviewers are acknowledged for their very useful comments
                      that helped greatly with the improvement of our paper.},
      abstract     = {A comparison study of nuclear magnetic resonance
                      relaxometry at high and low magnetic field (7 and 0.1 T) has
                      been initiated for investigating the influence of the
                      magnetic field strength, variable clay content, and
                      different degrees of saturation on the relaxometric
                      properties of four ideal porous media. The samples consisted
                      of medium sand with increasing fractions of kaolin clay
                      ranging from 0 to $15\%.$ Six different volumetric water
                      contents between saturation and theta = 0.05 were used.
                      Changes in water content of the samples were achieved by
                      slow evaporation. T-2 relaxation curves were monitored by
                      the Carr-Purcell-Meiboom-Gill sequence and were further
                      analyzed by inverse Laplace transformation, yielding T-2
                      distribution functions. Sand shows a slight continuous shift
                      with decreasing water content of a bimodal distribution
                      function of T-2 to faster relaxation at high and low
                      magnetic field. Sand-clay mixtures show broad, bimodal
                      distribution functions for both magnetic field intensities
                      which shift slightly with decreasing water content. Signal
                      amplitude behavior with variation of saturation degree was
                      also monitored. An expected proportionality of the total
                      signal amplitude with water content was observed for all
                      samples at 0.1 T, whereas at 7 T deviations occurred for
                      samples with a clay content higher than $5\%,$ which are
                      assigned to loss of signal in the first echo periods. The
                      relaxivity in unsaturated clay-based porous media is mostly
                      surface dominated, as the weak and comparable dependence of
                      1/T-2 on T-E at both field strengths shows. Nevertheless,
                      for a reliable determination of water content in mixed
                      systems with varying texture and saturation the employment
                      of multiecho sequences at low magnetic field strength are
                      preferable.},
      keywords     = {J (WoSType)},
      cin          = {ICG-4 / ICG-3},
      ddc          = {550},
      cid          = {I:(DE-Juel1)VDB793 / I:(DE-Juel1)ICG-3-20090406},
      pnm          = {Terrestrische Umwelt},
      pid          = {G:(DE-Juel1)FUEK407},
      shelfmark    = {Environmental Sciences / Limnology / Water Resources},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000269036900002},
      doi          = {10.1029/2008WR007459},
      url          = {https://juser.fz-juelich.de/record/5798},
}