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| Titel |
Non-invasive investigation of the saturated/unsaturated zone with magnetic resonance sounding - a field example at the testsite Fuhrberger Feld near Hannover, Germany |
| VerfasserIn |
S. Costabel, U. Noell, C. Ganz |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250068212
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| Zusammenfassung |
| Magnetic resonance sounding (MRS) is a non-invasive geophysical method for groundwater
prospection that uses the principle of nuclear magnetic resonance (NMR) in the
Earth’s magnetic field. Its unique property distinct from other hydrogeophysical
methods is the direct sensitivity to the amount of water, i.e. to the amount of 1H
nuclei in the subsurface. Because MRS is normally used to investigate the water
content of the saturated zone and to characterize aquifer structures, the standard
application is optimized for 1D-measurements in depths from several to several tens
of meters. However, our investigations show that MRS has also the potential to
contribute substantially to the study of groundwater recharge if the sensitivity of
the method for the unsaturated zone and for the transition to the saturated zone
is increased by using a modified measurement setup and adjusted interpretation
schemes.
We conducted MRS test measurements with the focus on the very shallow subsurface in
the range of some few decimeters down to the groundwater table in a depth of 3 m. The test
site is located in the area Fuhrberger Feld about 30 km north-east of Hannover, Germany,
which comprises an unconfined sandy aquifer of 20 to 30-m thickness. Previous studies have
discovered the soil physical characteristics of the site with tension infiltrometer
measurements and tracer irrigation experiments in the field, as well as with water retention
measurements in the laboratory. In addition, several infiltration experiments with dye tracer
were conducted and monitored with electrical resistivity tomography (ERT), tensiometers and
TDR devices.
For the MRS measurements at the testsite, a serious challenge was the intense
electromagnetic noise consisting of large spiky radio signals and harmonic components,
respectively. A special combination of new processing techniques was developed to isolate
and interpret the NMR signals with amplitudes of approximately 5 to 14 nV. The standard
inversion of the MRS data shows the ground water table at the correct depth and
furthermore, increased residual water in the topsoil, which is verified by the water
retention measurements in the lab. However, the amount of water at shallow depth
down to 30 cm is difficult to quantify and to allocate exactly in depth due to the
limited resolution properties of the method in this depth range. A new inversion
scheme that parameterizes the capillary fringe using the van-Genuchten model
was applied to the data. These results are in good agreement with the laboratory
measurements.
In order to develop MRS as a method for monitoring groundwater recharge
processes, we combine hydraulic simulations and MRS forward modeling. Our
numerical experiments suggest that the common MRS measurement scheme must
be modified to enable faster repetitions, i.e., to resolve fast infiltration processes
accordingly in time. For such modifications one must accept losses in the spatial
resolution of the method. Compared to non-invasive ERT measurements with a 2D or
3D resolution in the decimeter range, the resolution properties of MRS are much
worse. However, the direct sensitivity of the MRS method to the water content
is an important benefit, whereas the quantification of water with ERT methods
remains a serious problem. Therefore, we anticipate therefore that combining both
methods could be the key for non-invasive monitoring of groundwater recharge in the
future. |
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