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| Titel |
Multi-offset ground-penetrating radar imaging of a lab-scale infiltration test |
| VerfasserIn |
A. R. Mangel, S. M. J. Moysey, J. C. Ryan, J. A. Tarbutton |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 16, no. 11 ; Nr. 16, no. 11 (2012-11-05), S.4009-4022 |
| Datensatznummer |
250013555
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| Publikation (Nr.) |
 copernicus.org/hess-16-4009-2012.pdf |
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| Zusammenfassung |
| A lab scale infiltration experiment was conducted in a sand tank to evaluate
the use of time-lapse multi-offset ground-penetrating radar (GPR) data for
monitoring dynamic hydrologic events in the vadose zone. Sets of 21 GPR
traces at offsets between 0.44–0.9 m were recorded every 30 s during a
3 h infiltration experiment to produce a data cube that can be viewed as
multi-offset gathers at unique times or common offset images, tracking
changes in arrivals through time. Specifically, we investigated whether this
data can be used to estimate changes in average soil water content during
wetting and drying and to track the migration of the wetting front during an
infiltration event. For the first problem we found that normal-moveout (NMO)
analysis of the GPR reflection from the bottom of the sand layer provided
water content estimates ranging between 0.10–0.30 volumetric water content,
which underestimated the value determined by depth averaging a vertical
array of six moisture probes by 0.03–0.05 volumetric water content. Relative
errors in the estimated depth to the bottom of the 0.6 m thick sand layer
were typically on the order of 2%, though increased as high as 25% as
the wetting front approached the bottom of the tank. NMO analysis of the
wetting front reflection during the infiltration event generally
underestimated the depth of the front with discrepancies between GPR and
moisture probe estimates approaching 0.15 m. The analysis also resulted in
underestimates of water content in the wetted zone on the order of 0.06
volumetric water content and a wetting front velocity equal to about half
the rate inferred from the probe measurements. In a parallel modeling effort
we found that HYDRUS-1D also underestimates the observed average tank water
content determined from the probes by approximately 0.01–0.03 volumetric
water content, despite the fact that the model was calibrated to the probe
data. This error suggests that the assumed conceptual model of laterally
uniform, one-dimensional vertical flow in a homogenous material may not be
fully appropriate for the experiment. Full-waveform modeling and subsequent
NMO analysis of the simulated GPR response resulted in water content errors
on the order of 0.01–0.03 volumetric water content, which are roughly
30–50% of the discrepancy between GPR and probe results observed in the
experiment. The model shows that interference between wave arrivals affects
data interpretation and the estimation of traveltimes. This is an important
source of error in the NMO analysis, but it does not fully account for the
discrepancies between GPR and the moisture probes observed in the
experiment. The remaining discrepancy may be related to conceptual errors
underlying the GPR analysis, such as the assumption of uniform
one-dimensional flow, a lack of a sharply defined wetting front in the
experiment, and errors in the petrophysical model used to convert dielectric
constant to water content. |
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