ASSESS test site
A testbed for high-resolution electromagnetic geophysical methods
Fig. 1 - Left: Construction of the testbed. Middle: Top-view of the completed testbed. Right: On top of the testbed, with the automatic weather station at the left wall and the pumping well next to it.
DesignThe testbed consists of five uniform layers artificially constructed in a concrete box 20 m long, 4 m wide, and 2 m deep (Fig. 1). The five layers were built from three different sands. The geometry (Fig. 2) of the layers varies along the long axis and is constant along the lateral axis. The top of the box is open to normal atmospheric forcing, all the other boundaries are impermeable. The water level is controlled through a pumping well in one of the corners with rapid transmission throughout the box guaranteed by a gravel layer at the bottom. 32 TDR sensors grouped in 4 profiles are logged periodically to provide information on the distribution of the soil water and its dynamics. Precipitation, air temperature, wind speed, and net radiation are logged right at the box.
2 - Layer geometry of the testbed (20 m*4 m*2 m). The topmost and
lowest interfaces indicate the surface and the upper edge of the gravel
layer, respectively. Red markers show the positions of the TDR sensors.
Water Content Dynamics
Since the hydraulic properties of the sands differ, soil water content is in general discontinuous across the layer boundaries. This leads to easily detectable reflections of GPR signals (Fig. 4). Rainfall events induce sharp infiltration fronts that propagate rather quickly through the box (Fig. 3). The capillary fringe shows a dynamic evolution from the ponding of water. But more important, pumping events are causing rapid changes in the water content at the lower observation points (e.g. 23rd of September at S5 and S6).
|Fig. 3 -
Precipitation (top) and water content dynamics (bottom, second profile
from the left, sensors are numbered from top to bottom). Current data is available, if you are interested, please contact Angelika Gassama.
Various GPR observations were carried out since the construction in June 2010. Two exemplary results can be found in Fig 4. In both images one can clearly identify the reflections from the layer interfaces, the main geometrical features, the direct waves and the bottom reflection. On the left image you can also see steep reflections from the side walls, which are not visible for the Mala system because of the more narrow radiation pattern. This also causes the weaker reflections from the dipped interfaces.
4 - Common offset radargrams. Left: 400 MHz IDS at 0.94 m antenna
separation. Right: 800 Mhz Mala at 1.05 m antenna separation. Click on the images to see enlarged
versions of the radargrams.
Patrick Klenk, Stefan Jaumann, Kurt Roth
Klenk, P., Jaumann, S., and Roth, K. (2015a). Quantitative high-resolution observations of soil water dynamics in a complicated architecture with time-lapse Ground-Penetrating Radar, Hydrol. Earth Syst. Sci., 19, 1125-1139. DOI: 10.5194/hess-19-1125-2015.
Klenk, P., Keicher, V., Jaumann, S., and Roth, K. (2014b). Current limits for high precision GPR measurements, in 'Proc. 15th International Conference on Ground Penetrating Radar (GPR2014), 734-738, 30 June-04 July 2014, Brussels, Belgium. DOI: 10.1109/ICGPR.2014.6970524.
- Dagenbach, A., Buchner, J.S., Klenk, P., and Roth, K. (2013). Identifying a parameterisation of the soil water retention curve from on-ground GPR measurements, Hydrol. Earth Syst. Sci., 17, 611-618. DOI: 10.5194/hess-17-611-2013.
- Buchner, J.S., Wollschläger U., Roth K. (2012), Inverting surface GPR data using FDTD simulation and automatic detection of reflections to estimate subsurface water content and geometry, Geophysics, 77, H45-H55, doi:10.1190/geo2011-0467.1
- Buchner, J.S., Kühne, A., Antz, B., Roth, K. and Wollschläger, U. (2011): Observation of volumetric water content and reflector depth with multichannel ground-penetrating radar in an artificial sand volume, Proceedings of the 6th International Workshop on Advanced Ground Penetrating Radar (IWAGPR), 22-24 June 2011, Aachen, Germany, doi:10.1109/IWAGPR.2011.5963910
- Antz, Benny, Entwicklung und Modellierung der Hydraulik eines Testfeldes für geophysikalische Messungen, Diploma Thesis, Heidelberg University, 2010.
- Kühne, Alexander, Experimentelle Untersuchung der zeitlichen Variabilität des Bodenwassergehalts mit GPR, Staatsexamensarbeit, Heidelberg University, 2010.
- Bogda, Florian, Untersuchung der Bodenwasserdynamik mit Mehrkanal-GPR, Staatsexamensarbeit, Heidelberg University, 2011.
- Dagenbach, Andreas, Untersuchung der hydraulischen Bodeneigenschaften durch GPR: Analyse der Kapillarsaumreflexion durch numerische Simulationen, Diplomarbeit, Heidelberg University, 2012
- Jaumann, Stefan, Estimation of effective hydraulic parameters and reconstruction of the natural evaporative boundary forcing on the basis of TDR measurements, Diploma-Thesis, Heidelberg University, 2012
- Buchner, Jens, Constructive Inversion of Vadose Zone GPR Observations, PhD-Thesis, Heidelberg University, 2012
- Klenk, Patrick, Developing Ground Penetrating Radar for Quantitative Soil Hydrology, PhD-Thesis, Heidelberg University, 2012