LIAG operates a hydrogeophysical testsite about 25km Northeast of Hannover. Numerous information from boreholes, laboratory data and field measurements are available and make it the perfect place to try new methodods for subsurface characterization.
In order to test and to validate geophysical methods at a well-known site, we started establishing a test field for shallow aquifer structures in the frame of the project "new methods in hydrogeophysics" in 2009. Initially, four drillholes (ENG01-04) were drilled in a forest area close to the village Engensen, approx. 25km Northeast of Hannover (Binot, 2008). They showed two quaternary aquifers (0-12 and 16-23m depth), separated by a till layer with an undulating surface. Both aquifers consist of fine-grained to medium-grained glaciofluviatile sandy sediments with thin layers of gravel, silt and peat.
First, a meadow area around ENG03 was chosen to drill several cored boreholes (ENG03, ENG08, ENG20) and groundwater wells for monitoring groundwater levels and as a basis for geophysical investigations. Since 2017 the investigations focused on the neighboring forestal area around ENG01, where further boreholes were drilled (Binot, 2017).
A wide range of geophysical field measurements was done on both sub-areas:
The testfield and the first core measurements were first described by Holland et al. (2010). A more detailed description can be found in chapter 6 of the dissertation of Dlugosch (2014). Numerous qualification works comprised groundwater level measurements (Kuntzer, 2010), hydraulic tests on cores (Sass, 2010), surface NMR (Dlugosch, 2014) and mapping of the till aquitards using GPR (Helms, 2018). A continuous focus has been on deriving hydraulic conductivity from geophysical measurements (Attwa & Günther, 2013; Dlugosch, 2014; Jiang et al., 2020; Skibbe et al., 2021). Another important issue is the combination of different methods by coupled inversion (Günther et al., 2010; Jiang et al., 2020; Skibbe et al., 2021).
In order to characterize the lateral variability of hydrogeophysical parameter, methods such as surface-NMR and ERT are suitable. However, these methods do not allow for imaging sharp boundaries or thin layers and mainly provide only smooth paramter distributions. In contrast, GPR measurements can provide sharp boundaries. Consequently, SIMAR targets to implement the information about sharp boundaries from GPR relections as structural contraint into surface-NMR inversion. The approach has already been sucessfully applied for ERT inversion by including seismic reflections into the mesh generation and lowering the smoothness contrains across these boundaries.
Results from Jiang et al. (2020):
Left: 2D distribution of water content (a,c) and relaxation timeT2* (b,d) derived from MRT smooth inversion using (a,b) and GPR reflectors as structural constraints (c,d). The black lines in (c,d) represent the average GPR reflectors from three GPR-profiles aty=0, +10, +20 m used as sharp boundary constraints in the inversion. The white lines give the reflectors from each GPR-profile used to obtain the averaged boundary. The horizontal dashed black lines are the approximate bounds of the second aquifer and plotted only for orientation, but are not used as constraints.
Right:Sediment log including radar facies interpretation (a), depth profiles of relaxation times T2 and T2* (b) and θ(c) at drilling location Eng35B. The figure gives the relaxation time distribution (yellow-red shades), cutoff-time (grey dashed) and hence obtained θ (green crosses) from laboratory NMR, smooth (light blue) and constrained(dark blue) MRT inversion extracted from the 2D model at the location of the drilling and geometric constraints from GPR and water table measurements (black).
Binot , F. (2008): Vier neue Bohrungen im Mess- und Testgebiet des GGA-Instituts nördlich von Schillerslage bei Burgdorf (German), Niedersachsen. – GGA-Bericht, Archiv-Nr. 0128085; Hannover.
Sass, S. (2010): Applicability of geophysical measuring methods for determination of K values in comparison to conventional methods. - 83 S., diploma thesis, Universität Hannover.
Kuntzer, M. (2010): Höchauflösende Grundwasserspiegelmessungen im Bereich des geophysikalischen Testfeldes Schillerslage (German). - 57 S., BSc thesis, Universität Hannover.
Dlugosch, R., Müller-Petke, M. Günther, T., Yaramanci, U. (2010): Aquifer characterisation by Magnetic Resonance field and laboratory measurements. – Ext. abstr., 16 th EAGE Near Surface, 6.-8.9.2010; Zürich.
Holzhauer, J., Günther, T., Yaramanci, U. (2010): Examination of seismoelectric observations at the test site Schillerslage and laboratory. - 16th European Meeting of Environmental and Engineering Geophysics of the Near Surface Geoscience Division of EAGE, 06.-08.09.2010; Zurich.
Mueller-Petke, M., and U. Yaramanci, 2010, QT inversion — Comprehensive use of the complete surface NMR data set: Geophysics, 75(4), WA199–WA209, doi:10.1190/1.3471523.
Holland, R., Dlugosch, R., Günther, T., Sass, S., Holzhauer, J., Sauer, J., Binot, F. & Yaramanci, U. (2011): Das hydrogeophysikalische Testfeld Schillerslage (German). - DGG-Mittlg. 1/2011, 51-54;Potsdam, PDF
Attwa, M. & Günther, T. (2012): Application of spectral induced polarization (SIP) imaging for characterizing the near-surface geology: An environmental case study at Schillerslage, Germany. Australian Journal of Basic and Applied Sciences 6(9), 693-701.
Attwa, M. & Günther, T. (2013): Spectral induced polarization measurements for predicting the hydraulic conductivity in sandy aquifers. Hydrol. Earth Syst. Sci. 17, 4079-4094, doi:10.5194/hess-17-4079-2013.
Dlugosch, R. (2014): Aquifer characterisation using nuclear magnetic resonance: Ph.D. thesis, Berlin University of Technology, PDF.
Binot, F. (2017): Dokumentation und geologische Interpretation weiterer Bohrungen im Mess- und Testgebiet des LIAG im Raum Engensen-Schillerslage bei Burgdorf, Niedersachsen (German). - LIAG-Bericht, 30 S., 10 Abb., 10 Anl., 10 Tab., Archiv-Nr. 0135003; Hannover.
Helms, J. (2018): Das geophysikalische Testgebiet Schillerslage: Eine 3D-geologische Modellierung auf der Basis von Georadar und Bohrdaten (German), MSc thesis, Leibniz-Universität Hannover.
Wiederhold (2018): Reflexionsseismische Untersuchungen - Scherwellenseismik Schillerslage. - Kurzbericht für Projekt TOPSOIL (German), LIAG-Bericht, Archiv-Nr. 0135310; Hannover.
Jiang, C., M. Müller-Petke, Q. Wang, and J. Igel (2018): Two-dimensional QT inversion of complex magnetic resonance tomography data: Geophysics, 83(6), JM65–JM75, doi:10.1190/geo2017-0756.1.
Grombacher, D., Dlugosch, R., Grunewald, E., Müller-Petke, M., Auken, E. (2018): Frequency cycling to alleviate unknown frequency offsets for adiabatic half-passage pulses in surface nuclear magnetic resonance, Geophysics, 83(5), JM29-JM38, 10.1190/geo2017-0701.1
Jiang, C., Igel, J., Dlugosch, R., Müller-Petke, M., Günther, T., Helms, J., Lang, J. & Winsemann (2020): Magnetic resonance tomography constrained by ground-penetrating radar for improved hydrogeophysical characterisation, Geophysics 85(6), JM13-JM26, doi:10.1190/geo2020-0052.1.
Skibbe, N., Günther, T. & Müller-Petke, M. (2021): Improved hydrogeophysical imaging by structural coupling of two-dimensional magnetic resonance and electrical resistivity tomography. Geophysics 86 (5), WB135-WB146, doi:10.1190/geo2020-0593.1.