Magnetic Resonance Sounding to determine hydrogeological parameters
Fig. 1: GMR instrumentation during field survey.
Magnetic Resonance Sounding (MRS) is the only geophysical method which is capable to directly detect free protons in the subsurface. A very common source for free protons is water which makes MRS very suitable for shallow hydrologic studies to depths down to 100m.
The method is based on the principle of nuclear magnetic resonance (NMR). The protons are excited with a magnetic field which fits their resonance frequency in the earth magnetic field (Lamorfrequency approx. 1-3 kHz). After the end of the exciting pulse the exponential decaying electromagnetic signal from the excited protons is measured. For a MRS several measurements with increasing pulse strength are applied. Therefore as typical for a sounding the signal from the excited protons comes from different depths. The amplitudes of the measured decay curves can be directly inverted to water content and under saturated condition porosity of the subsurface. The decay times T2* yield information about the surface-to-volume ratio of the pore space and accordingly hydraulic conductivity (K), e.g. after Seevers (1966) which can be estimated after a local calibration of the equation.
The LIAG mainly carry out MRS field measurements to improve the determination of hydraulic parameters. Therefore we apply a field equipment of the new generation of instruments (GMR from Vista Clara Inc.) as shown in Fig. 1. The new instrumental features are full signal sampling (50 k Hz), a decreased instrumental dead time and four receiver channels. Additionally to soundings, efficient 2D and 3D applications are therefore possible (Fig. 2).
Fig. 2: 3D NMR survey on a frozen lake close to Clausthal-Zellerfeld.
So far we focused our work on the new LIAG test site Schillerslage. But also additional surveys were applied on other locations like on the North Sea island Borkum during studies on the LIAG research field Groundwater systems and hydrogeophysics.
The MRS measurements at the test site Schillerslage were spectrally inverted with the QT-Inversion scheme (Müller-Petke 2010). The result can be seen in Fig. 3.
Fig. 3: Borehole description (left) and partial water content (PWC) distribution of T2* derived from QT-inversion of surface-NMR data (centre). T2 decay time distributions (right) from laboratory-NMR on samples (blue) compared to the QT-inversion result at respective depth (red).
The subsurface can therefore be roughly divided into two aquifers (2-12m and 16-20m), whereas the first one can be subdivided into two parts with different T2* times. The inversion result was qualitatively confirmed by laboratory NMR measurements.
The hydrogeological parameters water content and hydraulic conductivity derived from the QT inversion of MRS data was compared to results obtained from another common inversion scheme and laboratory measurements (Fig. 4).
Fig. 4: Borehole description (left), comparison of total water content (mid left), (log-mean) T2(*) (mid right) and hydraulic conductivity (right) over depth derived from different methods and inversion schemes.
The water contents up to approx. 38 vol.% derived from MRS match the results from laboratory NMR and porosities from grain size analyses very well. For the second aquifer the derived water contents from MRS is slightly underestimated in favour of an overestimation of the layer thickness. This is due to the degreasing resolution of the used MRS setup at this depth. The hydraulic conductivities estimated from T2* times after Seevers were, after calibration, in good agreement with measurements on drilled cores, laboratory samples and estimations from grain size analyses. Observed differences at depth at about 10m will be further studied. After this site specific calibration a spatial extrapolation of K values obtained from further MRS measurements in the surrounding is possible. Further information can be obtained at the following link (Ext. Abstract EAGE2010).
References
Müller-Petke, M. and Yaramanci, U. [2010] QT-Inversion - Comprehensive use of the complete surface-NMR dataset. Geophysics 75 (4), WA199-WA209.
Seevers, D. [1966] A nuclear magnetic method for determining the permeability of sandstones. Society of Petrophysicists and Well Log Analysts, 7th Annual Logging Symposium Paper L.
Products & Publications
MRS group
Raphael Dlugosch
Mike Müller-Petke
Thomas Günther
Jobst Liebau
Products and Publications
A project of the topical research field ground water systems - hydrogeophysics