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Magnetic resonance sounding: new method for ground water assessment
1Hydrogeology, Department of Water Resources, International Institute for Geo-Information Science and Earth Observation. lubczynski@itc.nl
Ground Water
|March 24, 2004
Summary
Magnetic Resonance Sounding (MRS) offers a water-selective geophysical approach for determining subsurface hydraulic properties. While effective in resistive environments, signal attenuation in conductive media presents limitations for accurate hydrogeological parameter estimation.
Area of Science:
- Geophysics
- Hydrogeology
- Nuclear Magnetic Resonance
Background:
- Classical geophysical methods often face ambiguity in determining subsurface water content and hydraulic properties.
- Magnetic Resonance Sounding (MRS) utilizes the nuclear magnetic resonance (NMR) principle for a water-selective approach.
- MRS offers reduced ambiguity in characterizing subsurface free water and hydraulic media properties.
Purpose of the Study:
- To demonstrate the application of MRS in obtaining hydrogeological parameters through case examples.
- To analyze the capabilities and limitations of MRS technology in various subsurface environments.
- To provide guidelines for MRS survey planning based on signal-to-noise ratio (S/N) variability.
Main Methods:
- Application of Magnetic Resonance Sounding (MRS) surveys in two distinct hydrogeological settings: Delft (Netherlands) and Serowe (Botswana).
- Comparison of MRS data with borehole data for validation of hydrogeological parameter estimations.
- Analysis of signal attenuation effects in different geological and electrical resistivity environments.
Main Results:
- In Delft, MRS accurately determined effective porosity (≈28%) and specific drainage (≈20%) in a shallow sand aquifer (high S/N = 73).
- MRS interpretation was hindered below 39m in Delft due to signal attenuation in low resistivity (≈10 Ωm) media.
- In Serowe, MRS successfully defined hydrogeological parameters (>74m depth) in a fractured sandstone aquifer despite a low S/N (0.9) due to negligible signal attenuation in a resistive environment.
Conclusions:
- MRS is a valuable tool for subsurface hydrostratigraphy description, hydrogeological system parameterization, and well siting.
- Key limitations include S/N ratio dependence, signal attenuation in conductive environments, magnetic field non-uniformity, and instrumental constraints.
- MRS survey success is primarily determined by the S/N ratio in resistive locations, with spatial and temporal variability of signal and noise impacting results.