MRS in brief

Magnetic Resonance Sounding (MRS)

Magnetic Resonance Sounding (MRS) also denoted referred as Surface Nuclear Magnetic Resonance method (SNMR) is a geophysical application of Nuclear Magnetic Resonance (NMR) to groundwater investigation. The resonance behavior of proton magnetic moments in the geomagnetic field ensures that the method is selectively sensitive to groundwater. A non-invasive detection of subsurface water is the competitive advantage of the MRS. Developed in early eighties, MRS is already established method, but still not all the problems with the method applications are resolved. Encouragement of geophysicists and hydrologists to use this method in their practical work and attraction of new researches to contribute to further developments is the main goal of the MRS workshops.

For outside observer, the MRS method is similar to the Transient Electromagnetic method (TEM). MRS measurements comprise generation of a pulse of alternating current in a wire loop and measuring of the MRS response when the pulse is cut off. The amplitude, the relaxation times and the phase of the magnetic resonance signal are measured. Measurements are repeated with different amplitudes of the current in the loop, which makes a data set. Then, inversion of these data reveals variations of the water content and of the relaxation time in the subsurface. The loop sizes may vary between a=20 m and a=150 m. Loops larger than 80 m usually have one turn and small loops may have 2 to 5 turns of wire. Each transmitting loop is made of electrical cable with a cross-section of more than 6 mm2. Such a thick wire allows generating large current pulses (up to 600 A). Special attention should be paid to the electrical insulation of the wire because of a high pulse voltage (up to 4 kV). The volume affected by each Tx loop depends on the loop size and shape. If MRS loops are separated at the distance of more than the length of the loop side, then these measurements should make a 1-D survey. When loops are set side by side or closer then measurements can be interpreted as a 2-D profile. A 3-D field setup comprises a set of loops around investigated area.

Detection of groundwater is the most reliable parameter provided by MRS. If the magnetic resonance signal is not observed and the external electromagnetic noise is low then MRS points at the absence of an aquifer formation. The threshold of water detection in rocks with the hydraulic permeability more than 10^-5 m/s depends on the measuring conditions but typically the water content should be more than q>0.005. Water in rocks with the hydraulic conductivity less than 10^-6 m/s (clay, for example) is undetectable with MRS. In the MRS log, low permeable rocks are shown as waterless intervals. The water content given by MRS inversion allows estimating the effective porosity and the relaxation time allows prediction of the hydraulic conductivity. MRS can be compared with with other popular surface geophysical methods.

The table shows that electric and EM methods provide variations of the electrical resistivity in the subsurface. Seismic methods provide variations of the elastic properties of rocks. These physical parameters have an indirect and often a non-unique link with groundwater. In the contrary, the MRS signal is generated by groundwater molecules. It allows a more reliable interpretation of MRS results in terms of groundwater.

For review of Magnetic Resonance applications, see for instance:

- Legchenko, A., 2013, Magnetic Resonance Imaging for Groundwater, Wiley-ISTE 978-1-84821-568-9.

- Behroozmand, A. A., K. Keating, and E. Auken, 2015, A review of the principles and applications of the NMR technique for near-surface characterization: Surveys in Geophysics, 36, 27 –85, doi: 10.1007/s10712-014-9304-0

Link to the 7 previous special issues following the previous MRS workshops:

- Journal of Applied Geophysics, 2020, vol. 179,  https://www.sciencedirect.com/journal/journal-of-applied-geophysics/vol/179/issue/1

- Geophysics journal, 2016,  Vol. 81, issue 4, https://doi.org/10.1190/geo2016-0519-SPSEINTRO.1

- Near Surface Geophysics 2014, vol 12, issue 2, https://doi.org/10.3997/1873-0604.2014010

-  Near Surface Geophysics 2011, vol. 9, issue 2, https://doi.org/10.3997/1873-0604.2014010

- Journal of Applied Geophysics, 2008, vol. 66, https://doi.org/10.1016/j.jappgeo.2008.07.002

- Near Surface Geophysics 2005, vol.3, issue 3, https://doi.org/10.3997/1873-0604.2005007

- Journal of Applied Geophysics, 2002, vol 50, https://www.sciencedirect.com/journal/journal-of-applied-geophysics/vol/50/issue/1