Improving the sNMR signal-to-noise ratio using cost-effective EM shielding

Surface Nuclear Magnetic Resonance (sNMR) is a geophysical method suited for hydrogeologists, due to its unique ability to directly sound groundwater. Nevertheless, its use is highly limited in urbanized areas due to the ubiquitous ambient electromagnetic noise that perturb the signal originating from the groundwater response to the sNMR excitation.
In this study, we investigate the efficiency of a cost-effective shielding made of cattle fences to reduce the noise impact on sNMR measurements. The main issue with EM shielding in sNMR is the size of the antennas required to acquire signal from deep aquifers, since the depth of investigation is roughly equal to the diameter of the transmitting antenna. Therefore, classical shielding methods are very difficult to implement at a reduced cost. We emit the hypothesis that using a cattle grid lying above the antenna should reduce the noise impact on the measurements. A proof of concept field experiment was first conducted on the University of Liège Campus, showing that using a simple 25mm grid reduced the noise by 40%. Further experiments were held inside the walls of the Orval abbey. There, multiple shapes of shielding where tested: domes and tunnels.
Several aspects were investigated: the ease of installation and the efficiency of the used setup in filtering the noise. Building a tunnel above the cable of the receiving loop was the easiest to achieve, whereas the construction of a dome covering the full antenna revealed complex to set in place. Shielding proved more efficient with tunnels above the receiving antenna cable than a full dome covering the whole loop. Using tunnels like shields, we were able to reduce the noise measured on the receiving loop by 80%, using superposed tunnels with different grid sizes.
Those experiments show that an efficient and easy shielding could be achieved to reduce the noise impact on measurements by a factor of 5. Such approach is promising to enable the use of sNMR in more diverse environments. Nevertheless, demonstrating that the shielding does not perturb the NMR signal transmission and reception remains to be investigated. The sNMR signal-to-noise ratio could also be improved using the central-loop configuration with a shielding only on the (small) central loop.