MRS measurements and inversion in presence of EM noise

Abstract

The Magnetic Resonance Sounding method (MRS) is based on the resonance behavior of proton magnetic moments in the geomagnetic field. As the signal generated by the protons is very small, the method is sensitive to electromagnetic noise and this is one of the major limitations for practical application. The frequency of the magnetic resonance signal is directly proportional to the magnitude of the geomagnetic field, and varies between 800 Hz and 2800 Hz around the globe. Whilst natural noise within this frequency range is generally not very large (excepting magnetic storms or other temporary disturbances), the level of cultural noise (electrical power-lines, generators, etc.) may be very high. Both the depth of investigation and resolution of the MRS method depend on signal to noise ratio. If measured data are corrupted by noise, it will have an effect on the accuracy and reliability of MRS results. Consequently, the MRS signal has to be measured with an acceptable signal to noise ratio. For improving the signal to noise ratio different filtering techniques could be applied. Selection of the filtering scheme depends on the noise origin. In any case, application of the stacking is necessary. A large number of repetitive measurements are often required for stacking and consequently one sounding may take from one to twelve hours. In this paper, efficiency of different filtering techniques, inversion strategy and influence of non-filtering noise on MRS results are discussed.