Contribution of Electromagnetic Shielding and the Bloch–Siegert Effect to Magnetic-Resonance Sounding

Abstract

Abstract —Calibration experiments of magnetic-resonance sounding (MRS) were carried out on the ice of the Ob’ Reservoir. For the first time, a comparative quantitative analysis of the contribution of electromagnetic shielding and the Bloch–Siegert effect to the MRS signal was carried out, and the asymmetry of this contribution when detuning from the resonant frequency was investigated. The results of theoretical calculations without regard to the Bloch–Siegert effect differ about three times from the experimental data at the maximum intensity of the radio frequency pulse for the experimental data. At the same time, when taking into account the interference of electromagnetic shielding and the Bloch–Siegert effect, the results of calculating the amplitude and phase of the signal best approximate the field experimental data. Based on the good agreement between the experiment and the theoretical calculations taking into account the interference of electromagnetic shielding and the Bloch–Siegert effect, it is demonstrated that this model should be used to solve the direct and inverse problems of MRS of underground aquifers. The results obtained allow us to calibrate the MRS method and use it not only as an indicator of underground water but also as a means of quantitative measurement. The necessity of taking into account the Bloch–Siegert effect in real field conditions is demonstrated by the example of MRS studies in the Ebro River valley (Spain). When taking into account only electromagnetic shielding (without regard to the Bloch–Siegert effect), the signal amplitude differs nearly twice at the maximum pulse intensity for experimental data, and the signal phase differs 12 times at the maximum pulse intensity. Therefore, in previously published works, approximation of the experimental data using the model without regard to the Bloch–Siegert effect was made by adding layers at depths close to the maximum for this type of antenna (approximately 50 m). In reality, however, these layers do not exist; they appear only as an artifact of the model without regard to the Bloch–Siegert effect. The best approximation of the experimental data can be achieved by taking into account the interference of electromagnetic shielding and the Bloch–Siegert effect.