Bloch–Siegert Effect in Magnetic-Resonance Sounding

Abstract

The magnetic-resonance sounding (MRS) method is intended for groundwater prospecting. The Earth’s magnetic field used in MRS is of the order of 5 × 10⁻⁵ T. The radiofrequency (RF) field produced in MRS by the surface antenna is linearly polarized and may be comparable with the geomagnetic field. The use of RF pulses at Larmor frequency leads to the appearance of the mean resonance offset in rotating frame (Bloch–Siegert shift) which is caused by the influence of the off-resonance counter rotating component of the RF field. The solution of the inverse problem for ground water distribution must take account of the proton spin resonance frequency shift, because an inaccurate kernel of the respective integral equation may lead to significant errors in the estimation of aquifer characteristics. The value for Bloch–Siegert effect with one order of approximation higher than usual has been calculated. The MRS technique with 100-m-diameter antenna has been used at the frozen Ob reservoir near Novosibirsk, Russia, to detect subice water to prove the improved physical model of MRS. There is a good agreement between measured and calculated data taking into account the Bloch–Siegert and the next-order effects. New physical model accounting the Bloch–Siegert effect allowed a significant improvement of the reliability of the MRS data interpretation.