Bloch–Siegert Effect for Surface Nuclear Magnetic Resonance Sounding Experiments in the Unsaturated Zone

Abstract The Bloch–Siegert effect is relevant for NMR experiments where components of the excitation pulse other than circularly polarized component have an influence on evolution magnetization spin system under consideration. For linearly fields this happens at amplitudes higher roughly one tenth magnitude static magnetic field. Since surface nuclear resonance (SNMR) experiments, also called magnetic-resonance-soundings (MRS), are conducted in relatively low local field earth, can quickly become relevant. This especially case SNMR unsaturated zone, due to short relaxation times fast pulses high intensity must be used. To describe effect, we use average Hamiltonian approximation obtained by Magnus expansion up fifth order, as well solution Bloch equations. results these approximations tested against simulations and it shown that they only valid limited ranges amplitude. sensitivity kernels described verified with experimental data a small scale sensor water containers.