Enhanced Detection of Water and Mud Inrush Hazards in Tunnel Engineering: A Multi-Off-Resonance Strategy for Underground Magnetic Resonance Sounding

Water and mud inrush represent some of the most catastrophic geological hazards encountered in tunnel engineering. Underground Magnetic Resonance Sounding (UMRS) holds significant potential for prospecting hydrogeological parameters within adverse geological bodies. The implementation of the method is limited, however, by the challenge of undesired frequency offsets between the assumed and true Larmor frequencies and poor signal-to-noise ratios in the tunnel environment. For the adaptation of UMRS to the tunnel environments, accurate modeling considering the off-resonance effects and magnitude enhancement of received signals is required. The traditional UMRS application assumes that on-resonance excitation is valid for any circumstance. Neglecting the effects of undesired frequency offsets produces a significant influence on amplitudes and phases of UMRS signals, as demonstrated by our models. Moving beyond the on-resonance excitation condition, we focus on a primary study of a novel multi-off-resonance excitation method using a broadband pulse, in which the off-resonance effects are exploited for improving signal magnitudes of UMRS. To implement the method we proposed, a new excitation pulse with several spectral peaks in a finite bandwidth is presented. Each spectral peak of the excitation spectrum contributes to the response voltage according to its spectral amplitude and offsets to Larmor frequency. The spectrum of the new excitation pulse can be modulated according to demands. The feasibility of the excitation pulse and method are supported by synthetic experiments using three different pulse parameters. Significant magnitude enhancement in the sounding curves is presented in the occurrence of undesired frequency offsets with different magnitudes. Furthermore, the method we proposed provides signal enhancement for the deeper water occurrence in the presence of an undesired frequency offset. We note that the present study is a theoretical and numerical proof-of-concept investigation. Experimental validation, including laboratory-scale physical model tests and field tunnel measurements, is planned as future work once suitable transmitter instrumentation becomes available.