Research On Full-relaxation Signal 3D Forward And Inversion Of Tunnel Nuclear Magnetic Resonance

Recently years,with the tunnel construction focused on western mountainous areas of complex terrain geological conditions and karst areas,a large number of deep tunnel projects emerged.There is an urgent need for a method to accurately predict disaster water in tunnels.As a geophysical method of direct water exploration,nuclear magnetic resonance(NMR)has achieved a few applications in water exploration on the ground.In order to further explore the application of this method in disaster water forecast in tunnels,to achieve the location and occurrence state of water abnormalities,this paper attempted to establish a three-dimensional forward and inversion method based on NMR full-relaxation signals.The traditional NMR kernel function is calculated mostly based on the uniform half-space or layered dielectric,ignoring the excitation magnetic field affected by underground resistivity distribution.In this paper,the finite element method was used to simulate the 3D kernel function distribution in front of the tunnel face with underground electrical abnormal bodies fully considered: Starting from double-rotation equation satisfying the magnetic field,the solution of double-rotation equation was transformed into the solution of the functional stagnation.The transmitting current source was loaded by introducing a pseudo-delta source to avoid singularity.The influence of "weak solution" was eliminated by forced introducing of divergence condition.The excitation field component perpendicular to the direction of the geomagnetic field could be calculated with the help of two sets of rotation matrices.The exponential decay term was added to the kernel function to simulate the NMR free induction decay(FID)signals.In order to investigate the influence of surrounding rocks,geomagnetic field,launch coil and water-bearing structure parameters on the NMR FID signal characteristics,we discussed parameters such as surrounding rock resistivity,geomagnetic field inclination,launch coil turns,water body scale,water body resistivity,the distance between water body and tunnel face and water body horizontal relaxation time(T2).Comparative analysis showed that low resistance surrounding rock and geomagnetic field inclination have a considerable impact on the NMR signal.The launch coil turns Significant changes the phase.Large scale and close distance water-bearing structure have a great influence on the NMR signal while local low-resistivity abnormal bodies have a certain influence.The water body horizontal relaxation time affects the signal decay rate.These simulations provide valuable references for 3D inversion of tunnel NMR.The least squares objective function containing penalty terms was established with its solution constrained.The 3D inversion was realized by using the optimization algorithm based on inequality constraint.The multi-turn small coil model with side length of 6 meters and side length of 4 meters were simulated respectively to explore their effective detection depth.Results showed that the effective detection depth of 6 meters coil is about 25 meters while 4 meters coil is about 18 meters under the model parameters in this article.The water-bearing and mud-bearing anomaly models were simulated respectively to explore resolution.Results showed that front-after distribution water anomalies have stronger resolution than up-down distribution water anomalies.Model calculations showed that the 3D inversion method achieved good results.Not only to locate aqueous anomalies in front of the tunnel face,but also to provide water content information under different relaxation time.The method of this paper could indirectly get the existence state of disaster water and realize the prejudgment of sudden flood disaster,which provides valuable reference for tunnel disaster management.