The Research On The Theory And Techniques Of Multi-components Observation In NMR Advanced Detection

Potentially serious water-disaster situations are being confronted during the construction process of many underground engineering projects in China.The existing geophysical methods used to detect water-bearing structures in front of the tunnel-face are indirect.These methods are based on the difference in some physical parameter such as conductivity,density,magnetism,elasticity,and thermal conductivity between the water-bearing structure and normal strata with the purpose of detecting the extent of hazardous water.In addition,the construction environment of underground engineering projects is complex,and many water-bearing structures may be well-concealed.The accuracy of these methods is therefore not very satisfactory.However,because of its characteristics in exploring water directly,the quantitative inversion,and the abundant information,nuclear-magnetic-resonance(NMR)technology has been proposed by some experts in our country to detect water in front of the tunnel face.To develop a new geophysical method for hazard water detection in underground engineering sites,in this paper,the basic theory and techniques are presented,and field tests of an advanced NMR detection system is described and discussed.While the idea of applying NMR in advanced detection has just been proposed,the theoretical system is not complete.Hence,from Maxwell’s equations,a formula is derived for the magnetic field transmitted by the working coil over the entire space domain.Using the Hankel transform of the Bessel function and its bulk asymptotic properties,a numerical calculation for the excitation of the magnetic field was performed.Next,from the calculation of the vertical component of the magnetic field in the underground space and the decomposition of the elliptical polarization,a forward simulatioin of the response signal for NMR predictions was performed.Also,by setting up a special high resistivity model,the correctness of the forward simulated signals is verified using the commercial software Samogon.Moreover,based on the amplitude of the NMR response signal,we define the exploration distance used in advanced NMR detection and analyze the influence rules regarding the geographical location,resistivity,and loop parameters within this distance.The results indicate that the exploration distance varies greatly because of the different geographical location.Generally,because the geomagnetic field at high latitudes is stronger,the exploration distance is much greater.The effect of resistivity on the detection distance is apparent mainly in the low resistivity range;when the resistivity is greater than 100 Ω m,the change in resistivity has almost no effect on the exploration distance.The dependence of the exploration distance on loop parameters is mainly through the effective area of the loop.The simulated results illustrate that larger effective areas provided a greater exploration distance.As long as the effective area of the excitation and the receiving loops remained constant during operations,the detection distance does not change significantly.The working space to undertake NMR measurements is often very limited.To collect as much information as possible in this limited space,multi-component observations are an effective way to overcome the problem.However,there are several issues related to such observations for advanced NMR detection and they are discussed in detail in the paper.First,combined with the models of the four typical hazardous water sources,namely,Ordovician limestone water,water-bearing faults,goaf water,and water-bearing collapse columns,I analyze the signal characteristics under different excitation components and different receiving components.The research results show that,when water-bearing structures were excited by different components,plate-like water bodies,such as Ordovician limestone water and water-bearing fault,could be effectively excited using both X,Y and Z component direction.However,of the three components,only the component for which the loop plane is parallel to the water layer needs the minimum optimum pulse moment.In other words,this component provides the most energy saving.For models with a more limited water volume,such as goaf water and water-bearing collapse columns,the best excitation component was determined by the position of the body of water.For a body of water located in front of the tunnel-face,the best excitation component is the Z component;when the body of water is located either side of the tunnel,the Y component is the most appropriate to used.However,when the body of water is located above or below the tunnel floor,whether the X component is the best direction depends on the tunnel azimuth.Second,for Ordovician limestone water and water-bearing fault,when the excitation direction aligns with the Z component and the NMR signals are received in all directions,the Z component of the received signal is always the strongest being the best receiving direction.Similar to the multi-component excitation result,for goaf water and water-bearing collapse columns,the component with the best reception was also related to the location of the body of water.Moreover,while receiving the signal from a water-bearing fault,although the signal obtained by the X component was weaker,it was nevertheless sensitive to changes in the dip angle of the fault when compared with the signal received in the Z component.For goaf water,the Y-component signal was not as strong as the Z-component signal,although it was more sensitive to the change in angle between the goaf water and the tunnel than the Z component.The technology associated with advanced NMR detection is still in its infancy,and the methods used in data acquisition in this practical stage are not clear.In this paper,the imaging effects of several multi-components observation methods in different models were discussed with very fast simulated annealing algorithm.A summary of various problems needing attention in the future work is given.The technology of NMR advanced detection is still in its infancy,and the data acquisition method in its practical stage in the future is not clear.In this paper,the imaging effects of several multi-components observation methods in different models were discussed with very fast simulated annealing algorithm,and the following problems need to be noticed in the future work were summarized.(1)The NMR advanced detection method studied the whole space problem,and the NMR signals produced by the water body in the symmetrical position in the underground space had good similarity.This similarity would result in a false anomaly which was symmetrical to the real model in the inversion process.The shielding technique provided a good idea to solve the problem of multi-solvability in the inversion process.(2)By comparing and analyzing the imaging results of several multi-components observation methods,the fan-shaped observation modes employing a vertical rotation and a horizontal rotation of the working loop were found to give the best imaging results but low working efficiency.In addition,when the quantity of data is the same,the image quality associated with the three components of the receiving mode was close to that from the two fan-shaped observation modes mentioned above.(3)If an Ordovician limestone water layer is below the tunnel floor and 2D inversion imaging is performed directly with multi-components observation data,the imaging result would not be very good.In this instance,a 1D detection of the X component should be performed before the 2D imaging.The 1D inversion result of the Ordovician limestone water may then be used to constrain the water distribution range in the 2D inversion process thereby improving the imaging quality.(4)In addition,measuring the tunnel azimuth during advanced NMR detection was found to be necessary.In the inversion process,if the value of the tunnel azimuth value was set too far from its actual direction,the accuracy of the inversion result is greatly affected.To test the correctness of the above research results,some field tests were performed in the process of the study.The signals collected in the simulated tunnel in Handan were in agreement with the results of the proposed model,which was established taking the known body of water in the test field in account.Moreover,the water distribution obtained from the inversion result was consistent with actual situations at the test site.The experimental results proved the reliability and effectiveness of this advanced NMR detection method.The results from two groups of multi-component excitation tests performed in Wuhan showed that the multi-component excitation mode was able to delineate the main locations of water,but there is still a difference between the inversed water content and water content of the actual water-bearing structure.In conclusion,the above research results provided a theoretical basis and techniques for advanced NMR detection and thus have some theoretical and practical value.Advanced NMR detection offers a means to detect hazardous water sources during the construction stage of underground engineering projects thereby mitigating risk and ensuring safety in tunnel boring operations.