Research On Extracting The Off-resonance Effects By Sweeping Frequency In Tunnel Nuclear Magnetic Resonance

In recent years,as the country to promote the construction of high-speed railway,more and more tunnels emerge in Western of China where the altitudes are higher.In the process of tunneling,the unknown situation in front of the face of the tunnel needs to be effectively detected.In addition,the mountainous peaks,continuous fluctuations and complex geological conditions in the western of China will lead more difficult problems to the construction of the tunnel.With the improvement of people's life and science technology level,various geophysical exploration methods can achieved their own effective exploration methods.In the detection and prediction of water resources and disaster water,nuclear magnetic resonance(NMR)can perform a better role in most geophysical methods because it can effectively excite hydrogen protons in water.This method can effectively detect aquifers in shallow surface.It is a more effective method to directly detect the water storage and spatial distribution of aquifers in some areas in water prospecting and tunnel disaster prediction field.Due to the hydrogen protons in water can only transition to high energy level when they are under the specific frequency of Larmor frequency in magnetic field,that is to say,they can be successfully excited.Therefore,in most nuclear magnetic resonance methods,the Larmor excitation frequency under the magnetic field is normally used to excite hydrogen protons in water,and the Larmor frequency of hydrogen protons is determined by the magnitude of the local geomagnetic field.The nonuniform distribution of geomagnetic field,the magnetic materials or rock arounding,and even the diurnal variation of geomagnetic field all can lead to any bad influence in the geomagnetic field,which is often neglected by the constructors,mistakenly believing this value is representing the true geomagnetic field.Therefore,the excitation frequency calculated with such an "original" geomagnetic field will be different from the Larmor frequency of hydrogen protons in real environment.Sometimes hydrogen protons can not successfully transit to high energy level under artificial magnetic field due to an excessive deviation,resulting in some inaccurate effects,which brings any problems for the analysis and interpretation in lots of tunnel constructions.In this paper,according to the actual environment in the tunnel,we use the numerical modeling similaring to the track and other magnets in the tunnel to prove the influence of several magnets by three-dimensional forward modeling of the static magnetic field.It is confirmed that a certain scale of ferromagnetic materials will produce tens to hundreds nanotesla of influence on the static geomagnetic field.Then,three-dimensional forward simulations based on the vector finite element method in the excitation field are carried out to verify the accuracy and practicability of NMR forward modeling.On the basis of the above,a new sweeping frequency NMR method,by changing the emission waveform,is proposed to improve the traditional method.If the Larmor frequency coincides with one of the sweeping frequencies in this process,the on-resonance signal will be generated.Then the effective NMR signal can be successfully extracted by this new excitation method.