Research On Denoising Method And Device Of Magnetic Resoance Sounding Based On Nearby Reference Coil

With the rapid development of China's economy,the demand for water resources exploration and underground engineering disaster water exploration continues to increase.Magnetic resonance technology is a geophysical method to detect water bodies directly,but when disturbed by strong electromagnetic noise,the signal-to-noise ratio obtained by magnetic resonance sounding detection instruments is low,which seriously affects the accuracy and reliability of inversion interpretation.Increasing the number of superposition can improve the signal-to-noise ratio,but the degree of improvement is limited and the collection time has increased,which reduces the work efficiency.Using the multiple remote reference coils can also improve the data quality.While the laying of the remote reference coils is complex,and the correlation with the detection coil is low,and the noise cancellation capacity is limited,so it is not suitable for the environment with uneven spatial distribution of strong electromagnetic noise.This paper carries out a series of studies on the above problems,and the main work is as follows:1.The relationship between the kernel function and the changes of the number of turns,water content,aquifer position and aquifer thickness of transmitting and receiving coils has analyzed by numerical calculation.It has concluded that the separation of multi-turn transmitting and receiving coils is beneficial to improve the excitation effect and signal acquisition ability,and the effects of different levels of external noise on magnetic resonance signal parameter fitting have analyzed.By adopting different denoising methods to remove different types of external noise,it has conclude a large number of system noise residues in the data after data processing,and the residual noise should be further suppressed or eliminated to provide a theoretical basis for the work of this paper.2.For the traditional reference noise cancellation(RNC)method when using the remote reference coil,the environmental random noise correlation decreases with distance,resulting in limited noise suppression ability of the remote reference coil.I proposed a nearby reference noise cancellation(NRNC)method based on modified transfer function,which improves the noise cancellation ability by improving the noise correlation between the reference coil and the detection coil.The working principle and implementation steps of noise cancellation have described in detail.To verify the effectiveness of the NRNC method by using the simulation data.Compared with the traditional RNC method.It has proved that NRNC is superior to the traditional RNC method.Three nearby reference coil laying methods are given,and the effects of the number,position and direction of reference coils on the reference denoising effect are analyzed through the measured data in the laboratory,which shows that the noise elimination effect of nearby three-axis reference coil is the best.Compared with the power frequency harmonic modeling method,the NRNC method can not only effectively remove the power frequency harmonic interference,but also remote the random noise related to the detection coil,which is better than the power frequency harmonic modeling method in removing noise and improving signal-to-noise ratio.3.To solve the problem of low correlation between the original reference coil and the detection coil and the limited ability to improve the signal-to-noise ratio.A reference coil design and evaluation method based on correlation has proposed.To determine the electrical parameters of the reference coil,and a fixed three-axis reference coil that meets the 2m detection coil has designed.The correlation between the detection coil and the three reference coils is 0.321,0.379,and 0.347,respectively.The signal-to-noise ratio increased from 18.1 dB to 29.87 dB from the original reference coil,which improved the noise suppression effect of the RNC method.4.In order to solve the problem that the low working efficiency of the magnetic resonance system when using NRNC method.By analyzing the time consumption of each stage of the magnetic resonance instrument measurement sequence,the one-charge multiple-discharge working mode based on matching energy storage capacitor has proposed.By configuring the appropriate energy storage capacitor and carrying out only one charge,the transmission and signal acquisition from the maximum pulse moment to the small pulse moment.The working efficiency of the magnetic resonance groundwater detection instrument has improved without changing the original instrument structure.The average time of single launch and data acquisition has reduced from 10.54 s to 2.75 s,which the work efficiency increased by 4 times.5.Finally,field experiments have carried out on the three-axis nearby reference coil and the NRNC denoising method.The denoising results show that the nearby reference coil is superior to the remote reference coil,and the applicability of the designed three-axis reference coil to the nearby reference denoising method is verified,and it is further explained that the stronger the environmental noise is,the more significant the signal-to-noise ratio of the nearby reference denoising method is.It provides the research basis and technical guarantee for the magnetic resonance groundwater detection instrument to work in the urban and underground engineering environment with strong noise background.The main innovation lies as follows:1.In order to solve the problem that the noise suppression effect of the current reference denoising method is limited when the spatial noise distribution is uneven and the signal distortion occurs when the reference coil is close to the detection coil,a nearby reference coil noise suppression method based on modified transfer function is proposed.The noise of the first half of the detection channel has estimated by using the transfer function calculated from the second half of the data of the reference channel and the detection channel.The measured data processing results show that the magnetic resonance signal distortion has avoided while the correlation noise has removed,and the signal-to-noise ratio has improved to 18.1dB.2.A reference coil design and evaluation method based on correlation degree is proposed.By constructing the ratio relationship between the transfer coefficient of the reference coil and the detection coil and the product of the effective area,the optimal parameters of the reference coil are determined to increase the correlation of the coil to improve the noise suppression effect.For the 2m square 90-turn detection coil,a three-axis reference coil has designed and manufactured.The correlation of the coils is greater than 32%,reaching a significant correlation level.The field measured data processing results have improved the signal-to-noise ratio by 29.87 d B.3.Aiming at the problem of low working efficiency of magnetic resonance system when using the NRNC method,the one-charge multiple-discharge working mode based on matching energy storage capacitor has proposed.By configuring a suitable energy storage capacitor and charging the capacitor only once,it can be transmited from the maximum to small pulse moment transmission and signal collection,and the average single transmission and data collection time has reduced from 10.54 s to 2.75 s,working efficiency increase nearly 4 times.