Research On Forward Modelling And Signal Enhancement Technique Of Pre-polarization Magnetic Resonance Sounding For Groundwater

For near-surface hydrological investigations,magnetic resonance sounding(MRS)technology is considered an efficient tool.It could directly determine the distribution of subsurface aquifers and the related hydrological parameters,such as water content and porosity.In recent years,it has demonstrated significant potential for urban and underground engineering and detection.However,as the MRS measurement for underground water proceeds into the geomagnetic field(?0.05 m T)environment with alternating pulse excitation(AC),the signal responses are identified as extremely weak,particularly at only nanovolt levels(10^-9V).Evidently,the further developments and applications of MRS technology are limited by the low detection signal-to-noise ratio(SNR)problem.As a result,numerous studies have focused on noise suppression,which has been adopted to improve the SNR.Unfortunately,the related literature remains insignificant for the strong noise interferences,since the high environmental noise submerges the effective signal in the time and frequency domains.Hence,strengthening the SNR and resolution of MRS detection in principle is key to expanding this technology to more applications and fields.Recently,our research group proposed the pre-polarization MRS technique for subsurface aquifer detection,which can increase the background measurement field through DC transmission.Based on the instrument prototype,this method's ability to increase MRS signal amplitude under the geomagnetic field has been proven.Moreover,like the anisotropy of the subsurface pre-polarization field,further studies on high-precision forward modelling and the enhancement of effective exploration range should also be pursued.Thus,in view of these problems,this paper mainly focuses on the forward calculation and signal enhancement technologies of pre-polarization MRS for detections under high noise environments.Meanwhile,the author deduced the theoretical formulas of pre-polarization MRS for modelling simulations.Additionally,this paper developed the corresponding signal enhancement key technologies to increase the measurement SNR and resolution.Based on the field measurement results,the validity and reliability of the research outcomes were verified.The main contents of this paper are as follows:(1)In order to support the MRS data simulation and interpretation,the author established the high precision forward modelling of pre-polarization MRS.Subsequently,to calculate the forward responses,the kernel and signal expressions of pre-polarization MRS were deduced.Simultaneously,the forward modelling was simulated by combining the finite element technique and Curie's Law.Moreover,given that the instrument's pre-polarization transmitting system necessitates the large supply of power that produces high heat from the transmitting coil,the multi-source measurement mode was also proposed according to the thermal power principle.This new configuration could further improve detection sensitivity and resolution.(2)The author derived the pre-polarization field shutoff equations for the transmission system,which could provide considerable support for the instrument design.As one of the crucial issues involving MRS signal enhancement,the decaying law of the pre-polarization field,i.e.,the DC shutoff,should be determined under an inhomogeneous pre-polarization magnetic field distribution.Through theoretical derivation and simulation,the adequate shutoff condition was impossible to achieve by only using the hardware design.Thus,the Bloch equation between the magnetization of groundwater and the decayed pre-polarization field was introduced.By solving this equation based on the present DC shutoff circuit of the instrument system,we could generate a more accurate simulation of the magnetization distribution and updated modelling of pre-polarization MRS.As a result,calculating the signal responses of pre-polarization MRS by combing through the above scheme can effectively improve the forward and data interpretation accuracy.(3)The author proposed the measurement scheme that combines pre-polarization with adiabatic pulses,improving the effective depth and response amplitude of the pre-polarization MRS method.Under the inhomogeneous pre-polarization field distribution,the author introduced the excitation magnetization and signal responses formulas based on the adiabatic pulse with variations in the amplitude and frequency.Due to the complex excitation dynamic of adiabatic pulses with large values,the forward modelling was simulated by combining the technology of real-time rotation matrix and interpolation strategies.This technique could distinctly improve the calculation efficiency.In the end,based on the above theoretical formulas and computation strategies,the simulated experiments verified that the combined MRS detection scheme could further increase the measurement depth and signal enhancement amplitude.It also exhibited a higher resolution even with strong electromagnetic interference.Finally,the pre-polarization MRS field experiments were conducted in the suburbs and downtown Changchun city based on the discussed signal enhancement key technologies.Compared with traditional MRS detection,pre-polarization technology could certainly enhance the SNR even with high environmental noise and produce effective measurement and interpretations of the results.Overall,these outcomes turned out to be consistent with the established facts.Considering the undesired results from the traditional method,the effectiveness of this paper's technique and theory are fully substantiated.In essence,its research content evidently advances the reliability and practicability of MRS technology for groundwater,indicating great significance for the future developments and applications of this method.