地下磁共振响应特征与超前探测

Characteristics of underground magnetic resonance and advanced detection of hazardous water

地下(矿井和隧道内)超前探测灾害水源是磁共振测深(Magnetic Resonance Sounding,MRS)方法应用的新领域,在地球物理方法中是一个难题.本文在地面磁共振探测理论的基础上,建立地下全空间模型,推导直立线圈的磁共振响应信号表达式,对比国际标准模型验证了数值计算的准确性.引入旋转系数矩阵,计算任意地磁场方向和线圈方向的激发场垂直分量.研究了磁共振响应信号与线圈法向偏角和倾角的关系,指出当线圈法向方向垂直于地磁场方向时,磁共振响应信号最大.同时,研究表明磁共振超前探测距离与激发脉冲矩和接收灵敏度紧密相关,激发脉冲矩越大,接收灵敏度越高,则超前探测距离越大,但存在极限距离.在地下线圈尺寸受限的情况下,为使检测信号灵敏度为5nV时,超前探测距离达到30m,提出了边长2m线圈的匝数优化方案,共圈模式最少需要100匝,分离线圈模式最少需要10匝发射线圈和160匝接收线圈.仿真模型试验结果证明,随着噪声水平增大,磁共振超前探测距离和反演分辨率均减小.泽雅隧道探测实践表明,本文提出的地下空间磁共振理论在矿井和隧道环境中进行超前探测是可行的.

Advanced detection of underground hazardous water in mines or tunnels is a new application field of the magnetic resonance sounding（MRS）method,as well as a new challenge in geophysics.In this paper,on the theoretical basis of the surface MRS method,a model of underground MRS of whole space is constructed,and the expression of MRS signal with a vertical coil is derived.Numerical calculation and comparison with the international standard simulation model verifies the accuracy of this formula.By introducing the rotation coefficient matrix,the vertical component of the exciting field with arbitrary directions of the geomagnetic field and the coil is calculated.The relationship between MRS signal and coil declination angle,as well as inclination angle is studied.The results reveal that when the normal direction of the coil is perpendicular to the earth magnetic field,MRS signal is the largest.In addition,the advanceddetection distance of MRS is closely related to exciting pulse moment and receiving sensitivity.The larger exciting pulse moment and receiving sensitivity are,the longer advanced detection distance is.However,this distance has a limit value.In the condition of constrained coil size of 2 m,the receiving sensitivity of 5 nV and the advanced detection distance of 30 m,an optimization scheme of various coil modes with transmitting and receiving coincident coil or separated coil is proposed.The coincident coil mode needs 100 turns at least,while the separated coil mode needs10 turns of transmitting coil and 160 turns of the receiving coil.Tests on a synthetic model show that the advance detection distance and the inversion resolution of MRS decrease with the increasing noise level.The application of this method to the Zeya Tunnel demonstrates that the underground whole space MRS theory for advanced detection of hazardous water proposed in this work is feasible in mines and tunnels.