层状导电介质中地面核磁共振响应特征理论研究

A STUDY ON SURFACE NUCLEAR MAGNETIC RESONANCE OVER LAYERED CONDUCTIVE EARTH

水平层状导电大地中层状含水层地面核磁共振响应的数值计算技术涉及到导电介质中回线源磁场的计算以及地下含水层中质子磁矩在线圈中产生感应信号的体积积分 .文中采用直接数值积分技术 ,对具有振荡核函数的Hankel变换进行积分 ,以求取回线源磁场的径向与垂向分量 ,计算并研究了回线产生的径向与垂向磁场分量随空间位置的变化规律 .基于磁场空间分布特点 ,利用不等间距空间剖分技术计算地面核磁共振的体积积分 ,模拟了不同模型的地面核磁共振响应并讨论了其影响因素 .结果表明 ,结合能对任意层状导电介质中磁场进行稳定快速计算的直接数值积分技术与不等距空间模型剖分技术 ,可正确模拟地面核磁共振响应 .导电性是产生地面核磁共振信号相位的先决条件 ,但影响响应振幅强度与相位的因素还有含水层的埋深、厚度以及装置大小等 .

Numerical simulation technique of surface nuclear magnetic resonance (NMR) responses from stratified groundwater formation with finite conductivity is concerned with two problems. One is the integration of Hankel transform of a rapidly oscillating kernel function, and another is the volume integration of surface NMR signal induced in a receiver loop from the protons in buried aquifer. In this paper, a direct numerical integration method is applied to the oscillating kernel function integration to obtain radial and vertical components of magnetic fields from a large loop source, and the spatial characteristics of the fields are described. Due to the distribution of magnetic fields, an ununiform space interval subdivision method has been developed to estimate surface NMR response in the receiver loop in order to save computing time. The surface NMR responses from different models have been evaluated, and influences of model conductivity, aquifer depth and its thickness, and loop size have been discussed. Numerical results show that the combination of direct numerical integration and ununiform space interval subdivision is feasible to get surface NMR signal. And the conductivity of formation is the key to generate the phase of surface NMR signal, but other factors, such as groundwater depth and its thickness, loop size, can make the amplitude and phase of surface NMR response greatly different.