绝热脉冲磁共振地下水探测技术数值模拟及影响分析

Numerical simulation and analyze of magnetic resonance sounding with adiabatic pulse for groundwater exploration

磁共振测深技术能够定性、定量探测地下水,且信息量丰富,成本较低,在水资源探测领域有较好的发展前景.传统磁共振探测主磁场为地磁场,信号仅为纳伏级,极易被环境噪声淹没.绝热脉冲等信号增强方法是当前国际磁共振领域关注的研究热点.通过满足绝热条件的连续变幅、变频发射方式,能够有效增大磁共振信号幅度,改善探测信噪比.但绝热技术激发原理复杂、建模实现困难且计算量极大,相关理论报道较少.针对以上问题,本文从基本磁共振原理及绝热条件公式出发,推导了基于绝热脉冲技术的磁共振横向磁化强度及灵敏度核函数计算方式,通过插值实现了绝热脉冲的高效正演建模,并比较了几种不通发射参数对信号幅度的影响,为磁共振绝热脉冲仪器的研发提供理论支撑.相比于传统技术,在相同激发电流条件下,匹配80 ms发射时间,绝热技术能够在深部达到最大16.56倍信号增强效果.本文的研究结果,将为磁共振技术在城镇等复杂噪声环境地区的应用提供有力支撑.

Magnetic resonance sounding(MRS)technology used to detect groundwater directly and quantificationally,which owns the advantages of rich information and low cost.In these years,it has shown significant potential applications in hydrological detections.Considering the traditional measurements with the geomagnetic field,the nano-valt MRS signals are easy to suppress to environmental noise.As one of the MRS signal enhancement methods,the adiabatic pulse was quite popular recently.It is transmitted with variable frequency and amplitude pulse satisfying the adiabatic condition,which can enhance the signal amplitude and signal-to-noise ratio several times.However,there are only a few reports about this method,especially its geophysical modeling.Thus,we introduce the calculating progress of transverse magnetization and kernel function for this method.By employing the interpolation,the computation cost of forward modeling is obviously reduced.Moreover,several different settings of adiabatic transmitting factors are also proceeded to obtain certain results.In conclusion,a pulse moment with 80 ms transmitting duration and 30 quality factor produces a maximum signal amplitude that is enhanced 16.56 times for deep areas.The research in this paper could provide powerful support for MRS method used in noisy environments.