天然电场选频测深法在地下水勘探中的异常理论分析与实践应用

选频法是音频大地电场法的进一步应用与发展。本文通过实践应用说明选频法在浅层地下水勘探中的有效性,并对选频法测深极距(MN)与地下水埋深之间的关系开展对比分析和初步理论研究。首先,采用水平交变电场、交变磁场共同作用下的均匀半空间中低阻导电球体简化地质地球物理模型,对选频法测深曲线开展正演计算;然后,对选频法在广西“十二五”农村饮水安全工程应用中的131口钻井出水量情况进行统计,并对其中98口钻井的钻探情况开展详细列表统计分析,对比研究测深曲线异常处MN极距大小与实际钻探出水深度之间的关系。理论分析与实践应用表明,选频法在浅层地下水勘探中效果明显,是一种确定浅层地下水井位的有效方法;同时,实践统计结果表明,选频法测深法异常曲线处MN极距的大小与实际钻探出水深度之间存在1∶1的近似关系,验证了理论模拟计算结果的正确性;另外,本文的研究成果表明,在浅层(<200 m)天然电磁法勘探中,天然电场观测值的大小除了与大地的电阻率、信号的频率有关外,还与电极距大小是相关的。

Theoretical analysis of sounding anomaly and field application of the natural electric field frequency selection sounding method in groundwater exploration

The frequency selection method is a further development of the audio frequency telluric electricity field method. In this report, we illustrate the usefulness of the method in shallow groundwater exploration through field application examples. We studied the relationship between potential electrode spacing MN and groundwater depth and carried out preliminary theoretical analysis. Firstly, under the combined action of horizontal alternating electric and magnetic fields, we established a simplified geophysical model of low resistivity conductive sphere in homogeneous half space, and performed forward calculation on the sounding curve. We then recorded water yields of 131 rural drinking wells in Guangxi Province. In addition, we performed detailed tabular statistical analysis of 98 drilling wells, and compared the relationship between potential electrode spacing MN in abnormal sounding curve and actual drilling water depth. Theoretical analysis and field application show that the frequency selection method can be very useful in shallow groundwater exploration for determining shallow groundwater well location. At the same time, the tabular statistics show that there is 1∶1 approximation between the size of potential electrode spacing MN in the anomaly curve and actual drilling water depth, validating the theoretical simulation results. Moreover, we show that the observed value of telluric electric field is related not only to the Earth's resistivity and signal frequency, but also to the potential electrode spacing in a shallow (< 200 m) magnetotelluric exploration.