流体饱和岩样的震电勘探实验测量研究

Experimental studies on seismoelectric prospecting measurements in a fluid saturated rock sample

震电勘探结合了传统地震波勘探和流体饱和孔隙介质的动电耦合特性,利用地震波诱导的震电转换信号来评价被测介质的特征参数,是近年地球物理领域广泛关注的一种勘探测量新方法.与前人大多只测量介质外部流体相中的震电转换信号不同,本文针对固相介质内部震电波场的耦合特性,在实验室内开展岩样内部震电转换的观测实验研究.基于动电耦合机理,建立适用震电勘探测量的实验系统,制作内嵌测量电极的岩石样品,并通过流体中界面电磁波信号的比较,验证了镀银电极测量震电信号的有效性,进而开展岩样内部界面电磁波和伴随体波震电转换电场信号的实验测量,分析不同条件下两种转换波的传播特性和频率响应特性.实验结果表明:介质内部传播的界面电磁波信号和伴随纵波电场信号是可测量的,其均值在几十纳伏量级,并且幅值随源距的增大呈现指数规律衰减;随着声源频率的提高呈现先增大后减小的趋势,但伴随纵波电场信号对频率的敏感性更强.本文实验结果验证了Pride理论关于伴随震电场的预测,同时为震电勘探数据解释提供参考.

Seismoelectric exploration is a new prospecting method that combines the conventional seismic exploration with the electrokinetic coupling properties of fluid-saturated porous media to assess the media characteristics and has been widely focused on the geophysical field in recent years.Unlike most previous works that only measured and studied the interfacial electromagnetic(EM)waves in fluid phase outside the medium,we conduct the measurements inside the rock in the laboratory to investigate the seismoelectric coupling characteristics in solid phase.Based on the electrokinetic mechanism,we build an experimental system for seismoelectric measurements and embedded electrodes in a rock sample.The validity of silver-plated electrodes for measuring seismoelectric signals is verified by comparing the interfacial EM wave signals recorded in water with different kind electrodes.Then we measure the seismoelectric EM waves and the coseismic electric signals propagating inside the rock sample,and further analyze the propagation characteristics and frequency responses of these two conversion signals under different conditions.The experimental results show that the seismoelectric interfacial EM waves and the coseismic electric fields accompanying the P waves are measurable inside the media with mean values around tens of nanovolts.Their amplitudes both decay exponentially with increasing source-receiver distance and show an increase followed by a decrease with increasing source frequency,but the coseismic electric field accompanying the P wave is more sensitive to the source frequency.The results in this paper prove the prediction of Pride's theory on the coseismic electric field,and provide a reference for the data interpretation of the seismoelectric exploration.