地震监测中的电阻率层析技术

RESISTIVITY TOMOGRAPHY IN EARTHQUAKE MONITORING

将电阻率层析技术应用于地震监测 ,由视电阻率值重建出介质的真电阻率图像 ,取得了较好效果 .在宝坻台和昌黎台设立的两个试验点上于 1 998年 4月 1 4日ML5 .0和ML4 .4唐山两次地震前的 6天左右层析图像出现了明显的异常变化 :宝坻台的电阻率图像一改通常的在平稳背景上叠加小量随机变化的特点 ,出现了一种有序性特征的新图样 ;昌黎台也出现了反差加大、高低阻区集中分布的有序性特征 .上述异常图样均在震后消失 .层析图像在震前出现的熵值降低可能是这次地震活动的前兆 .地层导电结构和孔隙度的变化 ,是引起电阻率异常的直接原因 .研究表明 ,常规的单一极距的视电阻率测量之所以对真电阻率的变化不敏感且丧失空间分辨能力 ,是由于该物理量同时具有积分值和位函数这两个固有特征所决定的 .在地震监测中 ,改造现行观测系统 ,引入层析成像 ,是发展技术的方向 .

Resistivity tomography was applied to earthquake monitoring in the present study. Through inversion for apparent resistivity, the image of true resistivity was reconstructed. This method well reveals the nature of the variation of the relevant physical parameters during seismogenic process. At two stations in North China, Baodi and Changli, obvious anomalous changes appeared in the tomograms 6 days before the occurrence of two earthquakes with M L5.0 and M L4.4, respectively, in Tangshan on 14 April 1998. The distribution of true resistivity in the images of Baodi station showed a new pattern, i.e. an ordered state, instead of an ordinarily stable pattern with an even background. The resistivity image of Changli station showed distinct high and low resistivity areas and an ordered pattern, too. After these earthquakes, the above mentioned phenomena all disappeared, while the basic image pattern relating to geological structure beneath the stations did not change much. As an indicator of the ordered state of an image, the entropy of tomograms at two stations all decreased distinctly before the earthquakes, which might be the precursors to these earthquakes. It is inferred that the present resistivity anomalies were mainly attributed to the changes of the electrical conducting structure and porosity in the soil layer under increasing tectonic stress before earthquakes. The present result shows that in a conventional apparent resistivity survey with one fixed electrode spacing the observational results are not sensitive to the change of true resistivity and have not basic spatial resolution. These shortcomings result from an essential property of apparent resistivity being both an integral value and potential function. Adopting resistivity tomography in the earthquake monitoring has become necessary to improve our present observational system.