台址下表层介质电阻率变化对视电阻率观测值影响特征的初步研究

Primary Research of Interference on Observed Apparent Resistivity Value Due to Resistivity Variation in Surface Layer under Seismic Station

气候的交替变化将导致大地表层含水量发生变化 ,从而使该层介质的电阻率发生变化 ,进而使得观测结果 (即视电阻率 )发生变化 ,显然这对地震前兆观测而言是一种干扰。目前我国绝大多数前兆地电阻率台采用单极距观测方式 ,因此观测到的视电阻率变化应是表层变化 (主要是干扰 )和观测范围内深部变化 (地下介质应力与应变状态发生变化 )的综合反映。作者先依据台址下的水平层状模型进行了模拟计算研究 ,发现大地表层含水量的变化对单极距四极装置视电阻率观测值的影响有两个特征 :(1)无论台址下电阻率结构是下伏低阻还是下伏高阻 ,表层介质电阻率升高对观测值的影响都很小 ;(2 )表层电阻率降低对观测值的影响很明显 ,且对下伏高阻结构的影响大于对下伏低阻结构的影响。然后用视电阻率实际观测资料说明了将其特征用于预报的可能性。

The alternative change of weather will lead to the variation of water amount in surface layer and then the variation of resistivity in the same layer,therefore it will lead to the variation of apparent resistivity observed at seismic station. Obviously,this is a kind of interference to earthquake precursory monitoring. At present,the measuring systems with only one pair of fixed potential poles designed are used at most of the seismic station in China,so the variation of observed apparent resistivity is the synthetic response of true resistivity variation in surface and deep layers which are in the range of monitoring. Firstly,by analytical calculation,the authors find that the influence of water amount in surface layer on observed apparent resistivity posses two characteristics: (1) no mater the deep layers are resistive or conductive relative to the surface layer,the influence of resistivity increase in surface layer on apparent resistivity is very small; (2) the influence due to resistivity decrease in surface layer is quite distinct,and it is bigger in the case the deep layers are resistive relative to the surface one. Then the possibility of using the first characteristic in earthquake prediction is explained with observed data in seismic station.