井下地电阻率观测中布极参数的确定方法

Determination Method of the Buried Depths and Intervals of Electrodes for Borehole Geo-resistivity Observation

为压制地电阻率浅层干扰、突出深部以地震预报为目的的有用信息,选取了小江断裂中段一个场点作为实例,研究了井下地电阻率前兆观测中确定电极埋深、电极间距等布极参数的方法。结果表明:选择井下电阻率观测布极参数时要考虑影响系数和探测深度2个主要因素,即地下潜水位面以上各地层的影响系数应远小于深部各层,探测深度范围内最底层(受孕震影响最大的层)的影响系数应远大于其它各层,观测系统的探测深度最好不小于已有震例指示的地电阻率观测系统的探测深度。按照该方法选择了所给场点的井下电阻率观测布极参数,即电极埋深200 m、供电极距1 050 m、测量极距350 m,按对称四极布置,可获得最佳观测效果。

In view of the objective of suppressing shallow interference and highlighting the useful information from the depth,we selected a field site in the middle section of the Xiaojiang fault as an example,and developed a method for determining the parameters such as the buried depths and intervals of electrodes in the borehole geo-resistivity observation for seismic precursors. The research shows that two major factors,the influence coefficient and the probing depth,should be considered when selecting parameters for borehole geo-resistivity observation,namely the influence coefficients of all the layers above the water table should be far less than that of layers in the deep. The influence coefficient of the bottom-layer(the layer influenced the most by the seismogenic processes) within the detection range should be far greater than that of any other layers;and at the same time,it is also required that the probing depth of the observation system is not less than that indicated by the existing earthquake cases. Finally,according to this method,we selected the parameters of underground resistivity observation,i.e. the buried depth of the electrode is 200 m,the distance between the power supply poles is 1 050 m ,and the intervals of electrodes is 350 m. The optimal observation effect can be obtained by arranging the symmetrical quadrupole.