地电阻率多极距观测反演初步研究

Inversion of geoelectrical resistivity observed with multi-separation of electrodes

地电阻率多极距观测的目的是借助对观测数据的反演获得地下介质中不同层位真电阻率的变化.本文以3层结构为例,对地电阻率多极距观测的一维反演的效果进行了初步地理论研究.首先研究了一维地电阻率结构下观测数据一维反演的模拟效果,得到了各层电阻率值,且与真电阻率值很接近,说明在地下电性结构为一维的情况下,电阻率多极距观测可以区分出不同地层的电阻率变化.其次,考虑到台址下电阻率结构的复杂性,研究了上两层界面存在起伏的情况下,多极距观测数据一维反演的效果,结果显示:当电阻率变化较小时,各层反演得到的电阻率的变化与真电阻率十分符合;当上两层介质的电阻率变化较大时,各层反演得到的电阻率出现畸变,与真实电阻率的变化情况存在一定差别,表明浅层电阻率变化达到一定程度后将会影响对深部电阻率变化情况的正确判断.一般情况下,观测的时间间隔越短,则连续两次观测时段内各层介质的电阻率变化越小,因此缩短多极距观测的时间间隔可能是避免出现上述畸变现象的有效观测手段.

Preliminary theoretical studies were dealt with in this paper on the efficiency of one-dimensional inversion of apparent resistivity data observed with multi-separation array,for understanding the true resistivity variations with time inside the media.To this end a three-layer model has been taken as an example.Firstly,we did the inversion of the theoretical apparent resistivity data by forward modeling calculation for each layer in one-dimensional resistivity structure.The result shows a good consistency between the true resistivity variations and that obtained with inversion of each layer,suggesting that onedimensional inversion technology is adequate for recognizing the variations of resistivity in each layer.Secondly,more complex model was taken due to some interface fluctuation,which deviated from one-dimensional model to some extent.The same process of forward-inversion was conducted in the case of the fluctuation of interfaces of the three-layer model,the results showed that good fitting can be found only in the condition of non-large variations of resistivity of the first-and second-layers,otherwise the inverted resistivity of each layer will turn up distortion.Usually the shorter the time interval of observation,the smaller the medium resistivity change during the two consecutive observation.Therefore,to reduce the distortion,it could be available by taking short time interval of observation with multi-separation array.