Borehole geo-electric resistivity observation is an effective method to reduce or eliminate the earth's surface interference and as a result to improve the accuracy of observed data. It has been developed rapidly in our homeland in recent years. However

there are some key technical issues that need to be solved.Among them

the long-term stability of the configuration system is an important issue. The most important factor affecting the stability is the insulation performance of outdoor wire-line. This wire-line is often buried in a borehole deeper than 100 meters. Its insulation performance might deteriorated in a long-term observation

which thus may cause a wrong measured resistivity. Based on theoretical analysis and numerical calculation

we study quantitatively the influence of the insulation performance of outdoor wire-line

and then put forward the technical requirements. It might be a technical reference for the design

construction and operation of borehole geo-electric resistivity observation system at a station.