Simulation Research Of Focusing On The Current Method In Tunnel Ahead Exploration

The tunnel ahead detection with direct current(DC) resistivity method has been widely used in lateral logging, ahead prediction of tunnel construction, mining exploration and underground engineering because of its high exploration efficiency and simple outdoor operation. However, when used to tunnel ahead exploration, DC resistivity method is always influenced by the tunnel cavity and the side interference. Focusing on the current method has drawn attention of many researchers since the emergence of BEAM instrument. The thesis applies finite element simulation to study the detective principle, electric field distribution,electrode connection of focusing on the current method and the shape and position of the soil box. Furthermore, the thesis designs the powered measuring system and conducts field test.First of all, the article analyzes the effect of traditional three-pole tunnel ahead exploration, finding that measuring electric potential has no obvious resistivity anomalies on the same radius from abnormal body to working face, indicating that DC resistivity method exists error in judging the position and electrical properties of abnormal body.Secondly, the article introduces theoretical foundation and detective principle of focusing on the current method and puts forward a new comprehensive and scanning measurement system. The finite element simulation is used to analyze the feasibility of focusing on the current method.And then, the article provides a new connection method of powered measuring electrodes and shield electrodes, which connects shield and hob measuring electrodes with a small resistance and powers measuring electrodes via shield, not only guaranteeing the equal voltage of the shield electrodes and measuring electrodes, but also avoiding the voltage feedback control of the shield electrodes. Feasibility of this measurement method in qualitatively judging abnormal body is verified through simulation.Next, the article studies the soil box shape and position in the soil simulation experiment of focusing on the current method. Three simulated soil boxes with different shapes of spherical, cylindrical, conical and different positions are compared to the model of infinite far geological conditions. The conclusion is the conical shape soil box with certain centerposition offset can max approach the electric field distribution in infinite far geological conditions and it is easy to be machined.Finally, a complete measurement system is designed, which includes a variable frequency signal generator module, constant current driver module, computer system, and data acquisition module. This measurement system is tested in laboratory, Wuhan airport and Xi'an cited the Han to the Wei project.