Research on focusing and deflection effect of detection with DC focusingmultipoint current sources in tunnel

In order to increase the horizontal exploration depth of direct current method，find out accurately the specific direction of the abnormal body in front of the tunnel face，and reduce the influence of electromechanical equipment，track，terrain fluctuation，electromagnetic interference and other factors on the detection，forward modeling of focusing and deflection effect of detection with DC focusing multipoint current sources were carried out. According to the detection mechanism of the focusing electric method，the potential values of multi-point power sources in tunnel is solved by using the finite element method，and the differential equation of the electric field line of the space field is derived. We can build a three-dimensional geoelectric geometric model with COMSOL，then change the emission current intensity of the shield electrode and the main electrode to simulate the focusing distribution characteristics of the space current field and the change rule of the potential change percentage curve in different abnormal geological conditions by finite element method. The simulation results show that focusing effect detection can increase the distribution density of electric field lines in front of the tunnel face and make the detection of abnormal bodies more sensitive;when there is an abnormal body in front of the tunnel face，the effect of focusing effect detection is better than without focusing effect detection，and the detection distance can be increased by at least 50%,and as the shielding current increases，the detection effect is better,but the detection effect of the abnormal body 18 m away from the tunnel face has little change. When the abnormal body is a low-resistance body，the percentage of potential change is negative，otherwise it is positive. When the abnormal body deviates from the central axis of the tunnel，the detection effect deteriorates gradually with the increase of the deviation distance. At this time，by changing the size of shielding current to detect electric field used for deflection scanning detection，the specific direction of the abnormal body can be determined according to the changing rule of percentage of potential change of the measuring electrode in all directions around the main electrode. This research is of great significance for interpreting accurately the data of the specific direction of the abnormal body and creating the exploration technology of focusing electric method.