Research Of DC Focusing Resistivity Advanced Detection In Tunnel

Abstract:The tunnel advanced detection with direct current (DC) resistivity method plays an important role in mining, tunnel boring and underground engineering, which is characterized by its easy construction and high exploration efficiency. Using this method, many problems of hydrogeology survey, engineering geology and mineral exploration are solved. However, with the tunnel cavity and a variety of engineering facilities in tunnel, such as rail, construction machinery and so on, resistivity measurements are seriously affected. Eliminating the environmental influence is a key technology in tunnel advanced detection with DC resistivity method, which will benefit to accurate and efficient completion of geological exploration. A new method of advanced focus detection for DC resistivity in tunnel is researched and put forward (in this paper we call it DC focusing for short). Different circularity electrode groups are set up in the course of pole arrangement, which makes primary field current be of focusing function like searchlight, and decreases the flanking interference, and can be interpreted immediately, eventually attains focusing detection aim.Firstly, a numerical simulation method of finite element was used to evaluate the feasibility and effect of focus DC current advanced detection in tunnel for several models with2D axial-symmetric structure and3D full space structure. The2-D and3-D target model is divided into finite rectangular elements and finite hexahedron elements with bilinear variation in conductivity respectively, and non-uniform division technology is used to reduce the total mesh number, according to the actual situation of tunnel environment. Then linear interpolation was used to achieve the potential within each element for2-D model. For3-D model, we use bilinear interpolation for the conductivity. Finally, LDLT method is used for solving linear equations. Thus,2-D and3-D numerical simulation of tunnel DC resistivity is implemented.In the second part of this paper, the primary principle of advanced detection with DC resistivity is introduced and third kinds of surveying method of focus are given. And the influential factors and optimum surveying method of the tunnel for DC resistivity advanced focus detection are researched; the results show that the more the side anomalous body closes the tunnel, the greater influence of abnormal curve. The size of tunnel to advanced focusing of the influence can be ignored.A-M-A0array is superior to A-M array in suppress side disturbance respect. It can reach the best detect effect when the transmitting electrode spacing A0A is increased as far as possible in practical work, and the proportion between surveying electrode spacing A0M and A0A is A0M/A0A=0.6.Thirdly, the conductive paper experiments and the clay test are carried out on the basis of numerical simulation method, some effective factors and curve features of focused current advanced detection of tunnels are researched and analyzed by different electrode array and experimental parameters set. The parameters such as power distance, vertical distance (d) between abnormal body to the working face, the distance (L) between anomalous body to the tunnel middle and abnormal size. the detecting effect of the focused current electrodes layout are compared, and the applicability of advanced detection is verified. The results show that Forecast distance is determined by radius of electrodes and size of anomaly body. The bigger the electrode and the anomaly are, the earlier the anomaly body can be detected.The result shows that focus current method can reflect the anomaly obviously, and can be more effective to avoid the influence caused by side anomalies and construction machinery in tunnel. That means the method of focus current method is feasible and effectual in tunnel advanced detection.