Research of ANSYS Simulation Studied on AC BEAM Method in Geological Exploration

AC BEAM induced polarization method plays a very important role in advanced geological exploration. This study established simulation model based on BEAM double-pole detection device in the ANSYS electric simulation module environment. Simulation process includes pre-processing, solution, after processing and then summarizes the relationship between obstacle resistivity and measuring voltage of simulation. Changing location, offset angle, size of the obstacle and the emission electrode, study on the impact of measurement results. Build the experimental platform and compare the experimental results with the simulation results, the change trend of experimental and simulation results is same. The research results have present practical guidance for the BEAM application. Introduction ‘There is a need to tunnel, must be prospected before’ is the principle of all underground prospection engineering. Along with the increasing number of the tunneling engineering, tunneling technology is becoming more and more mechanization and automation. So the advanced prospection level is difficult to meet the requirements of fast and efficient tunneling engineering. And then the contradiction between "tunneling and prospecting" restricts the development of tunneling engineering. In recent years, scholars have made a deep research on the geological structure and strata waterbearing property of advance geological exploration working face, and achieved remarkable results. Usually the method of prospection is mainly including: seismic reflection wave, DC focusing method, magnetic resonance, transient electromagnetic, etc. But these methods in achieving the advanced exploration of tunnel still have a series of problems, such as the separating exploration, inefficient detection [1]. BEAM method is based on the induced polarization method working by the different physical and chemical properties of rock causing different geoelectric field . BEAM can be installed on the TBM machine, in order to prospect the geological conditions in front rapidly . Not only to make the detection more accurate, but also save the human resources, so the BEAM method has a good effect for tunneling geological structure and water-holding condition. ANSYS is general finite element analysis software, which integrates the structure, fluid, electromagnetism and heat in one [2]. It has the merits of greater generality, higher accuracy and the obvious advantages in the process of solving 3D electrical field problems. Operational Principle BEAM The working principle of BEAM device is based on the induced polarization method. The complex electrochemical reaction of underground rock called polarized electric field occurs under the action of artificial electric field, which increases with the increase of time, and forms a total electric field with artificial electric field. After a period of time, the total electric field tends to saturation, disconnect power, the polarized electric field disappears, and then the polarized electric field continues to discharge until decaying to zero. By measuring and calculating the size of the polarized electric field, the resistivity and moisture content of rock will be knew [3]. BEAM system has three electrodes: exciting electrode A0, guard electrode A1 and earth electrode B. In order to obtain the obvious effect of the induced polarization, the frequency (f1 and f2) of the excitation voltage must be two large differences in low frequency. The rock can be induced fully under low frequency voltage. The excitation voltage in frequency f1 is recorded as ΔU1, and the other excitation voltage is recorded as ΔU2 [4]. Percentage Frequency Effect (PFE) and Resistivity (R) are two important parameters of induced polarization method, and the geological structure can be obtained by these parameters. 1 / 1 1 I U f R  2 / 2 2 I U f R  (1)   % 100 1 / 2 1    f R f R f R PFE (2) % 100 × ΔU1 ΔU2 ΔU1  PFE (3) PFE is a characteristic parameter that measures the ability of storing electrical energy and reflects the degree of geological polarization. Rock property and porosity have direct influence on the polarization effect. When the rock mass is compact, with metal or water, the porosity is low and easy to be polarized, the value of PFE is higher than no obstacle. So the structure and properties of the geology can be judged according to the value of PFE. Detecting Device The devices used in the induced polarization method are: central gradient device, dipole device, near field source device and composite profiling device. According to the tunneling requirement, every electrode can be closely installed on the near field source device, which is convenient to control each electrode in the TBM [5]. In the BEAM tunneling process, obstacles in front of the tunneling, double-pole device included in near field source devices is used for tunneling progress. Double-pole device is shown in Figure 1. Figure 1. Detection device of double-pole. The detection process of double-pole device is that moving exciting electrode (A) and detecting electrode (M), changing the distance between AM, calculating the PFE of every structural layer aim to determine the location of obstacles [6]. Exciting electrode (B) and detecting electrode (N) are located in the "infinity" behind tunnel face (Infinity is a relative concept). In the actual detection process, B (N) is a iron very far behind the TBM machine. ANSYS Simulation Study Voltage distribution and electric-field distribution can be obtained in the atmosphere of ANSYS electric field module. The relationship between the voltage and the resistivity is obtained by analyzing the curve of the simulation voltage with the resistivity and shift position, preparing for experiments.