Study On Numerical Simulation And Distribution Regularity Of Transient Electromagnetic Field With Mine-Used Multi Small Loop

Mine transient electromagnetic method (MTEM) is one of mainly geophysical methods, which are used to predict water inrush structures nearby the roof, bottom and driving face in roadway. Compared with ground transient electromagnetic method, this technique has many advantages: high resolution, small volume effect, flexibility and high efficiency. However, relatively slow development of its fundamental theory makes it become an immediate problem to be solved. These theoretical problems, roadway effect, transient response for the typical geologic geophysical model in whole-space are studied using numerical modeling method in this paper.Analysis solution of whole-space multi-turn coils was derived; transient processes of primary, secondary and total field were analyzed; and calculation method of turn-off effects for linear current and half sine current in whole-space magnetic dipole source was also studied. The results indicate that central loop cannot be used because of singular point r=0 in magnetic dipole field. Secondary field of multi-turn coils has a similar attenuation regular with magnetic dipole source, but its magnetic value is 10 3 times than magnetic dipole value in"early time". Amplitude value of primary field induced electromotive force and time range of transient process are influenced by turn-off time; increase of inductance time, decrease of electromotive force amplitude value, and acceleration of total field attenuation are caused by the longer turn-off time."Early time"transient field is disturbed heavily by linear and half sine turn-off current; influence to transient field affected by the turn-off effect extends to"later time"for longer turn-off time; and compared with linear turn-off current, the half sine current has more heavy influence on transient field than linear current.Time domain finite-difference method was used to derive three-dimensional difference equation for transient electromagnetic field; and stability of 3D Euler method and Dufort-Frankel method was proved, and relationship between initial time and grid length in numerical calculation was obtained in this proof process. The results show that, 3D Dufort-Frankel method is unconditional stable; the diffusion velocity of transient field is related to sample interval and resistivity contrast. Numerical method of 3D transient difference equation, using non-uniform grid method and setting relevant time step length parameter, has a high accuracy by comparing numerical solution with theoretical solution of multi-turn coils; actually, the maximum error between numerical and analyzed solution in computational domain beyond the singular point is less than 12% at the moment 1.05ms.Based on Liao absorbing boundary condition, modified Liao absorbing boundary condition was derived; problem of roadway boundary condition was presented for the first time. The results indicate that, using the new boundary condition can preferably reflect the diffusion regular of transient field, and has a better absorbing effect on low frequency electromagnetic wave. The maximum error of new Liao absorbing boundary condition is less than 0.01, when choosing the parameterα=1,γ=0.98 and N=4, and the initial time step length less than 50μs. Roadway boundary condition has a good applicability and stability in difference calculation of diffusion equation.Roadway factors were defined; and relationship between roadway effect and equipments, geometry size of roadway, host rock conductivity, equipment position and turn-off time was studied by using roadway factors. The results show that, inductive electromotive force of transient electromagnetic field is much lower because of roadway effect; roadway effect increases with turn-off time. Compared with separated loop, data measured by coincident loop is influenced heavily by roadway space, but transient"later time"field of the two equipments is approximately equal. The bigger geometry size of roadway and the higher conductivity of host rock are, the heavier transient field affected by roadway effect and its boundary is. Roadway effect is same when coincident loop is located at the center of roadway roof, floor and side; and it is heaviest when coincident loop is located at the center of roadway plug.The method of plotting combining 3D slice with 2D slice was used; the diffusion regular and anomalous characteristics of apparent resistivity for typical geologic-geophysical models in whole-space were studied; the effect of physical model experiment was verified by using numerical method. The results indicate that, the strength and time of transient response is related to several factors in the low-resistivity layer: resistivity, geometric size, shape and the distance from roadway. Transient response of layered medium is determined by resistivity contrast and layer thickness. The influence to transient response at break points in fault is stronger. Numerical method has similar conclusions with physical experiment for the same geologic model.