Transient Electromagnetic Method Using Source Array For Tunneling Ahead Geological Prospecting Of High Resolution

Geological hazards,like water and mud inrush,during tunnel excavation,can result in delay of construction schedule,equipments damages,economic loss and tremendous casualties.With the recent development of tunneling ahead geological prospecting,the technique to prospect large-scale geological hazards has been widely studied and used in field cases.However,limited by the complexity of geological conditions and the environment in tunnnel,the existing geophysical exploration has some problems in the fine exploration and rapid interpretation of water storage,hydraulic structure and hydraulic connection ahead of the tunnel face.To approach fine exploration of the geological hazards,new method,technique and theory with higher resolution are desired.Transient electromagnetic method is a time domain electromagnetic prospecting method,which has been widely used in underground ahead geological prospecting for water and mud inrush.Based on the idea of array radiation in radar antenna,this paper proposed transient electromagnetic method using source array for tunneling ahead geological prospecting to approach high resolution detection of adverse geological sturctures.And the prospecting result can provide scientific guidance for Tunneling construction and hazards treatments.Array source transient electromagnetic in the face of a number of emission sources to form the array source,a number of points to collect the signal to form a "multi-source radiation,multi-point reception" work mode.Viewing from the aspect of mothod theory and prospecting result,transient electromagnetic method using source array has the following advantages: First,by shortening the power supply pulse width time in the emission source,widen the zero-crossing bandwidth of the radiation field source,enrich the high-frequency harmonics in the radiation field Secondly,by increasing the size of the array source,it can improve the overall radiation energy of the emission source,and then improve the signal-to-noise ratio and the detection depth of the response signal.In order to further shorten the forward calculation time of three-dimensional complex model,the heterogeneous parallel algorithm is optimized on the basis of predecessors' work,which makes the speed of the program increase about 5% on the basis of the original parallel acceleration.Based on the radiation source of the array source,the radiation source loading method with finite difference in the time domain is modified to realize the forward calculation of the finite difference time domain of the large-scale array source transient electromagnetic method.In this paper,three-dimensional time-domain finite difference method is used to simulate the forward-prediction method of array source transient electromagnetic tunnel: the transient response of the loop source and the array source is simulated and compared.It is verified that the array source has stronger ability of recognizing abnormalities,especially the parallel component response and the response component of the control surface has a better ability of resolving.It is proved that the scale of the extended array source and the length of the transmitting unit can be significant by simulating and analyzing the three-order transient electromagnetic three-component response under the excitation of the array of different parameters,it is found that the larger the scale of the geological disaster,the closer to the distance from the face,the lower the resistivity,the more obvious the transient response of the array source transient;the effect of the array source on the variation of the length of the disaster in the horizontal direction of the disaster is analyzed,The ?Bx/?t and the ?By/?t response components have obvious distinct advantages in the Y and X directions,respectively;the simulated array source is used to detect the thin faults in front of the face of the jaw,and the corresponding magnetic field distribution is analyzed,the fault has a clear response anomaly,can clearly detect the presence of thin faults in front of the face.Based on the results of forward modeling,the method of array source transient electromagnetic advance detection is helpful to solve the problem of advanced fine exploration in construction tunnel.Forward modeling results can be used as guidance for future array source instrument production.