Numerical Simulation Of Time Domain Borehole-to-surface Electromagnetic Method

Mine water disaster is one of the main safety disasters,which is the same as gas accident and fire accident.In all kinds of water accident types,the water in goaf occupies a large proportion.At the same time,the goaf water has the characteristics of strong suddenness,great destructive power and strong concealment,which is a great threat to the mine construction and production safety.Therefore,it is of positive significance to improve the detection ability of the water-logged mined-out area in coal field for the prevention and treatment of mine water disaster accidents.The time-domain well-ground electromagnetic method is an exploration technology which can be used to detect abnormal bodies near the well by placing a long conductor source in the well to launch the primary field and measuring the distribution of the induced secondary field on the ground.Compared with conventional ground transient electromagnetic method,the transmitting source of time-domain well ground electromagnetic method is closer to the abnormal body,which enhances the secondary eddy current field and improves the signal-to-noise ratio.The time-domain well-ground electromagnetic method combines many advantages of conventional surface electromagnetic method and electromagnetic logging method,and is an important method in the field of resources and engineering investigation.Therefore,it is of great theoretical significance to carry out a systematic and in-depth study of the time-domain well-ground electromagnetic method.This article first introduces the transient electromagnetic method and time domain well to the basic principle of electromagnetic method,derived the vertical electric dipole source in layered formation medium frequency domain electromagnetic response equation,and gives the underground vertical long wire source frequency domain electromagnetic field analytical expression,the time domain electromagnetic field is obtained by frequency-time conversion,then the induction electromotive force.Maxwell software is used to simulate the response characteristics of the 3D time-domain well-ground electromagnetic method,and the influence of different parameters of the geoelectric model on the response of the well-ground electromagnetic method is compared and analyzed.Through the calculation of a series of geoelectric models designed,the results show that the detection effect of time-domain well-ground electromagnetic method is related to the length of line source and the buried depth of abnormal bodies,and the appropriate length of line source should be selected for the abnormal bodies with different buried depths.In addition,the detection ability of the well ground electromagnetic method is also related to the intensity of the emission current,the greater the intensity,the stronger the detection ability.The shallower the buried depth of the low-resistivity abnormal body is,the closer it is to the emission source,and the greater the resistivity difference between the abnormal body and the surrounding rock is,the easier it is to be detected.The simulation results also show that the time-domain electromagnetic method has a higher sensitivity to the lateral scale change than the longitudinal(thickness change)detection of the watered-out goaf.In addition,the abnormal response gradually increases with the increase of water content in goaf.It can be seen that the time-domain geophysical electromagnetic method is an effective geophysical exploration technique for detecting water-logged goaf in coalfield,and it has a good development and application prospect.