Study On The Detection Of Water In Coal Gob Of Coalfield With Surface-Borehole High Precision Electromagnetic Method

For a long time, water disasters is one of most serious accidents in mines,along with gas accident,during which goaf seeper play a key role.Meanwhile,goaf seeper have some features,like suddenly happen,great in-situ water,strong damage and so forth,that make it as a main threaten to mine safety. Consequently,a good job of water-accumulating gob detection is significant for prevention of mines accident.Geophysical prospecting methods,such as seismic,electromagnetic,are commonly used in coalmine gob areas exploration,in which,although seismic method has a better result in prospecting faultage,gob,stratum,it cost a lot of money and take a long time to obtain result,on the other side, blasting activity is not appropriate to be carried out in pipeline spreaded everywhere.Electromagnetic detection obtained widespread application in coalfield hydro-geological exploration due to its water sensitivity,high resolution.Commonly used ground electromagnetic exploration methods for gob detecting include High-density electrical method, transient electromagnetic method and controlled source audio-frequency magnetotelluric sounding, induced polarization, etc. All of which can get relative good results in normal geological conditions,but for some special electrical conditions,ground electromagnetic method will show certain deficiencies. Signal is weak and interference is too big, which cause great difficulties for later data processing and interpretation.Therefore, to overcome the defect of conventional ground geophysical exploration,ground-well electromagnetism method draw more and more attention. Ground-well electromagnetism method combine cased well,producing well,mine well and ground based facility to carry out exploration work.Because the signals are from wells that reflect more information of underground,on the other side avoid a series of electromagnetic interference from ground,the anomaly response are more evident and can reflect better underground information.Ground-well transient electromagnetic method and well-ground potential method are kind of widely used and have good application results. Ground-well transient electromagnetic method is a variant of the transient electromagnetic method.This way of observation can get closer to object,has little impact from conductive layers and weak electromagnetic interference,thus can obtain strong anomaly response compared with conventional transient electromagnetic method.Many abroad research and actual application indicate that ground-well transient electromagnetic method has many advantages like fine direction, high detection accuracy,wide prospecting range,which contribute it to one of most effective method to explore deep concealed ore bodies. Well-ground potential method is a kind of new method developed in recent years.High power current is supplied into underground via cased well and measurement of potential anomalies caused by underground medium is done on the surface.Compared with the traditional resistivity method, the ground-well potential method has advantages like great exploration depth, wide detection range, high resolution, etc.This paper will study ground-well transient electromagnetic method and ground-well potential method,and analyses its application in mined-out area of water.The paper will start from basic theory,expound their basic principle andMethod content,study observed anomalies response characteristics under different geoelectric conditions of ground-well transient electromagnetic method,Use the finite difference algorithm to do forward modelling for ground-well potential method in three-dimensional space.At the same time,the paper introduced well-ground potential method of a coal in Inner Mongolia and process and interpretive its data,at last,deduced to some degree coal mined-out area location and its scale.Through the study of this article,both ground-well transient electromagnetic method and well-ground potential method are effective geophysical prospecting method for subsurface structure and have great results in detecting coalfield water-accumulating gob.