| Coal resources are still an important energy pillar of China's national economic construction.With the depletion of coal in the shallow coal field,coal mining continues to march to the deep strata.As a result,many coal mines have different depth,different levels of goaf,most of which are isolated,discontinuous,and there is water,which greatly increases the difficulty of detection,and brings hidden dangers for the safety production of coal mines.For these complex goafs,it is of great practical significance to seek effective exploration methods and reasonable geophysical data interpretation methods.In view of the complex situation of goaf in coal measure strata,this paper studies and analyzes the transient electromagnetic response characteristics of single-layer goaf,adjacent goaf and double-layer goaf under different conditions through physical simulation experiment and numerical simulation.The main conclusions are as follows:Physical simulation:(1)The results show that the electromagnetic response degree of low resistivity goaf is higher than that of high resistivity goaf,and the multi trace curves show "single peak" and "single concave" type respectively,and the electromagnetic response of the two kinds of goaf decreases with the increase of buried depth;(2)The resolution ability of TEM to different kinds of adjacent goaf is different.In horizontal resolution,adjacent low resistivity goaf > adjacent high resistivity goaf > adjacent high and low resistivity goaf.The resolution distance of adjacent low resistivity goaf is 15 cm ? 20 cm,i.e.1.5 times buried depth? 2 times buried depth;the resolution distance of adjacent high resistivity goaf is 20 ? 25 cm,i.e.2 times buried depth ? 2.5 times buried depth;the resolution distance of adjacent high and low resistivity goaf is 25 ? 30 cm,i.e.2.5 times buried depth ? 3 times buried depth;(3)Under a certain interval,the transient electromagnetic method can distinguish the double-layer high and low resistance goaf,and the longitudinal resolution distance is 15 cm,which means it is difficult to distinguish all kinds of double-layer goaf with different interval.There are some differences in electromagnetic response values,such as double-layer low resistance goaf >double-layer low high resistance goaf > double-layer high and low resistance goaf >double-layer high resistance goaf;Numerical simulation:(1)Transient electromagnetic method is more sensitive to low resistivity goaf than high resistivity goaf.The increase of buried depth will reduce their electromagnetic response.The greater the difference between the resistivity of surrounding rock and that of abnormal body,the more obvious the response of abnormal body.For the low resistance goaf,the smaller wire frame corresponds to the lower electromagnetic response value,but the low resistance response will be more obvious.Compared with no overburden,the existence of low resistivity overburden will increase the electromagnetic response value,but the thicker the low resistivity overburden is,the lower the resistivity is,and the lower the response of low resistivity body will be,which will affect the detection;(2)For adjacent mined-out areas,numerical simulations also show that the transient electromagnetic method has the strongest horizontal resolution capability for adjacent low-resistance mined-out areas(75 m?100 m,that is,1.5 times buried depth to 2 times buried depth).Adjacent high-resistance goaf is followed(100 m?125 m,ie 2 times buried depth?2.5 times buried depth),the adjacent high and low resistance goaf is the weakest(125 m?150 m,ie 2.5 times buried depth?3 times buried depth).For adjacent low-resistance mined-out areas,the low-resistance covering layer will increase the electromagnetic response value.The thicker the low-resistance covering layer,the lower the resistivity,and the harder it is to distinguish this type of mined-out area;(3)Transient electromagnetic method(TEM)can be used to distinguish the goaf with certain interval(75m,Equivalent to 0.75 times of the buried depth of the upper goaf),and it is difficult to distinguish other goafs.For the double-layer high-low group goaf,we further studied that the smaller the wire frame and the lower the resistivity of surrounding rock are conducive to the detection of this kind of goaf;the increase of the thickness of low resistivity overburden and the decrease of resistivity will greatly shield the detection of the lower goaf.The results of numerical simulation and corresponding physical model simulation are consistent.Field tests have verified some double-layer goafs,and it is known that transient electromagnetic method is difficult to distinguish double-layer low resistance goaf and double-layer high resistance goaf,but it can distinguish double-layer high and low resistance goaf under certain interval.It is consistent with our simulation results. |
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