The Application Of3D Transient Electromagnetic Migration Imaging In Coal Mine Goaf Area

Coal is one of the main energy resources of the national economy development. In thedomestic primary energy consumption and production, coal accounts for about three-quarters.However, massive coal mining caused abundant coal mine goaf and thus bringing aboutnumerous secondary disasters, making the detection of coal mine goaf of vital importance.Transient electromagnetic method (TEM) is developing fast in recent years, and themeasurement accuracy of TEM instrument increased rapidly with the fast development ofinstrument manufacturing industry and electronic industry. Currently, TEM is one of thepreferred methods of coal mine goaf detecting. The pseudo-seismic processing andinterpreting of TEM becomes the hot research spot of EM exploration gradually and has madesome breakthroughs, hence, we employ the pseudo-seismic processing and interpretingmethod into the area of coal mine goaf detecting for higher detecting accuracy.Transient electromagnetic3-D imaging technology is introduced in this paper. Based onthe mathematical transformation between TEM field and the wave field, we give theKirchhoff downward continuation method for the derivation of fictitious wave field by wavefield transformation and we also give the basic principles and implementation procedure ofTEM3D imaging technology through3D continuous fictitious velocity analysis which isbased on apparent longitudinal conductance method. In addition, the simulation result of acomplex model with two mined-out layers using3D FDTD method is also processed andinterpreted using the TEM3D imaging method. First, the apparent resistivity contour map and3D apparent resistivity diagram is given using traditional method; Second, the TEM responsesimulated by FDTD is transformed into TEM fictitious wave field according to wave fieldtransformation principle, and the diagram of3D wave field including slice map is given; Atlast, the migration imaging schematic diagram and slice maps of all the directions are giventhrough the Kirchhoff integral equation downward migration imaging of the fictitious wavefield data. Two low resistivity layers can be clearly seen from the apparent resistivity contourmap and3D apparent resistivity diagram, and the electrical interface can be easilydistinguished from the3D wave field diagram which is exactly the same as designed in the original simulating models. The spatial location of coal mine goaf is also clearly shown in3Dwave field diagram, and precise location can be derived from the slice map.The processing and interpreting of the theoretical models using TEM3D imagingtechnology are effective, and the processing and interpreting procedure is performed with thedetection of real coal mine goaf in Shanbei district. Part of the measured data of the entiredetecting area is selected and processed and interpreted, and the interpretation result provedthat this method is effective in the detecting of coal mine goaf.