Present study situation review of borehole transient electromagnetic method

In the borehole transient electromagnetic method (TEM), the receiving probe is placed in the borehole, which is different from either aerial or ground TEM. In such methods, as the probe will be closer to the detection target and far away from surface, not only stronger response signals can be observed, but also the interference of various electromagnetic signals outside the borehole can be attenuated effectively. It can meet the demand of meticulous detection with large burial, especially the detection demand of deep blind ore and water hazard. The present situation of using various methods in the study in forward modeling, inversion imaging, instrument and equipment as well as application cases was summarized, by reviewing the study history of various borehole TEMs (return line source surface-to-borehole TEM, electric source surface-to-borehole TEM, and tunnel borehole TEM). According to the study results, the study of the return line source surface-to-borehole TEM, with solid foundation and comprehensive development, has been applied frequently and successfully in production. Both the study of electric source surface-to-borehole TEM and that of the tunnel borehole TEM have not been widely promoted or applied currently due to their weak foundation and the limitations in terms of theoretical methods, detection equipment, inversion techniques, etc. The study of the both TEMs is still at the preliminary stage, and the related technologies still require further improvement. For the future development direction, the implementation can be commenced in terms including the three-dimension (3D) inversion with topography, rapid topographic correction, pseudo-random emission, emission with multiple radiation sources, interference elimination for casings and the combined interpretation using the multi-parameter geophysical method, so as to form the complete borehole TEM theory and interpretation method, and to provide theoretical basis for the detection of deep ores and the detection of the water-bearing structures in coal and tunnels.