Time-depth conversion of transient electromagnetic method used in coal mines

Accuracy of time-depth conversion in data processing of transient electromagnetic prospecting always affects the accu- rate positioning of water bodies in coal mines. In order to improve the accuracy of time-depth conversion, we established a mathe- matical model of time-depth conversion for a transient electromagnetic method based on the theory of "double smoke ring effect" of full space transient electromagnetic field transmission. Using a 3-layer as well as a 4-layer geo-electric model for roadway floors, we performed the time-depth conversion of theoretical curves of apparent resistance varying over time. In these curves, the depth corresponding to extreme value points is nearly the same as the depth of a geo-electric model. The position of water body deter- mined by our time-depth conversion method agrees well with the result of borehole drilling, indicating that the established time-depth conversion model can clearly improve the accuracy of spatial positioning of water bodies in coal mines.