Experimental study on response characteristics of transient electromagnetic method in shallow gob

This paper presented the results of an experimental study on response characteristics of transient electromagnetic method in shallow gob. Representative shallow gobs with known depth in Ordos mining area were chosen to perform the experiments. Single curve analysis of electromagnetic response characteristics was conducted under different wire frame sizes, emission currents, emission frequencies, and gains. The apparent resistivity of the known goaf and the normal coal seam was compared with the typical electromagnetic response profile to develop the identifying criteria for shallow gobs. The experimental results showed that for gob below 150 m depth, under the combinations of these conditions, the collect data had a high signal to noise ratio - emission frame below 180 m, emission current over 12 A, frequency above 25 Hz, and gain around 24～26. Applying the optimized parameters to field detections, the normalized potential curve near the gob was significantly different from that of the normal rock formation, the normalized potential decayed faster near the gob. From the apparent resistivity profile, the detection error of gob boundary was less than 10 m, which proved that the shallow gob could be accurately detected.