Small fixed-loop transient electromagnetic in tunnel forward geological prediction

Tunnel forward geological prediction is indispensable to tunnel construction. The transient electromagnetic method is widely used in the field of tunnel forward geological prediction. A small fixed-loop transient electromagnetic method adapted to the limited space of a tunnel face is proposed for tunnel forward geological prediction for the first time. This method makes use of a large number of non-central points inside the loop. The obtained numerical simulation and field data show opposite trends owing to mutual inductance and self-inductance. A correction method for the field value of non-central points is proposed. The influences of different parameters are verified and the correction function is established by a test method. The validity of the correction function is verified through controlled field tests. The small fixed-loop transient electromagnetic has been successfully applied to a tunnel under the Weiwu expressway. This method can effectively improve the detection depth and better detect unfavourable geological bodies, such as karsts in front of the tunnel face.