Small multi-turn coils based on transient electromagnetic method for coal mine detection

To improve the detection of underground anomalies, a transient electromagnetic method with coincident loop is devised and the derivation of late-time and all-time apparent resistivity formula for a coincident loop is presented. The derivation takes the induced electromotive force generated from the coincident loop in a homogeneous whole space into consideration. A translation algorithm for the all-time apparent resistivity in half space is extended into the whole space. The current turn-off affect the detection ability of the instrument and the interpretation of the data, so a general procedure to correct this effect is introduced to compute the transient electromagnetic response of the whole-space considering ramp turn-off time. In addition, unlike the conventional measurement method, the sector detection technique is used to improve the accuracy of the underground transient electromagnetic method. The results from two field examples show good consistence with hydrological and geological information, indicating that the robustness and reliability of our theory and technical scheme.