China Coal Technology and Engineering Group Chongqing Research Institute，Chongqing ，China

The ground-well transient electromagnetic method(TEM) adopts the methods of ground launching and well or roadway receiving,and the distance is too close to the anomaly,which has important research significance for improving resolution and increasing the depth of detection.In order to finely detect deep small anomalies,based on two-dimensional time domain(FDTD) finite difference and ground TEM detection principle,and under the condition of uniform medium,the large fixed source loop device was used as the launch field source to establish multiple field-electric models of plate-like and square-conductors in different depths and different positions; using ground-drilling (tunnel) observation methods for numerical simulation forward analysis,researches were conducted on vertical magnetic field intensity curves of small anomalies and response characteristics of multi-track curves of vertical magnetic field intensity difference value(EA);an interpretation method for accurate localization analysis of small anomalies using the intersection of the extreme point of vertical magnetic field intensity curve and the EA curve was proposed.The results show that the time-domain finite difference method can effectively and accurately calculate the established linear conductor field electric model,and can obtain the characteristics of the small anomaly response curve.The ground-drilling observation mode passes the extreme value of vertical magnetic field intensity curve.The longitudinal position of the anomaly can be determined and the plurality of anomalies can be distinguished,and the amplitude is smaller and smaller with time; the intersection of the EA curve in the ground-tunnel observation mode can determine the lateral position of the anomaly and distinguish the plurality of anomalies,and the closer the distance is,the larger the EA value is; the accurate positioning of small anomalies is realized according to the extreme pointsand intersections,and the vertical and horizontal resolution is improved.

国家科技重大专项基金资助项目（2016ZX05045002-003）