地-井瞬变电磁法线性导体正演及异常响应特征研究

Study on linear conductor forward modeling and characteristics of abnormal response in ground-well TEM

地-井瞬变电磁法采用地面发射、井中或巷道接收的方式,距离异常较近,可以提高对异常分辨力和增加探测的深度,为了对深部小异常进行精细探测,基于二维时域有限差分和地面瞬变电磁探测原理,在均匀介质背景条件下,以大定源回线装置作为发射场源,建立不同深度和不同位置的板状、方形导体的多个场电模型;采用地面、钻孔(巷道)2种观测方式,并进行数值模拟正演分析,对小异常体的垂直磁场强度和及差值EA多测道曲线响应特征进行研究;并利用垂直磁场强度曲线的极值点和EA曲线的交叉点,可对小异常进行准确定位作出解释。研究结果表明:时域有限差分法能够对建立的线性导体场电模型进行有效准确的计算,能够获取小异常的响应曲线特征;地面-钻孔观测方式下基于垂直磁场强度曲线的极值点,能够确定异常的纵向位置和区分多个异常,且幅值随时间增大越来越小;而地面-巷道观测方式下通过EA曲线的交叉点,能够确定异常的横向位置和区分多个异常,且至异常越近EA值越大;以极值点和交叉点对小异常进行准确定位,提高了纵横向分辨力。

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.