地孔瞬变电磁法超前探测数值模拟响应特征

为了避免传统井下瞬变电磁超前探测受巷道中电磁干扰信号、锚网、掘进机等影响,提出一种在地面布设发射回线,在煤矿井下掘进工作面或工作面水平钻孔中测量瞬变电磁场三分量信号的超前探测方法。该方法具有径向探测深度大、三分量接收探头离异常体近、异常场占总场的比值高而更易发现异常体等特点。基于瞬变电磁三维时域有限差分正演算法,对均匀半空间模型、均匀半空间含孤立异常体模型以及异常体位于水平钻孔不同方位的地孔瞬变电磁三分量的总场和异常场响应特征进行了系统研究。发现三分量总场和异常场之间总体符合烟圈效应,总场和异常场三分量衰减曲线或者多测道曲线形态随时间的变化特征可以采用不同时刻感应电流环相对于测点或者测线的位置来进行分析;在不考虑互感现象的情况下,地下接收的不同时刻的三分量响应完全可以看成是背景电流环和异常电流环的叠加,其中异常场的三分量形态可以采用垂直于一次场方向相同形状的电流环来拟合;另外当异常体位于水平钻孔不同方位时,异常场X分量均表现为S型,Y分量和Z分量表现为单峰异常,且不同象限异常场三分量组合形态不同,据此可以判断异常体所在钻孔的深度和方位。最后在地面进行了地孔瞬变电磁超前探测方法的相似性试验,验证了理论模型响应特征的正确性。

Numerical simulation response characteristics of down hole TEM for advanced detection

In order to avoid the problems that the traditional underground electromagnetic transient detection,the electromagnetic interference signal,anchor net and road header are affected by the roadway,an advanced detection method for measuring the three component signal of transient electromagnetic field in the underground coal working face or the horizontal borehole of the working face was proposed.The method has the characteristics that the radial detection depth is large,the three component receiving probe is close to the abnormal body,the ratio of the abnormal field to the total field is high,and the abnormal body is more easily found.Based on the transient electromagnetic three dimensional time domain finite difference forward algorithm,the total field and the anomalous field of the uniform three component model of the uniform hole in the uniform half space model,the uniform half space containing the isolated anomalous body model and the response characteristics of total field and anomalous field of abnormal body in different directions of the horizontal borehole were systematically studied.It is found that the three component total field and the anomalous field is consistent with the smoke circle effect.The three component attenuation curve of the total field and the anomalous field or the variation of the shape of the multi track curve with time can be analyzed by the position of the induce current loop relative to the measuring point or the measuring line at different times.Without considering the mutual inductance phenomenon,the three component response at different moments of underground reception can be regarded as the superposition of the background current loop and the abnormal current loop,wherein the three component form of the anomalous field can be modeled by the current loop of the same shape perpendicular to the first field direction.In addition,when the anomalous body is located at different orientations of the horizontal borehole,the X component of the anomalous field is S shaped,the Ycomponent and the Zcomponent are unimodal anomalies,and the different quadrant anomalies.The three component combination is different in shape,and the depth and orientation of the borehole where the abnormal body is located can be determined.Finally,the similarity test of the ground hole transient electromagnetic advanced detection method is carried out,which verifies the correctness of the response characteristics of the theoretical model.