基于等值反磁通的隧道TEM超前探测三维模拟及应用

Simulation and application of tunnel TEM advanced detection based on opposing-coils configuration

等值反磁通瞬变电磁方法是近年来发展起来的一种新的浅层瞬变电磁勘探方法,但其在隧道超前探测和三维数值模拟方面还鲜有研究.本文将从掌子面前方三维含水体模型的瞬变电磁响应方面分析其进行隧道超前探测的可行性和有效性.首先,从时间域麦克斯韦方程组出发,采用伽辽金方法推导了基于电场的有限元方程.通过非结构化四面体网格剖分,对异常体区域及发射源附近网格加密,以提高计算精度;计算了均匀半空间模型下的瞬变电磁响应,与解析解进行比较,最大误差不超过2%,验证了程序的正确性.在全空间隧道掌子面前方建立了球状单低阻异常体和球状双低阻异常体,分别采用中心回线装置和等值反磁通装置进行激发和模拟,结果表明:对于相同的目标体,等值反磁通方法得到的异常幅度要比常规中心回线瞬变电磁高3~5倍,对低阻异常体的灵敏度更高,发现异常的能力更强;但双向供电方法抵消了大部分一次场信号,测量的二次场信号很弱,需要大大提高测量精度才可以提高探测深度.设计了一个倾斜板状体薄层,通过等值反磁通装置模拟表明该方法对浅部薄层探测能力较好.最后,采用等值反磁通仪器,通过一个隧道超前探测的实例说明该方法在隧道水体病害超前探测方面有较好的实用性.

Opposing Coils Transient Electromagnetic(OCTEM) is a new shallow transient electromagnetic exploration method developed in recent years, but its application in tunnel advance detection and three-dimensional numerical simulation is rarely studied. In this paper, the feasibility and effectiveness of the tunnel advance detection based on the transient electromagnetic response of the three-dimensional water-bearing model in front of the tunnel face are analyzed. Firstly, based on Maxwell’s equations in time domain, the finite element equation based on electric field is derived by Galerkin method. The unstructured tetrahedral mesh is divided to refine the abnormal body area and the mesh near the transmitting source to improve the calculation accuracy.The transient electromagnetic response in the uniform half space model is calculated, and the maximum error is less than 2% compared with the analytical solution, which verifies the correctness of the program. In the whole space in front of tunnel face, we establish spherical single low-resistivity anomaly and spherical double low-resistivity anomaly, and adopt the central loop device and equal and reverse magnetic flux device to excite, the results show that for the same target, anomaly amplitude obtained by equal and reverse magnetic flux TEM is higher 3~5 times than conventional central loop TEM, and it has higher sensitivity for conductive body and stronger ability to detect target;However, equal and reverse magnetic flux method cancels out most of the primary field signals, and the measured secondary field signals are very weak, so the measurement accuracy can be greatly improved to improve the detection depth. An inclined plate body is established and simulated by the equivalent counter flux device. The results show that this method has better detection ability for shallow thin conductive layer. Finally, by using equal and reverse magnetic flux instrument, an example of tunnel advance detection shows that this method has good practicability in advance detection of tunnel water diseases.