仰拱病害的电磁波响应特征模拟及三维成像

为提高隧道仰拱病害的电磁波三维成像效率和探测精度，提出一种基于旋转交错网格（rotated staggered grid，RSG）数值模拟及偏移成像理论的波场分析方法，建立了仰拱典型病害的RSG数值模型，模拟了电磁波在隧道仰拱及隧道底层结构中的复杂传播情况，分析了电磁波的衰减、绕射、频散等电磁响应特征及道集信号，然后基于RSG偏移成像理论，并通过局部刻画和三维切片等技术，开展了隧道仰拱病害电磁波模拟数据的RSG三维成像研究. 结果表明:仰拱中钢筋与钢支撑混凝土介质的电场幅值均大于6.5 × 104 V/m，仰拱病害的电场幅值约为6.0 × 104 V/m，RSG三维成像及分析方法兼顾了电磁波横纵向分辨率，刻画出了软弱夹层介质内部的疏松情况和起伏形态，提高了电磁波对仰拱病害的成像精度. 

Simulation of Electromagnetic Response Characteristics and Three-Dimensional Imaging of Inverted Arch Diseases

A new wave field analytical approach, based on forward simulation and migration imaging theory using rotated staggered grid (RSG) algorithm, is proposed for enhancing the imaging efficiency and detection precision of inverted arch diseases in tunnelling using three-dimensional electromagnetic (EM) waves technique. The RSG algorithm was employed to establish an inverted arch numerical model with typical diseases. The complex propagation simulation process of EM waves was performed in the inverted arch and bottom structures, and the gathers and electromagnetic response phenomenon of the EM waves were analysed, taking their attenuation, diffraction, and dispersion into consideration. Next, based on the RSG imaging theory, the three-dimensional migration imaging of the simulated data of inverted arch diseases was performed, using the local depicted image processing and three-dimensional slicing technique. The results reveal an electric field of amplitude greater than 6.5 × 104 V/m at the steel bars and steel bracing concrete medium, which within inverted arch. However, the amplitude of electric field at the diseases underneath the inverted arch is approximately 6.0 × 104 V/m. Comparing the two values, it was concluded that the proposed approach provides higher imaging resolutions in both the lateral as well as vertical directions. Moreover, the extent of incompactness and undulation of the weak interlayer is well depicted, which will result in a great improvement of the imaging accuracy of EM waves while detecting inverted arch diseases.