电磁波CT技术探测溶洞的模拟分析与应用研究

目前应用电磁波CT技术进行溶洞探测时对于溶洞形态和填充状态的解释与判断尚缺乏成熟的理论依据。在FDTD(时域有限差分法）基础上，采用Matlab软件编程建立CT技术探测溶洞模型，在模型边界处特意设置PML完全匹配层和PEC完善导体。模拟分析和实验探测结果表明，（1）电磁波穿过溶洞时在溶洞边界处发生折射现象，溶洞内部发生反射、绕射和障碍增益现象，且电磁波能量的衰减主要由折射现象产生；（2）当溶洞内充填物有水存在时，因反射现象使一部分电磁波滞留在溶洞内部，该部分电磁波在溶洞内部不断发生反复的反射和绕射现象；（3）经过溶洞后电磁波波形的扰动主要由反射现象及滞留在溶洞内部电磁波造成；（4）当溶洞对电磁波吸收系数发生改变时需综合考虑填充物种类及地层含水率等因素的影响。最后应用模拟结果对实际工程试验层析成像数据进行了判断和解释，将所得判定结果与勘察资料进行对比，二者基本吻合。验证了模拟结果的准确性和应用电磁波CT技术探测溶洞的可行性，可为今后层析成像的结果解释和判断提供借鉴和理论依据。

Simulation analysis and application study of electromagnetic wave computed tomography in detecting karst caves

There is still a theoretical defect in interpretation and judgment of karst morphology and filling state in karst cave detection using electromagnetic wave computed tomography. In order to solve practical problems in the detection, on the basis of finite-different time-domain(FDTD) algorithm, a numerical CT detection model is established using Matlab. A perfectly matched layer(PML) and a perfect electric conductor(PEC) are set at the boundary of computation model to prevent electromagnetic wave getting into the model again after it passed through the boundary. Simulation and experiment results show that: (1) When the electromagnetic wave passes through karst cave, it refracts at the cave boundary. And within the cave, the phenomenon of reflection, diffraction, and obstacle gain occur. Energy attenuation of electromagnetic wave is mainly caused by refraction. (2) When the cave filled with water, the reflection makes a part of the electromagnetic wave remains in the cave inside, this part of wave reflects and diffracts repeatedly inside the cave. (3) After electromagnetic wave passing through the cave, the disturbance of the waveform is mainly caused by the reflection phenomenon and the electromagnetic wave trapped inside the cave. (4) We have to consider the influence of the filling type, water content and other factors when the electromagnetic wave absorption coefficient changes in karst caves. At last，the tomography data obtained in practical detection are judged and interpreted according to the simulation results. By comparing the interpretation of tomography with previous investigation data, the accuracy of the simulation results and the reliability of the application of electromagnetic wave CT technology in detecting karst caves are validated. This research provides a reference and a theoretical basis for interpreting and judging tomography results in the future.