电磁波CT技术探测溶洞的模拟分析与应用研究

城市地下空间开发与轨道交通的快速发展对低阻地层岩溶发育区、断裂带的孔间电磁波CT探测需求越来越大。由于低阻地层对电磁波吸收大，为保证探测距离，必须降低频率。而现有的孔间电磁波CT正反演方法基于高频射线理论对幅度测量信息进行计算，用于低阻地层的低频电磁波CT正反演计算误差会增大，影响反演分辨率。为此，提出了适用于低阻地层条件低频电磁波CT的幅度相位联合正反演方法。推导了二维分层介质中一维 Sommerfeld 积分形式的张量格林函数，将其应用于低频频段散射场正演计算，计算地表面波效应的影响，提取了地表面波影响的包络特征；利用基于Laplace算子约束的迭代反演方法提高了低频信号反演分辨率。将所提方法应用于杭州三江汇地区70 m距离与武汉龙泉山某地铁线岩溶发育区27 m距离反演，杭州反演结果与地层分层相符，在龙泉山成功探测到高度为1.1 m的溶洞，验证了该方法的正确性。

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

The rapid development of urban underground space development and rail transit has an increasing demand for inter hole electromagnetic wave CT detection in low resistivity Karst development areas and fault zones. Due to the high absorption of electromagnetic waves by low resistivity formations, frequency reduction is necessary to ensure the detection distance. The existing CT forward and inversion modeling methods for inter hole electromagnetic waves are based on high-frequency ray theory to calculate amplitude measurement information. The error in using CT forward and inversion modeling for low-frequency electromagnetic waves in low resistivity formations will increase, affecting the inversion resolution. A joint method based on amplitude and phase is used to realize forward and inverse calculation for low-frequency electromagnetic wave CT under low resistivity formation conditions. The tensor Green’s function in one-dimensional Sommerfeld integral form in two-dimensional layered media is derived, and the influence of surface wave effects is calculated. The envelope features of surface wave effects are extracted. The iterative inversion method based on Laplace operator constraints is used to improve the resolution of low-frequency signal inversion. The technology was successfully applied to the 70 m distance inversion in the Sanjianghui area of Hangzhou and the 27 m distance inversion in a Karst development area of a certain subway line in Longquan Mountain, Wuhan. The inversion results in Hangzhou were consistent with the formation stratification, and a Karst cave with a height of 1.1 meters was successfully detected in Longquan Mountain. This method has been fully validated.