面波全波形反演在地下浅埋障碍物探查中的应用研究

Application of surface wave based full waveform inversion on detection of shallow-buried obstruction

在岩土和隧道工程中,及时准确地确定地下障碍物的位置和范围可以有效避免经济损失和人员伤亡,因此,岩土工程领域越来越重视场地前期调查中的浅埋障碍物探测工作.面波法由于其高效性和高信噪比,被广泛应用于场地调查中.然而,传统的面波方法如多道面波分析法的应用理论假设为地下模型须为水平层状介质.这限制了多道面波分析法在地下障碍物探测中的分辨率.而通过直接对波动方程进行求解及利用地震波的全部振幅和相位信息,面波的全波形反演结果可以有效地反映地震波速在水平方向和垂直方向的变化情况,从而为判断地下障碍物范围提供了可能.目前,全波形方法大多应用于大尺寸的地球物理勘探中,在小尺度地球物理领域如岩土工程中的应用较少.本文在新加坡樟宜机场扩建工程中分别使用多道面波分析法,全波形反演,跨孔波速测试,贯入试验和岩心取样确定回填土中一块大型岩石的具体位置和范围.试验结果表明:多道面波分析法仅可反映出岩石的上界面深度,但是无法反映出岩石的下界面和侧边位置.而面波全波形反演可以准确反映出回填土中岩石的具体位置和大致形状.另外,本文分析总结了贯入试验,岩心取样,多道面波分析法和面波全波形反演的优缺点,提出了一种经济高效的工作流程,可为岩土工程中的地下障碍物探测提供一种新的解决方案.

The detection of underground obstruction can decrease economic loss and casualties in geotechnical and tunneling engineering. Therefore, detection of shallow-buried obstruction such as rock and sinkhole draws more attention in geotechnical engineering field recently. As an effective geophysical method, surface wave survey is widely used in geotechnical field investigation. However, a theoretical assumption of Multichannel Analysis of Surface Waves(MASW) is that the underground model must be horizontal homogeneous, which limits the resolution of MASW in the detection of underground obstruction. By using both phase and amplitude information of whole observed data, Full Waveform Inversion(FWI) provides a new opportunity to capture the ground profiling variation in both vertical and horizontal direction, which may delineate isolated buried small-scale objects. Most previous studies on FWI concentrate on large-scale geophysics, and studies on small-scale geophysics(e.g., geotechnical engineering geophysics) are still rare. To determine the location of a rock obstruction in Changi East Reclamation project, MASW, FWI, cross-hole wave velocity tests, penetration tests and borehole sampling were conducted in this work. Results show that MASW could delineate the upper surface of shallow-buried rock, but did not capture the bottom and lateral boundary of the rock. Results prove that FWI can determine the location and rough range of rock obstruction. Additionally, advantage and disadvantage of penetration tests and surface wave surveys were summarized. And thus an advanced workflow containing both geotechnical methods and geophysical methods were recommended in this work, providing significant reference to the detection of shallow-buried obstruction in geotechnical engineering.