AN ANALYSIS ON SIMULATED MASW RESPONSES FOR UNDERGROUND CAVITIES BASED ON 2D ACOUSTIC-ELASTIC COUPLED MODELING

摘要

As ground subsidence caused by cavities in urban area in South Korea, mainly in Seoul, is a serious problem, non-destructive geophysical survey gets more important for the detection go the cavities. Among various geophysical surveys, multichannel analysis of surface waves (MASW) survey can be an efficient method for detecting relatively large underground cavities located deeper than 10 m. Since the amplitude of surface wave is large enough to be detected by pad-style geophones even on pavement, MASW can be efficiently performed using land streamers being attached to an auto-vehicle. Even though the MASW survey is one-dimensional (ID) survey assuming the target underground to be ID-layered, the surface wave travels along 3D earth structure from source position. Thus, in order to analyze the applicability of MASW survey to the detection of underground cavities, we simulated synthetic MASW data for 2D cavity models constructed based on geologic settings of Songpa-gu, in which the largest number of sinkholes have occurred in Seoul. For the simulation, an acoustic-elastic coupled modeling algorithm was used considering the fact that only acoustic wave can propagate within the volumes of cavities, while both shear and acoustic waves propagate the volumes of solid underground. From synthetic MASW data for a 2D model, dispersion curves are obtained for each shot gather, and inverted to recover ID shear-velocity profiles, which are merged to make a pseudo-2D section of shear-velocity for the 2D model. Analyzing inverted pseudo-2D sections for each 2D model demonstrated not only the applicability of MASW survey in detecting underground cavities but also its limitations.