굴절 토모그래피를 이용한 터널부지 지반조사

Refraction Tomography over a Tunnel Construction Site

  Determining the depth and geometry of a sub-surface structure is a difficult task, especially in the steep and rugged mountain where even a drilling machine cannot be carried and installed on it. Potential field methods generally lack the necessary depth resolution, and seismic reflection data are much influenced by complex geometry of the structure. To address this problem, we have applied a surface 2-D tomographic refraction scheme that is based on an inversion method that incorporates appropriate damping and smoothing constraints. This scheme has been applied to a first-arrival traveltime data set collected over a tunnel route. Seismic data were collected along 10 profiles, and each profile consists of 5 shots and 24 receivers. Reliability of the resultant velocity tomograms were estimated on the basis of (1) plots of the difference between synthetic and observed traveltime(traveltime residual analysis), and (2) RMS errors with diverse input models. The sub-surface structure can be grouped in 2 layers, i.e. soil/weathered rock and basement. The thickness of the near-surface natural soil/weathered rock layer varied between 0 and 10 m, with occasional thickening to 30m. The depth to the basement could be traced to 9～11 m. Several low velocity zones which are developed in vertically have been found along the profiles. The low velocity zones are considered as weak and fracture zones, so that appropriate supporting methods must be considered when planning and great attentions must be paid during tunneling.