Abstract: Weathering cores (commonly known as "boulders") from the residues of orbicular or blocky granite are widely developed in southern coastal cities in China. "Boulder" often leads to stuck and damage of shield cutter head,and may cause problems such as surface uplift. Detecting "boulder" before shield construction for pretreatment is a challenge for a metro engineering survey. The different characteristics of seismic wave impedance of boulders and soil surrounded provide a theoretical basis for the application of seismic imaging method in a boulder exploration. In this study,we introduced the basic working principle and methods of three-dimensional seismic imaging; then we established a three-dimensional finite element model which contains the boulder to simulate seismic imaging in three-dimensional stratum formation and study the regulation of waveform cross-sectional view at different position of survey lines. The result shows that when the survey line lies in the boulder's projection area, the obvious reflection wave and diffracted wave appear. The existence of boulder can be thereby clearly detected,and the plane form and burial depth of the boulder can be determined after data processing. Finally, we applied this method to Xiamen subway shield zone. When a wave appeared abnormal, reflecting frequency became obviously low, amplitude got larger, oscillation and co-phasal axes similar to a short arc came into being at the same time, we drew the conclusion for the existence of an underground boulder. We confirmed the feasibility of the method through comparing the exploration results to the borehole data. This three-dimensional imaging method improves the visualization degree of the detection results,and provides a reference for geotechnical detection in a similar area.

Key words: 3D seismic method, metro shield zone, underground boulder, scanning detection mode, factor