基于三维地震映像法的地铁盾构区间孤石勘探及其应用

球状或块状花岗岩不均匀风化残留的风化核(俗称"孤石")在我国南方沿海城市普遍发育。"孤石"经常导致盾构刀盘卡住及损坏,且可能引起地表的隆起等问题,因此在盾构施工前对"孤石"进行探测便于提前处理,是地铁工程勘察的难题。孤石与周围土层波阻抗不同的特点为地震映像法在孤石勘探中的应用提供了理论依据。本文首先介绍三维地震映像法的基本工作原理及方法;其次,建立含有孤石的三维地层模型,模拟三维地层中的地震映像法试验,研究不同位置测线的波形剖面图规律,结果表明:当测线在孤石投影区域内的时候,会产生明显的反射波和绕射波,可清晰观察到孤石的存在,经过数据处理后可确定孤石平面性状及埋深;最后,利用该方法对厦门地铁盾构区间的孤石进行勘探,当发现波形异常,揭示反射频率明显变低、振幅增强、有震荡现象和类似于短弧形的同相轴时,可判断为地下孤石。通过对比勘探结果与钻孔资料,验证了该方法的可靠性。本文提出了地铁孤石的探测及三维成像方法,供类似地区进行地质勘探借鉴。

Boulder Detection in Metro Shield Zones Based on 3D Seismic Method,and Its Application

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.