HMT法探测花岗岩差异风化的应用研究

Application of HMT in detection of weathering difference in granite rocks

通常情况下花岗岩岩性稳定、岩石完整,是隧道工程的理想围岩.我国沿海地区花岗岩覆盖普遍,但因气候炎热,秋夏两季多台风暴雨,降雨量丰富,化学风化作用剧烈,使得花岗岩差异风化严重.风化作用不仅仅发生在近地表,而且深切入地下达数百米,常穿过隧道洞身.本文以新建厦门至深圳高速铁路(简称厦深高铁)某隧道工程勘察为例,采用探测深度大、工作方便的天然场源高频大地电磁测深法(HMT),探测花岗岩差异风化情况.四条相邻测线综合探测结果表明:根据HMT法二维反演剖面图中电阻率的差异,成功解释了已开挖段的施工塌陷范围,并预测了将要开挖段的强风化弱地质结构分布范围;通过垂向相对低阻异常特征定量推断了岩体风化带向下延伸情况,为隧道安全施工提供了科学指导.

Granite rocks are normally stable and complete, and seen as the ideal wall rocks for railway tunnel. Most of the coastal areas in China are covered by granite rocks. While in coastal areas, there are always so much hot and wet weather in summer and autumn, which causes much serious chemical weathering in the granite rocks. The weathering appears from the near surface to even hundreds of meters deep underground, always through the railway tunnel, which should be discovered before the tunnel construction. The nature source HMT （high frequency magnetotellruric） sounding method plays a good role to detect the weathering difference in the granite rocks underground. This paper shows an example. For the new high-speed railway lines from Xiamen to Shenzhen, the HMT method （with EH4 electromagnetic system） was employed to detect the weathering difference in the granite rocks underground. The 2D inversion resisitivity results of the four lines around the tunnel explain well the collapse range caused by the weathering, and forecast well the badly weathering place; the vertical relative low resistivity anomalies infer quantitatively the downward extension of the weathering, which provides scientific information for tunnel construction.