潮白河冲洪积扇下伏基岩面起伏及活动断裂电阻率特征显现——以雁栖湖开发区牤牛河区块为例

Characteristics of Rock Depth and Resistivity in the Alluvial-proluvial Fan of Chaobai River: A Case Study of Mangniu River Block in Yanqi Development Zone

潮白河冲洪积扇中上部下伏基岩面起伏及活动断裂分布,是影响重大工程规划和设计方案的重要因素。为怀柔雁栖湖经济开发区北京光源场地选址,开展以高密度电法为主的地球物理勘探。结果发现,不同于以往该区200~300 m第四系厚度等值线,燕山晚期宰相庄花岗岩体埋深由东北的30 m到西南的250 m,存在着一个近NW走向的埋深剧变梯度带,地下100 m以内电阻率从2 000多Ω·m变化到20Ω·m,相差2个数量级,说明第四系厚度剧烈变化和岩性及构造作用影响之大。在场地西侧500 m外为黄庄-高丽营断裂,其东侧200 m处发育与主干断裂平行的分支,断裂带角度陡而电阻率低。下伏基岩埋深梯度带产生不均匀沉降和活断层,影响到工程地基稳定性,随后该工程场址北移另建。

Both the fluctuation of the buried rock surface and the distribution of the active fault in the upper part of the alluvial-proluvial fan of Chaobai River are important factors for planning and design of major projects. Taking the site selection of Beijing synchrotron radiation facility（ BSRF） in Yanqi development zone as an example,high density resistivity method,one of the most commonly used geophysics exploration methods,was used to explore the characteristics of the site,the results show that,different from the contours of the Quaternary thickness between200 ~ 300 m in the site. The buried depth of the Zaixiangzhuang granite body,formed in Late Yanshanian,changes from 30 m in the northeast to 250 m in the southwest,indicating that there exists one gradient zone,which resulting in the ground resistivity within 100 m changed from more than 2 000 Ω·m to about 20 Ω·m,it＇s two orders of magnitude different. All of these indicate the existence of the dramatic changes of Quaternary thickness and the influence of lithology. About 500 m far away from the west side of the site,the Huangzhuang-Gaoliying fault spreads,whose parallel branch is located 200 m far from the west side,and the fault zone is vertically steep with low resistivity. According the above findings,the uneven settlement would occur when the major project is to be located at the gradient zone of the granite body. And the smaller distance to the active faults also affects the basement stability.The exploration result was accepted to an extent that a new place 5 km north away with more favorable geological conditions was selected.