高密度电法在地铁岩溶勘察中的应用

APPLICATION OF HIGH-DENSITY RESISTIVITY METHOD TO KARST INVESTIGATION OF METRO ENGINEERING

南京地铁3号线将穿越岗谷地和岩溶区,是建设施工过程中的几大难题之一。经过初步勘查,发现其中某段有多处溶洞。该段地处闹市区,交通流量大,建筑物密集,地下管线多,地质条件较为复杂。为了查明研究区岩溶发育情况,结合地质构造条件,经综合比选,采用高密度电法进行岩溶勘察,测量方法选择温纳剖面法。测量过程中将部分电极勘察点布设于沥青或混凝土路面上,接地条件相对较差,采用常规数据处理方法(数据反演),与实际钻探结果误差较大,不宜使用。因此在测量中采用改进电极与地面耦合条件以及加大供电电压的方法,提高电流穿透力,同时对所测数据直接绘制成等值线图。根据等值线图中低电阻率分布特征推断岩溶发育情况,其结果与钻探结果较为吻合,取得了较好效果。经与现场工程地质调查和钻孔资料对比分析,查明沿线岩溶空间分布。研究结果表明物探方法具有多解性,在开始物探工作之前,应对研究区地质和构造条件进行野外调查,对岩溶发育特征进行地质分析,以便更好的指导物探布线和数据解译工作;当接地条件较差时,可以采取堆土、洒水等方法解决电极耦合问题;对于数据反演解译存在误差时,可直接利用视电阻率等值线图进行解译,推测溶洞分布。

The No.3 line of Nanjing Metro would cross a valley and karst area,which is one of the several major geotechnical problems during the construction.A preliminary survey showed that there were many caves in the area.It＇s located in the downtown area where has heavy traffic and dense buildings,many underground pipelines and complicated geological conditions.In order to find out the karst development situation in a certain place for the engineering,the high-density resistivity method was used for karst prospecting after combination of geological structure and comprehensive election.The WN measurement was carried out.The electrical sense points are located on asphalt or cement concrete pavements.The condition of the sense contact to the ground surface was poor.So it had substantial errors in the back analysis process.For this reason,the electrode with ground coupling conditions were improved in the measurement.The power supply voltage was also increased to improve the current penetration.During testing,the isolines were rendered on the test data directly.According to the characteristics of low resistivity distribution from the isolines,the karst development situation was inferred.The results coincide with the drilling results relatively.The results were successful.After the contrast analysis between field engineering geological investigation and the borehole data,the spatial distribution of the karsts was identified.The results show that the geophysical method has multiple solutions.Before the geophysical work,a field investigation should be carried out for the geological and tectonic conditions in study area.Then geological analysis of the karst characteristics should be carried out in order to guide the work of geophysical wiring and data interpretation well.When the ground conditions are poor,the methods of piling soil up and watering can be undertaken to solve the problem of electrode coupling.To the existence of error of the back analysis,the isoline map of apparent resistivity can be used directly to infer the locations of karsts.