不同演化阶段的NAPLs污染区高密度电阻率法探测效果分析

Effect Analysis of Electrical Resistivity Tomography (ERT) Detection in NAPLs Contaminated Areas at Different Evolutionary Stages

应用高密度电阻率法(ERT)能够快速、实时探测非水相液体(NAPLs)污染区,但污染发生后不同时间段的NAPLs污染区在土壤中呈现不同的演化特征。为了解ERT调查剖面中NAPLs污染区自泄漏至形成污染羽全过程的异常区变化特征,本文归纳总结污染过程NAPLs饱和度和电阻率变化特点,构建不同演化阶段NAPLs污染区地质及电阻率模型,计算得到不同装置形式的电阻率理论探测剖面;通过对比分析,判断污染区不同演化阶段在ERT剖面中相应的异常特征。结果显示,污染物侵入量较少时,存在于非饱和带中的LNAPLs污染区在电阻率剖面中形成低电阻率异常区,随着污染物的积累,污染区转变为高电阻率异常区,且污染范围逐渐向下扩展。DNAPLs污染区在非饱和带中的电阻率变化特征与LNAPLs污染区具有一定相似性。在饱和带中,污染区表现为高电阻率异常,随着污染物向隔水底板迁移及饱和度的降低,污染区的高电阻率异常特征减弱。ERT剖面中难以识别饱和度低于0.10的污染区。施龙贝格装置水平方向分辨率较高,而偶极装置垂向分辨率较高,这为实地探测中不同范围的高、低电阻率异常区的解译以及不同污染区探测时采集装置的选择提供了参考。

Electrical resistivity tomography(ERT) can detect non-aqueous phase liquid(NAPLs) contaminated areasin real-time and quickly. However, NAPLs contaminated areas showed different evolution characteristics in the soilduring different time periods after pollution occurred. In order to understand the change characteristics of abnormal areas in the whole process from the leakage of NAPLs contaminated area to the formation of pollution plume in the ERT investigation. This paper summarizes the variation characteristics of NAPLs saturation and resistivity in the pollution process, and constructs geological and resistivity models of NAPLscontaminated areas at different evolutionary stages. Theoretical resistivity detection profiles of different devices forms are calculated. Through comparative analysis, the corresponding abnormal characteristics on the ERT profile at different evolutionary stages in the contaminated area are judged. The results show that when the intrusion amount is small, the LNAPLs contaminated area in the unsaturated zone forms an abnormal zone with low resistivity on the resistivity profile. With the accumulation of pollutants, the polluted area turns into the abnormal area withhigh resistivity, and the scope of contamination gradually expand downward. The variation characteristics of resistivity in the unsaturated zone of DNAPLs polluted area is similar to that of LNAPLs polluted area. In the saturated zone, the contaminated area presents high resistivity anomaly. With the migration of pollutants to the water-resistant floor and the decrease of saturation, the abnormal characteristics of high resistivity in the contaminated area weakened. It is difficult to identify the contaminated area with saturation below 0.10 on the ERT profile. The Schlumberger device has a higher horizontal resolution, while the dipole device has a higher vertical resolution. It provides a reference for the interpretation of abnormal areas with low or high resistivity in different ranges during field surveys and the selection of acquisition devices in different pollution areas.