Discussion on the suitability of electrical resistivity tomography method to detect DNAPLs contaminantion
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摘要: 重非水相流体（DNAPLs）的电阻率一般较高，在地下介质中与周围的地下水形成明显的电性差别。利用这一特性，非侵入式的高密度电阻率法（ERT）在探测DNAPLs污染场址中污染范围展现出一定优势。然而在实际场址的调查应用中，ERT有时无法探测到显著的高阻异常，或探测的高阻异常是由于介质的非均质性产生，从而导致对DNAPLs污染分布的误判。针对该问题，文章首先基于有效介质电阻率模型，计算了不同DNAPLs饱和度情形下的地层总电阻率值，从理论上解释了当DNAPLs污染程度较低时不存在显著高阻异常的原因。进一步构建理想ERT正演模型，探讨ERT探测DNAPLs污染的适宜性。研究结果表明：介质的非均质性（例如高阻的砾石等）对ERT探测具有很强的干扰，可造成DNAPLs分布范围的误判，因此传统静态ERT探测DNAPLs污染仅适用于均质介质或者非均质介质中DNAPLs污染程度较高的条件。对于非均质介质DNAPLs污染程度较低时，需采用基于时间域的差分反演方法屏蔽介质非均质性的影响，从而准确判定DNAPLs分布。Abstract: Due to the high resistivity of dense non-aqueous phase liquids (DNAPLs),there is a significant difference in electrical properties between the underground medium and the groundwater. Because of this difference, electrical resistivity tomography (ERT), which is a non-intrusive monitoring method, has shown its potential in detecting organic contaminated sites . However,in field survey ,the ERT couldn’t reveal the obviously high resistivity anomaly sometimes, or the high resistance anomaly is caused by the heterogeneity of the medium rather than the DNAPLs, thus caused the result of the miscarriage of justice.In order to explore the suitability of the ERT in DNAPLs contaminated site, this paper calculates the total true resistivity under different DNAPLs saturation conditions by using effective medium resistivity model. It is theoretically explained that there is no significant high resistance anomaly when the DNAPL saturation is small. Then, an ideal ERT forward model is established to disscuss the suitability of ERT. Results show that the heterogeneity of the medium (such as high resistance of gravel, etc) on the ERT detection has a strong interference, which may cause the miscarriage of justice of the DNAPLs distribution of the scope,and conventional static ERT detection of DNAPLs pollution is only applicable to the conditions which the DNAPLs saturation is high in homogeneous or heterogeneous medium. When the saturation of DNAPLs is small,if the DNAPLs saturation is low,the differential inversion method based on time domain can effectively weaken the influence of the heterogeneity of the medium, and recognize the distribution of DNAPLs accurately.
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Key words:
- ERT /
- effective medium resistivity model /
- heterogeneity /
- differential inversionmethod
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