Assessing the effect of offline topography on electrical resistivity measurements: insights from flood embankments

IF 2.8 3区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
Adrian White, James Boyd, Paul Wilkinson, Holly E Unwin, James Wookey, John Michael Kendall, Andrew Binley, Jonathan Chambers
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Abstract

Summary Electrical resistivity tomography (ERT), a geophysical imaging method, is commonly used on flood embankments (dykes or levees) to characterise their internal structure and look for defects. These surveys often use a single line of electrodes to enable 2D imaging through the embankment crest, an approach that enables rapid and efficient surveying compared to 3D surveys. However, offline variations in topography can introduce artefacts into these 2D images, by affecting the measured resistivity data. Such topographic effects have only been explored on a site-specific basis. If the topographic effects can be assessed for a distribution of embankment geometries (e.g. slope angle and crest width) and resistivity variations, it would allow for targeted correction procedures and improved survey design. To investigate topographic effects on ERT measurements, we forward-modelled embankments with different trapezoidal cross-sections sat atop a flat foundation layer with contrasting resistivity values. Each was compared to a corresponding flat model with the same vertical resistivity distribution. The modelling workflow was designed to minimise the effect of forward modelling errors on the calculation of topographic effect. We ran 1872 unique embankment forward models, representing 144 geometries, each with 13 different resistivity contrasts. Modelling results show that offline topography affects the tested array types (Wenner-Schlumberger, Dipole-Dipole, and Multiple-Gradient) in slightly different ways, but the magnitudes are similar, so all are equally suitable for embankment surveys. Three separate mechanisms are found to cause topographic effects. The dominant mechanism is caused by the offline topography confining the electrical current flow, increasing the measured transfer resistance from the embankment model. The two other mechanisms, previously unidentified, decrease the measured transfer resistances from the embankment model compared to a layered half-space but only affect embankments with specific geometries and resistivity distributions. Overall, we found that for typical embankment geometries and resistivity distributions, the resistivity distribution has a greater control on the magnitude of the topographic effect than the exact embankment geometry: the subsurface resistivity distribution cannot be neglected. 2D inversions are suitable when both the embankment is more resistive than the foundations and when the embankment's cross-sectional area is greater than 4 m2/m2 (area scaled to an embankment with a height of 1 m). Topographic corrections, 3D data acquisition or 3D forward models are required when these conditions are not met. These are demonstrated using field data from an embankment at Hexham, Northumberland, UK. Improving the accuracy of the resistivity values in ERT models will enable more accurate ground models, better integration of resistivity data with geotechnical datasets, and will improve the translation of resistivity values into geotechnical properties. Such developments will contribute to a better characterised and safer flood defence network.
评估离线地形对电阻率测量的影响:洪水堤坝的启示
摘要 电阻率层析成像 (ERT) 是一种地球物理成像方法,通常用于防洪堤(堤坝或防洪堤),以确定其内部结构的特征并查找缺陷。这些勘测通常使用单线电极对堤顶进行二维成像,与三维勘测相比,这种方法能够实现快速高效的勘测。然而,地形的离线变化会影响电阻率的测量数据,从而在这些二维图像中引入伪影。这种地形影响只针对具体地点进行过研究。如果能对堤坝几何分布(如坡角和坡顶宽度)和电阻率变化的地形影响进行评估,就能制定有针对性的校正程序并改进勘测设计。为了研究地形对 ERT 测量的影响,我们在电阻率值对比强烈的平坦地基层上建立了不同梯形横截面堤坝的正演模型。每个模型都与具有相同垂直电阻率分布的相应平面模型进行了比较。建模工作流程的设计目的是尽量减少前向建模误差对地形效应计算的影响。我们运行了 1872 个独特的堤坝正演模型,代表 144 种几何形状,每种形状有 13 种不同的电阻率对比。建模结果表明,离线地形对测试的阵列类型(温纳-施伦贝谢、偶极子-偶极子和多梯度)的影响方式略有不同,但幅度相似,因此都同样适用于堤坝勘测。发现有三种不同的机制会造成地形效应。最主要的机制是离线地形限制了电流流动,增加了从堤坝模型测得的传输电阻。另外两种机制之前尚未发现,与分层半空间相比,它们会降低路堤模型的实测传输电阻,但只影响具有特定几何形状和电阻率分布的路堤。总之,我们发现,对于典型的堤坝几何形状和电阻率分布,电阻率分布比确切的堤坝几何形状更能控制地形效应的大小:地下电阻率分布不容忽视。二维反演适用于堤坝电阻率比地基电阻率大,以及堤坝横截面积大于 4 平方米/平方米(按堤坝高度 1 米的面积比例计算)的情况。如果不满足这些条件,则需要进行地形校正、三维数据采集或三维正演模型。本文利用英国诺森伯兰郡海克萨姆的堤坝实地数据对这些方法进行了演示。提高 ERT 模型中电阻率值的精确度将有助于建立更精确的地面模型,更好地将电阻率数据与岩土数据集整合在一起,并改进电阻率值与岩土特性之间的转换。这些发展将有助于建立一个特性更好、更安全的防洪网络。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Geophysical Journal International
Geophysical Journal International 地学-地球化学与地球物理
CiteScore
5.40
自引率
10.70%
发文量
436
审稿时长
3.3 months
期刊介绍: Geophysical Journal International publishes top quality research papers, express letters, invited review papers and book reviews on all aspects of theoretical, computational, applied and observational geophysics.
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