探討入滲池內高電阻率邊界對於地電阻量測與土壤含水量推估之影響

Effects of the high resistivity boundary on the measurement of soil electrical resistivity and water content in a lysimeter

蘇昱豪(Yu-Hao Su)

指導教授 ： 許少瑜

國立臺灣大學/生物資源暨農學院/生物環境系統工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202103244

摘要

地電阻影像探測法(Electrical Resistivity Tomography)是一種非侵入式的技術，可用來量測地層下電阻率，以及土壤含水量的變化。然而地下水的結構物，例如:入滲池(lysimeter)，不透水混凝土邊界，會影響量測的電阻率。因此本研究藉由COMSOL Mutiphysics數值模擬軟體釐清不導電邊界對於量測電阻率影響的程度，並利用有限差分(finite difference)數值方法為基礎的三維反演方法及深度學習的隨機森林法，修正受邊界效應影響的電阻率資料，並比較兩種方法的修正結果。
COMSOL Mutiphysics模擬結果顯示，不導電邊界的存在，會造成量測的視電阻率增加，此效應稱為邊界效應(boundary effect)。邊界效應的影響程度與測線長及視電阻率剖面深度成正比關係，但與不導電邊界的距離成反比關係。
有限差分和隨機森林兩種方法均可有效的減少邊界效應的影響。本研究進一步分析和建構入滲池降雨實驗中，土壤電阻率和體積含水量的關係，並使用隨機森林修正受邊界效應影響的電阻率。修正前電阻率以及體積含水量關係曲線會隨著深度不同而有所差異，修正後的關係曲線則在深度上的差異有所減少。

關鍵字

地電阻 ； 邊界效應 ； 有限差分 ； 反演 ； 隨機森林

並列摘要

Electrical Resistivity Tomography(ERT) is a non-invasive technique that can be used to measure the changes in resistivity underground and the changes in soil moisture content. However, artificial hydraulic structures, such as lysimeter, usually consist of impermeable concrete with high resistivity inside the structures or around the boundary, significantly affecting the measured resistivity. Therefore, we clarify the influence of the non-conductive boundary on the measured resistivity with numerical simulation, COMSOL Mutiphysics. Furthermore, we used the finite-difference based on three-dimensional inversion method and the deep learning random forest method to modify the resistivity data affected by the boundary effect. The modified results of the two methods were compared.
The COMSOL Multiphysics simulation results showed an apparent boundary effect that a non-conductive boundary will increase the measured apparent resistivity. The boundary effect was directly proportional to the length of the survey line and the apparent resistivity profile depth but inversely proportional to the distance of the non-conductive boundary.
Both finite difference and random forest methods can effectively reduce the influence of boundary effects. Furthermore, this study analyzed and constructed the relationship between soil resistivity and water content in a lysimeter during an artificial rainfall experiment. The results show a discrepancy of the resistivity-water content relationships at different depths, but the difference is reduced when the resistivity is modified by random forest considering the boundary effect.

並列關鍵字

Electrical Resistivity Tomography ； boundary-effect ； Finite difference method ； inversion ； random forest

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