Focused time-lapse inversion of radio and audio magnetotelluric data

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• 作者：Marina Rosas Carbajal ; Niklas Linde ; Thomas Kalscheuer
• 关键词：Inverse theory ; Groundwater monitoring ; Time-lapse inversion ; Magnetotellurics ; Hydrogeophysics ; Iteratively reweighted least©\squares
• 刊名：Journal of Applied Geophysics
• 出版年：2012
• 出版时间：September, 2012
• 年：2012
• 卷：84
• 期：Complete
• 页码：29-38
• 全文大小：943 K

文摘

Geoelectrical techniques are widely used to monitor groundwater processes, while surprisingly few studies have considered audio (AMT) and radio (RMT) magnetotellurics for such purposes. In this numerical investigation, we analyze to what extent inversion results based on AMT and RMT monitoring data can be improved by (1) time-lapse difference inversion; (2) incorporation of statistical information about the expected model update (i.e., the model regularization is based on a geostatistical model); (3) using alternative model norms to quantify temporal changes (i.e., approximations of l₁ and Cauchy norms using iteratively reweighted least-squares), (4) constraining model updates to predefined ranges (i.e., using Lagrange Multipliers to only allow either increases or decreases of electrical resistivity with respect to background conditions). To do so, we consider a simple illustrative model and a more realistic test case related to seawater intrusion. The results are encouraging and show significant improvements when using time-lapse difference inversion with non l₂ model norms. Artifacts that may arise when imposing compactness of regions with temporal changes can be suppressed through inequality constraints to yield models without oscillations outside the true region of temporal changes. Based on these results, we recommend approximate l₁-norm solutions as they can resolve both sharp and smooth interfaces within the same model.