ABSTRACT In the framework of the Deep Electromagnetic Soundings for Mineral Exploration project, we conducted ground‐based long‐offset transient‐electromagnetic measurements in a former mining area in eastern Thuringia, Germany. The large‐scale survey resulted in an extensive dataset acquired with multiple high‐power transmitters and a high number of electric and magnetic field receivers. The recorded data exhibit a high data quality over several decades of time and orders of magnitude. Although the obtained subsurface models indicate a strong multi‐dimensional subsurface with variations in resistivity over three orders of magnitude, the electrical field step‐on transients are well fitted using a conventional one‐dimensional inversion. Due to superimposed induced polarization effects, the transient step‐off data are not interpretable with conventional electromagnetic inversion. For further interpretation in one and two dimensions, a new approach to evaluate the long‐offset transient‐electromagnetic data in frequency domain is realized. We present a detailed workflow for data processing in both domains and give an overview of technical obstructions that can occur in one domain or the other. The derived one‐dimensional inversion models of frequency‐domain data show strong multi‐dimensional effects and are well comparable with the conventional time domain inversion results. To adequately interpret the data, a 2.5D frequency‐domain inversion using the open source algorithm MARE2DEM (Modeling with Adaptively Refined Elements for 2‐D EM) is carried out. The inversion leads to a consistent subsurface model with shallow and deep conductive structures, which are confirmed by geology and additional geophysical surveys.

中文翻译：

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使用长偏移距瞬态电磁学进行深度探索：时域和频域中的场数据解释

摘要 在矿产勘探深电磁探测项目的框架内，我们在德国图林根州东部的一个前矿区进行了地基长偏移距瞬态电磁测量。大规模调查导致通过多个高功率发射器和大量电场和磁场接收器获得了广泛的数据集。记录的数据在几十年的时间和数量级上表现出很高的数据质量。尽管获得的地下模型表明电阻率变化超过三个数量级的强多维地下，但使用传统的一维反演可以很好地拟合电场步进瞬变。由于叠加的诱导极化效应，瞬态阶跃数据无法用传统的电磁反演来解释。为了在一维和二维上的进一步解释，实现了一种评估频域长偏移瞬态电磁数据的新方法。我们提供了两个域中数据处理的详细工作流程，并概述了一个域或另一个域中可能发生的技术障碍。推导出的频域数据一维反演模型表现出很强的多维效应，与传统的时域反演结果具有很好的可比性。为了充分解释数据，使用开源算法 MARE2DEM（二维 EM 自适应精化元素建模）进行了 2.5D 频域反演。