Improved ice quantification at alpine permafrost sites based on electrical and electromagnetic measurements of spectral induced polarization

由于气候变化，山地永久冻土目前正在发生重大变化，在世界各地的许多地点都观察到持续的变暖和融化。由于冻土特别是其地面冰含量既不能从地表直观地评估，也不能通过标准的遥感技术来评估，地球物理方法越来越多地用于研究冻土的时空动态。特别是电阻率层析成像（ERT）已被证明是表征和监测永久冻土发生的合适工具。然而，由于岩石的电阻率不仅取决于冰/水含量，还取决于孔隙度、孔隙连通性、矿物学、水化学和温度，因此几乎不可能通过ERT测量可靠地量化冰含量。利用冰具有电极化特征这一事实，我们提出了一种基于光谱诱导极化（SIP）空间分辨测量的高寒多年冻土区地下冰含量成像和量化的新地球物理方法。对应于冰显示最大极化的频率范围（100 Hz至100 kHz），我们计划通过时域电磁感应法（TEM）的间接SIP测量来补充直接SIP测量，以弥补在更高频率下直接SIP测量中可能缺乏的信息。SIP频谱将被描述为背景信号和特征冰响应的叠加，冰含量将根据（部分）冻结岩石的SIP特性的新开发的岩石物理模型来确定。提出的定量、空间分辨测定冰含量的新方法将在阿尔卑斯地区具有典型高山永久冻土特征的选定地点进行测试，这些地点可从以前的地球物理调查和现有的监测数据中获得校准和验证的独立信息。所涉及的岩石物理模型将通过SIP实验室测量在考虑的现场岩石样品的控制冻融实验过程中进行验证，并将独立分析相关模型参数。据我们所知，该项目代表了第一个基于SIP/TEM联合测量的高山永久冻土地下冰含量定量成像的综合研究。虽然实验室研究和岩石物理模型的发展有助于提高对（部分）冻结多孔介质电学性质的基本理解，但考虑到各种类型的高山永久冻土层，实地研究与全球变暖背景下有效的永久冻土层监测高度相关。

Mountain permafrost is currently undergoing substantial changes due to climate change, and consistent warming and thawing has been observed at many sites worldwide. As permafrost and especially its ground ice content can neither be assessed visually from the surface nor by standard remote sensing techniques, geophysical methods are increasingly being used to investigate the spatio-temporal dynamics of permafrost. In particular electrical resistivity tomography (ERT) has been proven a suitable tool to characterize and monitor permafrost occurrences. However, since the resistivity of rocks depends not only on ice/water content but also on porosity, pore connectivity, mineralogy, water chemistry and temperature, it is hardly possible to reliably quantify ice content from ERT measurements.Utilizing the fact that ice exhibits a characteristic electrical polarization signature, we propose a new geophysical methodology for the imaging and quantification of subsurface ice content at alpine permafrost sites based on the spatially resolved measurement of spectral induced polarization (SIP). Corresponding to the frequency range in which ice shows a maximum polarization (100 Hz to 100 kHz), we plan to complement direct SIP measurements by indirect SIP measurements by means of the time-domain electromagnetic induction method (TEM), in order to compensate a possible lack of information in the direct SIP measurements at higher frequencies. The SIP spectrum will be described as a superposition of the background signal and the characteristic ice response, and ice content will be determined on the basis of a newly developed petrophysical model of the SIP properties of (partially) frozen rocks.The proposed new methodology for the quantitative, spatially resolved determination of ice content will be tested at selected sites in the Alps with typical alpine permafrost characteristics, where independent information for calibration and validation is available from previous geophysical investigations and existing monitoring data. The involved petrophysical model will be validated by means of SIP laboratory measurements in the course of controlled freeze-thaw experiments on rock samples from the considered field sites, which will be independently analyzed for relevant model parameters.To our knowledge the project represents the first comprehensive study on the quantitative imaging of subsurface ice content in alpine permafrost terrain based on combined SIP/TEM measurements. While the laboratory studies and the petrophysical model development contribute to an improved fundamental understanding of the electrical properties of (partially) frozen porous media, by considering various types of alpine permafrost occurrences the field studies are highly relevant with a view to an effective permafrost monitoring in the context of global warming.