Monitoring rock freezing and thawing by novel geoelectrical and acoustic techniques
Monitoring rock freezing and thawing by novel geoelectrical and acoustic techniques
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DOI：
10.1002/2016jf003948
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发表时间：
2016-12
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影响因子：
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通讯作者：
J. Murton;O. Kuras;M. Krautblatter;T. Cane;D. Tschofen;S. Uhlemann;Sandra Schober;P. Watson
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文献类型：
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作者：
J. Murton;O. Kuras;M. Krautblatter;T. Cane;D. Tschofen;S. Uhlemann;Sandra Schober;P. Watson
需要对山体岩壁中的冻融循环和裂缝扩展进行自动监测，以提供关于落石危险的早期预警。传统的地电方法，如电阻率层析成像(ERT)，受到大而可变的欧姆接触电阻的限制，需要与插入岩石钻孔中的金属电极进行电流耦合，并且可能因岩石风化而松动。我们报道了一种新的实验方法，该方法结合电容电阻率成像(CRI)、ERT和微震事件记录来监测六块硬石灰岩和软石灰岩在模拟冻土上方活跃层和非冻土环境中季节性冻结岩石的冻融情况。我们的结果表明，CRI方法对冻融过程高度敏感；它产生的属性信息与常规ERT获得的信息相当，并为非电性长期地电监测提供了一条可行的途径，将该方法的优势扩展到软硬岩石环境。与ERT相比，CRI获得的接触阻抗受季节温度变化、孔隙水(液体或冻结)的聚集状态以及具有高基质电阻率的低孔隙率岩石的存在的影响较小。与声发射相比，微地震监测的优势在于，事件是在距破裂事件十米的相关场地距离内记录的。我们首次记录了约1000个微破裂事件，并根据频率和波形将它们分成四组。与以前主要关于冰川中的冰裂的研究相比，这些群体被归因于单阶段或多阶段的破裂事件，如裂缝合并。
Automated monitoring of freeze‐thaw cycles and fracture propagation in mountain rockwalls is needed to provide early warning about rockfall hazards. Conventional geoelectrical methods such as electrical resistivity tomography (ERT) are limited by large and variable ohmic contact resistances, requiring galvanic coupling with metal electrodes inserted into holes drilled into rock, and which can be loosened by rock weathering. We report a novel experimental methodology that combined capacitive resistivity imaging (CRI), ERT, and microseismic event recording to monitor freeze‐thaw of six blocks of hard and soft limestones under conditions simulating an active layer above permafrost and seasonally frozen rock in a nonpermafrost environment. Our results demonstrate that the CRI method is highly sensitive to freeze‐thaw processes; it yields property information equivalent to that obtained with conventional ERT and offers a viable route for nongalvanic long‐term geoelectrical monitoring, extending the benefits of the methodology to soft/hard rock environments. Contact impedances achieved with CRI are less affected by seasonal temperature changes, the aggregate state of the pore water (liquid or frozen), and the presence of low‐porosity rock with high matrix resistivities than those achieved with ERT. Microseismic monitoring has the advantage over acoustic emissions that events were recorded in relevant field distances of meters to decameters from cracking events. For the first time we recorded about 1000 microcracking events and clustered them in four groups according to frequency and waveform. Compared to previous studies, mainly on ice‐cracking in glaciers, the groups are attributed to single‐ or multiple‐stage cracking events such as crack coalescence.