Advancing Electrical Resistivity Tomography as an Environmental Monitoring Tool in Seasonally Frozen Ground: Linking Lab and Field Scales

When time-lapse electrical resistivity tomography (ERT) is used as an environmental monitoring tool, the effects of temperature variability should be taken into account to avoid erroneous interpretations of subsurface processes. In regions where the near-surface freezes seasonally, removing the effects of temperature is a non-trivial task. One limiting factor is the poorly understood relationship between electrical resistivity and temperature under frozen conditions. A laboratory experiment was conducted that quantified this relationship and related resistivity to unfrozen water content and fluid resistivity using a modified version of Archie’s equation. A second limiting factor is that the standard inversion has a limited ability to resolve sharp boundaries and large contrasts in resistivity, like those seen between frozen and unfrozen regions. A synthetic time-lapse ERT modelling study showed that because standard ERT inversion techniques were unable to accurately recover the resistivity of the frozen surface layer, temperature corrections applied to these models performed poorly. In this synthetic experiment, modifications to data acquisition (burying electrodes or reducing electrode spacing) and regularization strategy (increasing lateral smoothness, reducing regularization across boundaries, or using an L1 model misfit norm) did not appreciably improve the outcomes of temperature corrections in partially frozen ground. A hybrid inversion strategy was developed to incorporate prior information about the geometry of the frozen layer in the inversion. The hybrid inversion used a parametric approach with only depth and a single resistivity to describe the frozen layer, while the rest of the model space was described with a large number of voxels. A synthetic experiment showed that the hybrid inversion improved resolution of a frozen surface layer and features beneath it compared to a standard smooth inversion. The improved resistivity models resulted in lower errors in the temperature-corrected data. However, resolution of the frozen layer was still limited by the information content of the data, regardless of inversion strategy. Altogether, this study improves our understanding of the physical processes and relationships that govern resistivity at subzero temperatures, highlights limitations of standard data processing strategies, and demonstrates the efficacy of a hybrid inversion approach for ERT data collected in partially frozen ground

Initiation of an international database of geoelectrical surveys on permafrost to promote data sharing, survey repetition and standardized data reprocessing 

Geoelectrical methods are widely used for permafrost investigations by research groups, government agencies and industry. Electrical Resistivity Tomography (ERT) surveys are typically performed only once to detect the presence or absence of permafrost. Exchange of data and expertise among users is limited and usually occurs bilaterally. Neither complete information about the existence of geophysical surveys on permafrost nor the data itself is available on a global scale. Given the potential gain for identifying permafrost evidence and their spatiotemporal changes, there is a strong need for coordinated efforts regarding data, metadata, guidelines, and expertise exchange. Repetition of ERT surveys is rare, even though it could provide a quantitative spatio-temporal measure of permafrost evolution, helping to quantify the effects of climate change at local (where the ERT survey takes place) and global scales (due to the inventory).Our International Permafrost Association (IPA) action group (2021-2023) has the main objective of bringing together the international community interested in geoelectrical measurements on permafrost and laying the foundations for an operational International Database of Geoelectrical Surveys on Permafrost (IDGSP). Our contribution presents a new international database of electrical resistivity datasets on permafrost. The core members of our action group represent more than 10 research groups, who have already contributed their own metadata (currently > 200 profiles covering 15 countries). These metadata will be fully publicly accessible in the near future whereas access to the resistivity data may be either public or restricted. Thanks to this open access policy, we aim at increasing the level of transparency, encouraging further data providersand fostering survey repetitions by new users.The database is set up on a virtual machine hosted by the University of Fribourg. The advanced open-source relational database system PostgreSQL is used to program the database. Homogenization and standardization of a large number of data and metadata are among thegreatest challenges, yet are essential to a structured relational database. In this contribution, we present the structure of the database, statistics of the metadata uploaded, as well as first results of repetitions from legacy geoelectrical measurements on permafrost. Guidelines and strategiesare developed to handle repetition challenges such as changing survey configuration, changing geometry or inaccurate/missing metadata. First steps toward transparent and reproducible automated filtering and inversion of a great number of datasets will also be presented. Byarchiving geoelectrical data on permafrost, the ambition of this project is the reanalysis of the full database and its climatic interpretation