Permafrost monitoring by reprocessing and repeating historical geophysical measurements (REP-ERT)

Measurements using electrical resistivity tomography (ERT) make it possible to detect permafrost, i.e. continuously frozen ground, thanks to the very different electrical properties of frozen and unfrozen subsoil. However, for financial and logistical reasons only very few continuous ERT monitoring installations on permafrost exist worldwide. One of the exceptions are six permafrost sites in the Swiss Alps that have been constantly monitored with ERT in the context of the Swiss Permafrost Monitoring Network (PERMOS) since 2005, enabling analysis of the long-term change in the subsurface ice content and associated thawing and freezing processes. In contrast, there are many permafrost sites (estimated at more than 500) where one-off ERT measurements have been performed in the past. The aim of the REP-ERT project is to demonstrate the potential of these datasets for the climatological analysis of permafrost areas and to preserve them for future repeat measurements by incorporating them into a shared database.

Research aims:

1. Initiate and compile a database for historical ERT data on permafrost
2. Develop and apply suitable QA/QC criteria for data filtering and inversion (Mollaret et al. 2019)
3. Develop and conduct a protocol for regular repetitions of ERT measurements in a climatic context (repetition rate > 10 years)
4. Initiate first steps to include electrical resistivities as proxy for ice content changes in the international GTN-P (Global Terrestrial Network - Permafrost) database, where up to now only single-point and sparse borehole temperatures and active layer thicknesses are included.

 Study area: In a first step the study area is focused on the European Alps (location of the Swiss sites shown in Figure 1), but the project aims to encourage a joint international data base and survey and processing routines for all ERT surveys on permafrost worldwide (see Figure 2 for the location of all the profiles already integrated on the database).

Figure 1 (a) Map with location of 200 single ERT surveys on permafrost in Switzerland, which have been compiled in the REP-ERT project and which are already contained in the IDGSP data base. (b)-(d): close-ups of specific regions in the Valais Alps, Engadine and Bernese Alps for illustration. The maps are underlain with the colour-coded modelled permafrost distribution currently available under map.geo.admin.ch. The database is constantly updated by adding further survey locations and their metadata.


Figure 2 The map shows the locations of more than 400 ERT surveys on permafrost which are currently contained in the IDGSP data base.

• Project Overview

  Geophysical methods and especially electrical techniques have been used for permafrost detection and monitoring since more than 50 years. However, only after the development of 2-dimensional tomographic measurement and processing techniques in the late 1990’s, i.e. Electrical Resistivity Tomography (ERT), these methods became generally available and were applied on many mountain permafrost sites in the European Alps. Due to the large contrast in electrical resistivity between unfrozen and frozen material, ERT is well suited to detect, but also to monitor frozen ground, and more specifically the ground ice content (Hauck 2002).

  Within the Swiss permafrost network PERMOS, operational ERT measurements are conducted since 2005 for the monitoring of the changes in subsurface ground ice content at six permafrost stations in the Swiss Alps on a yearly basis (Hilbich et al. 2008, PERMOS 2019). A thorough analysis of this data set has shown its high quality and robustness against potential error sources related with the harsh high mountain field conditions and has indicated common climatic trends at all sites, i.e. a decreasing trend of mean specific resistivity since the first measurements in 1999 (Hauck 2002, Mollaret et al. 2019).

  Because of the comparatively large efforts needed for continuous and long-term ERT monitoring, only a very small number of operational ERT monitoring sites exist worldwide in permafrost terrain. However, a much larger number (estimated to be > 500) of permafrost sites with singular ERT measurements exist, many of them published in the scientific literature (for a review see Hauck 2013). These data sets are neither included in a joint database nor have they been analysed in an integrated way. The GCOS Switzerland funded project REP-ERT addresses this important historical data source. Whereas singular ERT data from different permafrost occurrences are not easily comparable due to the local influence of the geologic material on the obtained electrical resistivities, their use as baseline for repeated measurements and subsequent processing and interpretation in a climatic context is highly promising and could be effectuated with low efforts (e.g. Isaksen et al. 2011). The project plans therefore to establish a database of historical ERT surveys on permafrost, quality criteria, repetition protocols and processing routines in order to be able to interpret repeated ERT surveys in a climatic context.

  Although the proposed project is primarily focused on permafrost occurrences in Switzerland for financial reasons, the approach and processing routines will be developed together with international collaborators with the aim to adopt them in an international context as being currently discussed in international initiatives (Lewkowicz et al. 2017).

   

  1. References

  Hauck, C. (2002): Frozen ground monitoring using DC resistivity tomography. Geophysical Research Letters, 29 (21): 2016, doi: 10.1029/2002GL014995.

   

  Hauck, C. (2013): New concepts in geophysical surveying and data interpretation for permafrost terrain. Permafrost and Periglac. Process. 24, 131–137, doi: 10.1002/ppp.1774.

   

  Hilbich, C., Hauck, C., Delaloye, R. & Hoelzle, M. (2008): A geoelectric monitoring network and resistivity-temperature relationships of different mountain permafrost sites in the Swiss Alps. Proceedings Ninth International Conference on Permafrost, Fairbanks, Vol. 1, Kane D.L. and Hinkel K.M. (eds), Institute of Northern Engineering, University of Alaska Fairbanks, 699-704.

   

  Isaksen, K., Ødegård, R.S., Etzelmüller, B., Hilbich, C., Hauck, C., Farbrot, H., Eiken, T., Hygen H.O., Hipp T. (2011): Degrading Mountain Permafrost in Southern Norway: Spatial and Temporal Variability of Mean Ground Temperatures, 1999–2009, Permafrost and Periglacial Processes 22(4), 361–377.

   

  Lewkowicz, A.G., Douglas, T. and Hauck, C. (2017): Towards a Global Permafrost Electrical Resistivity Survey (GPERS) database. In EGU General Assembly Conference Abstracts (Vol. 19, p. 12241).

   

  Mollaret C, Hilbich C, Pellet C, Flores-Orozco A, Delaloye R, and Hauck C. (2019). Mountain permafrost degradation documented through a network of permanent electrical resistivity tomography sites, The Cryosphere Discuss., https://doi.org/10.5194/tc-2018-272, in review.

   

  PERMOS 2019. Permafrost in Switzerland 2014/2015 to 2017/2018. Noetzli, J., Pellet, C., and Staub, B. (eds.), Glaciological Report (Permafrost) No. 16-19 of the Cryospheric Commission of the Swiss Academy of Sciences, 104 pp, doi:10.13093/permos-rep-2019-16-19. 

• Collection of metadata

  A list of about 50 metadata fields were defined for each survey (half of them are optional). The detailed information about each field is explained in the metadata form (link below). The different metadata fields can be found in the database structure section (below).

  Call for geoelectrical metadata

  The call for metadata is open and will remain open in the future. Please submit the metadata of your electrical profiles via email to  through the metadata form. Metadata can be submitted at any time. (Feel free to contact ertdb for any question).
  
  Selected metadata fields extracted from the database can be downloaded here.

  Please find here the data policy defined with the IDGSP IPA Action Group members.

   

• Collection of resistivity data

  Harmonization and standardization of data format was done using the unified data format (http://resistivity.net/bert/data_format.html), which is transparent and transportable.

  Call for geoelectrical data

  The resistivity data is welcome in any format (standard and self-explanatory formats are of course preferred). BUT to simplify the upload of resistivity data into the database, the unified data format  (link above) is preferred (with the topography included at the beginning of the file).

  Further information about the data format and examples can be found here.

  Please find here the data policy defined with the IDGSP IPA Action Group members.

• Database structure

  A server hosted at the University of Fribourg has been dedicated to the database (maintained by the IT service of the University allowing for a long-term archive). We used the open source relational database management system PostgreSQL (together with Psycopg2 - a PostgreSQL database adapter for the Python programming language). An important task of this project was to develop the structure of the database. First, we made a list of all the content, that was considered necessary and optional in the database. And then, we built the different tables of data and metadata and defined how the tables related to each other. The final structure of the database is shown in Figure 2. It is built in a way that new tables (or new fields in existed tables) can be added in the future if needed. Therefore, the database structure stays flexible and may integrate new features.

  Figure 2 shows how the 17 main tables are related to each other’s. 11 secondary tables are not represented in Figure 2 for a better readability. The secondary tables are used to reduce the storage needs on the server, by linking certain type of fields to a unique numeric id (e.g. each country is linked to a unique number).

  Figure 2 Structure of the database resi_base including 17 related main tables categorized in 5 groups: metadata (blue), data - raw and inverted (green), filtering parameters (yellow), inversion parameters (orange) and data quality (red). (Mollaret et al., in preparation)

• Access to the database

  For database robustness and security reasons, the database cannot be directly accessed by anyone outside from the University of Fribourg.  However, we worked on a public user-friendly web interface to visualize most of the content of the database (including metadata and resistivity data).

  Dash (an open-source framework for building data apps) is currently used to develop a public, free and user-friendly access to the database content (see Figure 4 an exemplary visualization). Heroku will be used to deploy the data app.

  The link to the dash app will appear here as soon as technical issues are solved.

  

  Figure 4 Exemplary print screen of the dash app under development showing the visualization of ERT surveys with their selected associated metadata on a map. This example is from Hockenhorn mountain top in the Berner Oberland, Switzerland

• Guidelines for ERT survey repetition

  The guidelines were developed (together with international collegues members of the IDGSP IPA Action Group) to define criteria and priorities for choosing which ERT surveys on permafrost should be repeated first.

  Criteria

  The most criterion is the completeness of metadata information of the measurements to repeat, especially regarding the location, precise electrode position and survey geometry.

  Priorities

  • data sets with high data quality, i.e. low filter ratio (avoid repeating measurements where most of the measures are missing e.g. because of bad electrodes contacts)
  • the biggest time lag? That's to say the oldest measurements without repetition... probably something that has not been repeated in the last 10 or more years?
  • minimum time lag: 5-10 years
  • accessibility of the site
  • permafrost-related criteria: we would like to cover many climatic regions/latitudes/mountain ranges as well as landforms (not only rock glaciers etc.)
  • regions with sparse data
  • repeat measurements that has been already published

  Guidelines

  • Take place in the same period (+/- 2 weeks up to 1-month shift) of previous measurements: as a priority, but still accept deviations.
  • Same array(s) but the repetition survey can contain additional quadrupoles to improve the resolution for future repetitions
  • Repetition with the same settings. Preferably e.g. same electrode spacing.
  • Same instrument not important (we made tests with different instruments and the results were not sensitive to it)
  • Consider "possible repetition quality" - how good can the survey be repeated? Important for moving rock glaciers, rockfalls, etc.
• Publications

  The GCOS-funded project REP-ERT was presented at numerous conferences: Swiss Geosciences Meeting 2019, EGU Austria 2020, GELMON Austria 2020, EGU Austria 2021, Regional Conference on Permafrost (RCOP) USA 2021 and EGU Austria 2022.

  Peered-reviewed publications

  Etzelmüller, B., Guglielmin, M., Hauck, C., Hilbich, C., Hölzle, M., Isaksen, K., Noetzli, J., Oliva, M., Ramos, M.: Twenty years of European Mountain Permafrost dynamics – the PACE Legacy. Environmental Research Letters, 15(10), 104070, https://doi.org/10.1088/1748-9326/abae9d, 2020

  Hilbich, C., Hauck, C., Mollaret, C., Wainstein, P., and Arenson, L. U.: Towards accurate quantification of ice content in permafrost of the Central Andes – Part 1: Geophysics-based estimates from three different regions, The Cryosphere, 16, 1845–1872, https://doi.org/10.5194/tc-16-1845-2022, 2022.