Resolution capacity of geophysical monitoring regarding permafrost degradation induced by hydrological processes

Geophysical methods are often used to characterize and monitor the subsurface composition of permafrost. The resolution capacity of standard methods, i.e. electrical resistivity tomography and refraction seismic tomography, depends not only on static parameters such as measurement geometry, but also on the temporal variability in the contrast of the geophysical target variables (electrical resistivity and P-wave velocity). Our study analyses the resolution capacity of electrical resistivity tomography and refraction seismic tomography for typical processes in the context of permafrost degradation using synthetic and field data sets of mountain permafrost terrain. In addition, we tested the resolution capacity of a petrophysically based quantitative combination of both methods, the so-called 4-phase model, and through this analysed the expected changes in water and ice content upon permafrost thaw. The results from the synthetic data experiments suggest a higher sensitivity regarding an increase in water content compared to a decrease in ice content. A potentially larger uncertainty originates from the individual geophysical methods than from the combined evaluation with the 4-phase model. In the latter, a loss of ground ice can be detected quite reliably, whereas artefacts occur in the case of increased horizontal or vertical water flow. Analysis of field data from a well-investigated rock glacier in the Swiss Alps successfully visualized the seasonal ice loss in summer and the complex spatially variable ice, water and air content changes in an interannual comparison

New insights on permafrost genesis and conservation in talus slopes based on observations at Flüelapass, Eastern Switzerland

The talus slope at Flüelapass was the first mountain permafrost study site in Switzerland in the 1970s and the presence of ice-rich permafrost at the foot of the slope has been investigated in the context of several studies focusing on the role of snow cover distribution. We review previously developed hypotheses and present new ones using various data sources, such as temperature measurements in boreholes, a subaquatic DEM generated from unmanned aerial system (UAS) photogrammetry, terrestrial laser scan measurements of snow depth, geophysical ground investigations and automatic time-lapse photography. From this combination of data sources together with observations in the field, an interesting sequence of geomorphologic processes is established at Flüelapass. As a result we show how mass wasting processes can initiate the genesis and long-term conservation of ice-rich permafrost at the base of a talus slope

Ice content and interannual water storage changes of an active rock glacier in the dry Andes of Argentina

The quantification of volumetric ice and water contents in active rock glaciers is necessary to estimate their role as water stores and contributors to runoff in dry mountain catchments. In the semi-arid to arid Andes of Argentina, active rock glaciers potentially constitute important water reservoirs due to their widespread distribution. Here however, water storage capacities and their interannual changes have so far escaped quantification in detailed field studies. Volumetric ice and water contents were quantified using a petrophysical four-phase model (4PM) based on complementary electrical resistivities (ERT) and seismic refraction tomographies (SRT) in different positions of Dos Lenguas rock glacier in the Upper Agua Negra basin, Argentina. We derived vertical and horizontal surface changes of the Dos Lenguas rock glacier, for the periods 2016?17 and 2017?18 using drone-derived digital elevation models (DEM). Interannual water storage changes of −36 mm yr−1 and +27 mm yr−1 derived from DEMs of Difference (DoD) for the periods 2016?17 and 2017?18, respectively, indicate that significant amounts of annual precipitation rates can be stored in and released from the active rock glacier. Heterogeneous ice and water contents show ice-rich permafrost and supra-, intra- and sub-permafrost aquifers in the subsurface. Active layer and ice-rich permafrost control traps and pathways of shallow ground water, and thus regulate interannual storage changes and water releases from the active rock glacier in the dry mountain catchment. The ice content of 1.7?2.0 × 109 kg in the active Dos Lenguas rock glacier represents an important long-term ice reservoir, just like other ground ice deposits in the vicinity, if compared to surface ice that covers less than 3 % of the high mountain catchment.Fil: Halla, Christian. Universitat Bonn; AlemaniaFil: Blöthe, Jan Henrick. Universitat Bonn; AlemaniaFil: Tapia Baldis, Carla Cintia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Trombotto, Dario Tomas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Hilbich, Christin. University Of Fribourg; AlemaniaFil: Hauck, Christian. University Of Fribourg; AlemaniaFil: Schrott, Lothar. Universitat Bonn; Alemani

Mountain permafrost degradation documented through a network of permanent electrical resistivity tomography sites

Mountain permafrost is sensitive to climate change and is expected to gradually degrade in response to the ongoing atmospheric warming trend. Long-term monitoring of the permafrost thermal state is a key task, but problematic where temperatures are close to 0 ∘C because the energy exchange is then dominantly related to latent heat effects associated with phase change (ice–water), rather than ground warming or cooling. Consequently, it is difficult to detect significant spatio-temporal variations in ground properties (e.g. ice–water ratio) that occur during the freezing–thawing process with point scale temperature monitoring alone. Hence, electrical methods have become popular in permafrost investigations as the resistivities of ice and water differ by several orders of magnitude, theoretically allowing a clear distinction between frozen and unfrozen ground. In this study we present an assessment of mountain permafrost evolution using long-term electrical resistivity tomography monitoring (ERTM) from a network of permanent sites in the central Alps. The time series consist of more than 1000 datasets from six sites, where resistivities have been measured on a regular basis for up to 20 years. We identify systematic sources of error and apply automatic filtering procedures during data processing. In order to constrain the interpretation of the results, we analyse inversion results and long-term resistivity changes in comparison with existing borehole temperature time series. Our results show that the resistivity dataset provides valuable insights at the melting point, where temperature changes stagnate due to latent heat effects. The longest time series (19 years) demonstrates a prominent permafrost degradation trend, but degradation is also detectable in shorter time series (about a decade) at most sites. In spite of the wide range of morphological, climatological, and geological differences between the sites, the observed inter-annual resistivity changes and long-term tendencies are similar for all sites of the network

Semi-automated calibration method for modelling of mountain permafrost evolution in Switzerland

Permafrost is a widespread phenomenon in mountainous regions of the world such as the European Alps. Many important topics such as the future evolution of permafrost related to climate change and the detection of permafrost related to potential natural hazards sites are of major concern to our society. Numerical permafrost models are the only tools which allow for the projection of the future evolution of permafrost. Due to the complexity of the processes involved and the heterogeneity of Alpine terrain, models must be carefully calibrated, and results should be compared with observations at the site (borehole) scale. However, for large-scale applications, a site- specific model calibration for a multitude of grid points would be very time-consuming. To tackle this issue, this study presents a semi-automated calibration method using the Generalized Likelihood Uncertainty Estimation (GLUE) as implemented in a 1-D soil model (CoupModel) and applies it to six permafrost sites in the Swiss Alps. We show that this semi-automated calibration method is able to accurately reproduce the main thermal condition characteristics with some limitations at sites with unique conditions such as 3-D air or water circulation, which have to be calibrated manually. The calibration obtained was used for global and regional climate model (GCM/RCM)-based long-term climate projections under the A1B climate scenario (EU-ENSEMBLES project) specifically downscaled at each borehole site. The projection shows general permafrost degradation with thawing at 10 m, even partially reaching 20 m depth by the end of the century, but with different timing among the sites and with partly considerable uncertainties due to the spread of the applied climatic forcing

Geophysical monitoring systems to assess and quantify ground ice evolution in mountain permafrost

Permafrost bildet ein wesentliches Element der globalen Kryosphäre. Der Alpine Permafrost weißt Temperaturen nur knapp unter 0°C auf und ist damit besonders sensibel für die prognostizierten Klimaänderungen im 21. Jahrhundert. Der Eisgehalt im gefrorenen Untergrund gehört zu den Schlüsselparametern, welche die Stabilität von Hängen in Periglazialgebieten kontrollieren. Basierend auf dem Bedarf zur langfristigen Beobachtung der Entwicklung des Gebirgspermafrostes trägt die vorliegende Arbeit zur Entwicklung eines Monitoringansatzes bei, der Aussagen über zeitliche Änderungen des Eisgehalts zulässt. Im Rahmen dieser Studie wurde ein geophysikalisches Monitoring-Netzwerk bestehend aus permanenten geoelektrischen und refraktionsseismischen Profilen an vier morphologisch unterschiedlichen Standorten in den Schweizer Alpen aufgebaut. Das Ziel dieser Arbeit war es, das Potenzial geophysikalischer Methoden (geoelektrische Tomographie und refraktionsseismische Tomographie) für ein operationelles Langzeit-Monitoring der Permafrostentwicklung im Hochgebirge im Rahmen der globalen Erwärmung zu untersuchen. Die Ergebnisse weisen auf ein enormes Potenzial beider Monitoringansätze zur Erfassung und Charakterisierung klimainduzierter Permafrost- bzw. Bodeneisdegradation hin. Durch eine kombinierte Analyse von Widerständen und Bohrlochtemperaturen konnte der starke Zusammenhang zwischen beiden Parametern für alle Standorte nachgewiesen werden. Damit konnte die generelle Anwendbarkeit des geoelektrischen Monitorings für morphologisch unterschiedliche Standorte mit variierenden Oberflächenbeschaffenheiten und Eisgehalten demonstriert werden. Die Interpretation der seismischen Geschwindigkeitsänderungen in time-lapse Tomogrammen stimmt mit den Geoelektrikdaten sowie mit den Bohrlochtemperaturen überein, ergibt aber gleichzeitig auch komplementäre Informationen, welche die Interpretation der Geoelektrikdaten wesentlich unterstützen können. Einen wesentlichen Vorteil des refraktionsseismischen Monitorings bietet die Möglichkeit, unter bestimmten Voraussetzungen zweifelsfrei Eisdegradation (oder saisonaler Eisschwund) identifizieren zu können

Petrophysical joint inversion applied to alpine permafrost field sites to image subsurface ice, water, air, and rock contents

Quantification of ground ice is crucial for understanding permafrost systems and modeling their ongoing degradation. The volumetric ice content is however rarely estimated in permafrost studies, as it is particularly difficult to retrieve. Standard borehole temperature monitoring is unable to provide any ice content estimation, whereas non-invasive geophysical techniques, such as refraction seismic and electrical resistivity measurements can yield information to assess the subsurface ice distribution. Electrical and seismic data are hereby complementary sensitive to the phase change. A petrophysical joint inversion was recently developed to determine volumetric water, air, ice and rock contents from electrical and seismic data using a petrophysical model, but was so far only tested on synthetic data and one proof-of-concept field example. In order to evaluate its applicability on different types of permafrost materials and landforms (bedrock, rock glacier, talus slope), we apply this petrophysical joint inversion scheme to five profiles located in the northwestern Alps. The electrical mixing rule (Archie's second law) was hereby identified as a source of model uncertainty, as it applies only when the electrolytic conduction is the dominating process. We therefore investigate and compare four petrophysical models linking the electrical resistivity with the ground constituents: Archie's law, Archie's law with an additional surface conduction factor, a model considering only surface conduction, and the geometric mean model. In most cases, the three first resistivity relations yield largely comparable results, whose reliability is discussed. The geometric mean model better resolve high ice content, as it is less influenced by the ice-rock ambiguity. We perform a systematic analysis of the regularization parameters and then evaluate our results with validation data including thaw depths and ice contents derived from borehole measurements. Geophysical surveys have generally a lower resolution than borehole data, but have the advantage of providing spatio-temporal information in 2D or 3D. The joint inversion results are in relatively good agreement with the validation data for all sites from ice-poor to ice-rich conditions, when choosing the most adequate resistivity model and porosity initial value. Additional forcing constraints (e.g., porosity range constraint) based on site knowledge can improve the model parameter estimation

Applicability of electrical resistivity tomography monitoring to coarse blocky and ice-rich permafrost landforms

The inversion and interpretation of electrical resistivity tomography (ERT) data from coarse blocky and ice-rich permafrost sites are challenging due to strong resistivity contrasts and high contact resistances. To assess temporal changes during ERT monitoring (ERTM), corresponding inversion artefacts have to be separated from true subsurface changes. Appraisal techniques serve to analyse an ERTM data set from a rockglacier, including synthetic modelling, the depth of investigation index technique and the so-called resolution matrix approach. The application of these methods led step by step to the identification of unreliable model regions and thus to the improvement in interpretation of temporal resistivity changes. An important result is that resistivity values of model regions with strong resistivity contrasts and highly resistive features are generally of critical reliability, and resistivity changes within or below the ice core of a rockglacier should therefore not be interpreted as a permafrost signal. Conversely, long-term degradation phenomena in terms of warming of massive ground ice at the permafrost table are detectable by ERTM

Multi-methodological reconstruction of the lake level at Morgarten in the context of the history of the Swiss Confederation

In AD 1315, the Habsburgs fought against the Swiss Confederation at Morgarten. Since historical records are very limited, the battle has been the subject of very controversial discussions. Its location and outcome seem likely to have been influenced by the landscape and the size of the Aegeri lake, but only sparse and contradictory information are available. Numerical, semi-quantitative and relative dating techniques were applied to reconstruct the lake’s dimensions and the landscape. Results obtained from radiocarbon dating of mires (last sedimentation phase of the lake), geomorphic mapping, geoelectrics, soil maps (surface age indication) and archaeological findings were pieced together and gave an astonishingly good consensus. In the Lateglacial, the lake level was higher (750–760 m a.s.l.): because of a catastrophic event, it decreased by 25 m. About 5500 BP, the lake level was at 732 m a.s.l., and since the Roman period, it has varied between 724 and 727 m a.s.l. At the time of the battle the lake was at 726 m – that is, about 2 m higher than today. Together with the cooler climate, the greater extension of the fens and larger lake, the valley floor was wet and unpleasant. If a Habsburg army had to cross this region, they would likely have preferred to walk on a more accessible trail along the footslopes (where they probably were attacked). Precise landscape reconstruction provides new input for historical research. Details about the exact location of the battle, however, remain unclear, and the myth of the battle at Morgarten persists

Comparison of Simulated 2D Temperature Profiles with Time-Lapse Electrical Resistivity Data at the Schilthorn Crest, Switzerland

The Schilthorn Crest in the Bernese Alps, Switzerland, is a prominent permafrost research site. Topographic and transient effects influence the temperature field below the east-west oriented crest. Measured T(z)-profiles in boreholes, however, do not provide sufficient information for a comprehensive description of the subsurface temperature distribution. We combine ground temperature measurements, electric resistivity tomography (ERT) monitoring, and numerical modeling to investigate the 3-dimensional thermal regime below the crest. The modeled temperature field of a north-south oriented cross section agrees well with ERT results along the same profile. The simulated thermal regime below the Schilthorn is characterized by generally warm permafrost, with the coldest zone below the upper part of the north-facing slope, and permafrost a little below the surface on the south-facing slope. The combination of temperature modeling and measurements and geophysical monitoring bears potential to improve simulation and validation strategies