BGS has won a NERC Technology Proof of Concept research grant to adapt its Capacitive Resistivity Imaging technology for the remote spatial and temporal monitoring of permafrost.
Melting permafrost is not only a cryospheric indicator of global climate change, but also presents a threat to the stability of civil engineering infrastructure such as transport networks, buildings and settlements.
In alpine regions, thawing permafrost can precipitate rock falls and related geohazards such as flooding.
Our research partner in this collaborative project is Dr Julian Murton, University of Sussex. Dr Murton is an international authority on Arctic Quaternary geology and the physical modelling of permafrost processes. He is Director of the UK Permafrost Laboratory — a unique world facility in which our validation experiments will be undertaken.
The experimental work will be supported by Dr Michael Krautblatter, University of Bonn, who acts as a Visiting Researcher to this project. Dr Krautblatter is a leading expert on the application of conventional ERT to alpine permafrost, and has expertise in temperature-calibrated resistivity imaging of permafrost rocks.
The thermal state of permafrost is controlled by the freezing or thawing of pore water.
These processes can be directly related to changes in ground resistivity and can therefore be tracked remotely using permanent in situ sensors and wireless telemetry (GSM, GPRS) in a similar manner to our Automated time-Lapse Electrical Resistivity (ALERT) field observatories.
The project will adapt BGS-designed Capacitive Resistivity Imaging (CRI) technology as this uses non-contacting, capacitive sensors to image the subsurface.
This approach eliminates the need for intrusive galvanic probes and overcomes the problem of the extremely high or variable contact resistances that are associated with frozen ground.
Our existing CRI instrumentation was designed for one-off manual surveys with a small number of mobile sensors. The potential advantages of permanent temporal monitoring with fixed CRI sensors remain unexplored.
The work will include numerical simulations to predict and optimise the electromagnetic behaviour of distributed capacitive sensor networks required for the volumetric imaging of permafrost both at the field and laboratory scales. Based on the results, a viable multi-sensor automated time-lapse CRI measurement system will be designed.
Conceptual ideas and technical solutions incorporated in ALERT and existing CRI technology will guide our research. A functional bench-top prototype will be developed and the technical feasibility of automated operation using wireless telemetry will be demonstrated.
Proof of concept and prototype capability will be validated in laboratory experiments that simulate permafrost growth, persistence and thaw in bedrock.
Water-saturated rock samples (450 mm high, 300 mm x 300 m wide) of varying porosity will be monitored in the state-of-the-art Permafrost Lab at the University of Sussex.
The lower half of each sample will be maintained at temperatures below 0°C (simulating permafrost) and the upper half will be cycled above and below 0°C (simulating seasonal thawing and freezing of the overlying active layer).
The samples will be instrumented with both capacitive sensors and conventional galvanic sensors in order to compare results between both resistivity methods.
Time-lapse capacitive imaging of the samples during ten successive freeze-thaw cycles of the model active layer will test all functionality of the prototype instrumentation and will provide representative datasets.
Experimental control and calibration of resistivity images will be provided by contemporaneous temperature and moisture content measurements on the sample, using platinum resistors and time domain reflectometry methods.
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