Imaging of active layer characteristics through quasi-3D inversion of frequency-domain electromagnetic soundings

Imaging of active layer characteristics through quasi-3D inversion of frequency-domain electromagnetic soundings

Thalhammer, Marlene (TU Delft Civil Engineering & Geosciences)

Wagner, Florian (mentor)
Uhlemann, Sebastian (mentor)
Wellmann, Florian (graduation committee)
Manen, Dirk Jan Van (graduation committee)

Delft University of Technology
ETH Zürich
Rheinisch-Westfälische Technische Hochschule

Applied Geophysics | IDEA League

2022-08-17

The active layer thickness has become an important indicator in climate change research as permafrost degradation has long been documented. The thawing of permafrost causes the release of greenhouse gases accelerating Arctic warming. Monitoring and quantifying spatial and temporal changes of the active layer are challenging but crucial for reliable climate projections. Geophysical methods offer a non-invasive investigation of electrical properties and their distribution in permafrost areas, revealing phase transitions from water to ice. Subsurface electrical resistivity images can be obtained through inversion of electromagnetic data, yet are inherently ambiguous because of the ill-posed nature of the inverse problem. Since regularization methods offer the possibility to stabilize the inversion, lateral and spatial constraints are incorporated in the inversion algorithm to produce quasi-2D and quasi-3D subsurface models. The developed methodology is evaluated based on synthetic data sets to determine suitable inversion parameters, which are subsequently applied to a field example from the Seward Peninsula, Alaska. Laterally constrained inversion methods based on a few-layer starting model succeed in resolving sharp interfaces in quasi-layered environments. In more complex settings minimum-structure models can retrieve accurate subsurface representations leveraging on vertical and horizontal smoothness constraints. Enforcing lateral and spatial consistency between neighboring soundings thereby yields a similar degree of model smoothness. The inverted field data confirms the conclusions drawn from the synthetic study, as meaningful three-layered models with regard to electrical resistivities are recovered, indicating resistive snow overlying the conductive active layer and highly resistive permafrost. However, the inversion results imply that the snow layer has a significant effect on the predicted model. The implemented constraints help in reducing the ambiguity of the models, but uncertainties introduced by limited data availability cannot be overcome. The potential of adopting spatial and lateral constraints to the inversion is shown, although it becomes evident that additional a priori information needs to be integrated in the objective function in order to comprehensively image the active layer.

Geophysics
Permafrost
Electromagnetic induction
Inverse Modelling
Regularization methods

http://resolver.tudelft.nl/uuid:1a311a47-6f15-42c0-86e0-4b7a75ddc2cd

Student theses

master thesis

© 2022 Marlene Thalhammer