Near-surface geophysical investigations of saline permafrost

Permafrost is warming on the global scale by approximately 0.3°C per decade and as a result, Arctic communities are fighting many social, economic, and environmental challenges with very limited resources. Degrading permafrost presents challenges where natural hazards such as thaw slumps, major rock slides, and slope destabilization, are becoming more common along with more slowly developing differential settlements caused by progressive warming and deepening of the active layer, contribute to damages to housing and transportation infrastructure. Mitigation and solutions to all of these issues require subsurface information. Geophysical mapping has been proven a valuable tool for achieving cheap and reliable subsurface information but faces a unique set of changes in permafrost, especially saline permafrost conditions.

This dissertation reports the work of my PhD project taking on the challenges of Near-surface geophysical mapping of saline permafrost in Greenland. I present a unique data set collected near Ilulissat. The survey consisted of three geophysical methods namely Electrical Resistivity Tomography (ERT) data, continuous Transient Electromagnetic (TEM) data, and shallow seismic data for Multichannel Analysis of Surface Waves (MASW) of both Rayleigh and Love waves. The survey features coincidental lines of all three methods with borehole validation of varying quality. The dataset presents the opportunity to compare the method’s mapping capability. In the dissertation, the collected data is presented along with interpretations and comparisons of all data types, whereas the journal papers are method specific. The main research focus and development have been on the TEM and MASW methods since the ERT is the most applied and well-understood under permafrost conditions.

The collected TEM data was highly affected by induced polarization (IP) effects. Decay curve shapes varied widely, and the survey shows decay shapes not previously reported in scientific literature. The work demonstrates the challenges of the IPTEM invention problem and shows its high non-uniqueness and starting model dependency. A structural understating of the decay shapes is presented, which fits the observation of a high spatial dependency of the decay shapes. This allows for a qualitative interpretation of the data. The data also provides insides into the causes of IP in TEM data from permafrost conditions, where a consistent explanation is presented.

MASW on the permafrost condition from Ilulissat is found to be a worst-case example of known challenges of the method. No inversion of surface wave dispersion data is found to handle the data changes from saline permafrost. Therefore, I have developed a novel probabilistic inversion approach for joint inversion of Multimodal MASW data for Rayleigh and Love waves. The inversion is demonstrated on a synthetic example relevant to Ilulissat and was applied to a real data set as well. The inversion results do not fit the validation data, most likely due to 2- and 3-dimensional effects resulting in unreliable dispersion data extracted from the seismic shot gathers. The joint inversion of Rayleigh and Love waves does show well-defined inversion results and the potential for unique solutions to the otherwise highly non-unique individual Rayleigh and Love wave inversions.

Regardless of all the challenges, all methods produce data from where the presence of saline deposits can be interpreted. All methods are consistent in their interpretation and found to image the subsurface. The problem is our understanding of and capability to handle those signals. Geophysical mapping is still highly challenging in saline permafrost conditions, and no quantitatively reliable mapping method has been found. Here, I present some advances toward the end goal of a methodology that can supply fast and cheap subsurface information for Arctic communities.