Radio-magnetotelluric and controlled-source magnetotelluric surveys on a frozen lake : Opportunities for urban applications in Nordic countries

In a novel approach, we have carried out controlled-source and radio-magnetotelluric measurements in the frequency range of 2-250 kHz on a frozen lake located over a planned major multi-lane underground road tunnel near the city of Stockholm. The aim was to gain a better understanding of the resistivity variations above and, potentially, within the crystalline bedrock. Previous studies on the lake water using the boat-towed radio-magnetotelluric technique at the higher end of the frequency band lacked resolution at depth and could not provide conclusive information about bedrock level and potential fracture systems within the bedrock. Taking advantage of Nordic winters, we measured four profiles on the frozen lake complementing the previously acquired boat-towed radio-magnetotelluric data utilizing a double horizontal magnetic dipole transmitter that generated signals down to 1 kHz. The new resistivity models, incorporating the lower frequency data, show improvements and deeper penetrations based on a combined analysis of penetration depth, data misfits and sensitivity studies. The resistivity models also show better correlation with the available high-resolution shallow water seismic reflection data and the geological observations. A potential fracture system within the bedrock can also be inferred better in the new models. The idea of running similar surveys on frozen lakes can be further exploited in similar conditions in countries such as Sweden, where approximately 7% of the land is covered by freshwater bodies and poorly explored for infrastructure planning projects