Development of RMT techniques for urban infrastructure planning Stockholm Bypass (Förbifart) case study

Mehta, S. 2017. Development of RMT techniques for urban infrastructure planning. Stockholm Bypass (Förbifart) case study. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1547. 53 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0046-7. The tensor radio-magnetotelluric (RMT) method has extensively been used in near-surface investigations to obtain resistivity models of the subsurface. The main objective of this thesis is to further develop the RMT survey technique for a less paid attention and challenging environment namely on shallow water bodies and in the urban environment. The other objective is to develop a new processing technique to enhance the resolution and sensitivity of the tensor RMT method. For the first time a data acquisition system called ‘boat-towed RMT’ is introduced that has the capability to measure tensor RMT data on water bodies like lakes and rivers. A RMT survey carried out on Lake Mälaren near the city of Stockholm shows the capability and efficiency of the boat-towed RMT system. The resistivity models obtained from the RMT data are consistent from one line to another and show good correlation with the existing geological and drill core data. In general, a three-layer resistivity model was obtained that has a conductive layer interpreted as lake sediments, which is sandwiched between high resistive bedrock and resistive water column. A coherent discontinuity of low resistivity zone was observed in bedrock across all the lines. It was interpreted to originate from a major fracture zone striking in the direction of water bodies. However, due to the lack of penetration, RMT method alone was insufficient to provide a conclusive interpretation of this. Synthetic analysis was performed and showed that lower frequencies using controlled-sources are required to obtain the desired penetration depth. We took the advantage of the Swedish winters and carried out controlledsource RMT measurements on frozen lake at the same location. The new controlled-source models have enough depth penetration to delineate fractured bedrock. Furthermore, in order to improve the resolution and sensitivity of tensor RMT data, a new processing technique was developed that preserves the identity of each transmitter and allows improved resistivity model of the subsurface. These new acquisition and processing techniques should be useful in many different applications for urban infrastructure planning projects especially in Scandinavia where 7% of the land is covered by fresh water bodies and is poorly explored for these purposes. Suman Mehta, Department of Earth Sciences, Geophysics, Villav. 16, Uppsala University, SE-75236 Uppsala, Sweden.