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Boat-towed radio-magnetotellurics: A new technique and case study from the city of Stockholm
SGU.
SGU.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
2015 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 80, no 6, B193-B202 p.Article in journal, Editorial material (Refereed) Published
Abstract [en]

We have developed a new data acquisition system and technique to measure the radio magnetotelluric (RMT) signals from distant radio transmitters with the objective of mapping and modeling electric resistivity structures below a river or lake. The acquisition system is towed by a boat; therefore, we call the technique boat-towed RMT. The data acquisition is fast with a production rate of approximately 1  km/hr using a nominal sampling spacing of 10–15 m. Given the ample number of radio transmitters available in most parts of the world, the method can be used for near-surface studies of various targets. We have developed boat-towed RMT measurements on Lake Mälaren near the city of Stockholm in Sweden to determine the feasibility of the method. Approximately 15 km of RMT data were collected during three days above a planned 60-m-deep bypass tunnel with the goal of providing information on the bedrock depth and possible weak zones within the bedrock. The measured resistivity and phase data were of high quality with errors on the order of a few percent. The resistivity models from 2D inversion of the data showed a good correlation with available geologic data in resolving bedrock depth and also resistivity layering within the lake. Resistivity maps derived from the dense 2D models suggested a northeast–southwest-striking low-resistivity zone at less than a 30-m depth. The zone likely represents fractured crystalline bedrock. The boat-towed RMT technique is well suited for water bodies with moderate electric resistivity such as in brackish and freshwater environments.

Place, publisher, year, edition, pages
2015. Vol. 80, no 6, B193-B202 p.
National Category
Geophysics
Identifiers
URN: urn:nbn:se:uu:diva-267995DOI: 10.1190/GEO2014-0527.1ISI: 000368347800021OAI: oai:DiVA.org:uu-267995DiVA: diva2:875249
Funder
Swedish Research Council Formas
Available from: 2015-12-01 Created: 2015-12-01 Last updated: 2017-12-01Bibliographically approved
In thesis
1. Boat-towed RMT for urbanunderground infrastructure planning: Stockholm Bypass (Förbifart) case study
Open this publication in new window or tab >>Boat-towed RMT for urbanunderground infrastructure planning: Stockholm Bypass (Förbifart) case study
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Publisher
37 p.
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-267997 (URN)
Presentation
2015-12-02, Hambergsalen, Uppsala University, Uppsala, 07:25 (English)
Opponent
Supervisors
Available from: 2015-12-04 Created: 2015-12-01 Last updated: 2015-12-04Bibliographically approved
2. Development of RMT techniques for urban infrastructure planning: Stockholm Bypass (Förbifart) case study
Open this publication in new window or tab >>Development of RMT techniques for urban infrastructure planning: Stockholm Bypass (Förbifart) case study
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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 controlled-source 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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 53 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1547
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-328398 (URN)978-91-513-0046-7 (ISBN)
Public defence
2017-10-13, Hambergsalen, Geocentrum, Villavägen 16,, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2017-09-22 Created: 2017-08-22 Last updated: 2017-10-17

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