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Boat-towed RMT for urbanunderground infrastructure planning: Stockholm Bypass (Förbifart) case study
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Place, publisher, year, edition, pages
2015. , 37 p.
National Category
Geophysics
Identifiers
URN: urn:nbn:se:uu:diva-267997OAI: oai:DiVA.org:uu-267997DiVA: diva2:875252
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
List of papers
1. Boat-towed radio-magnetotellurics: A new technique and case study from the city of Stockholm
Open this publication in new window or tab >>Boat-towed radio-magnetotellurics: A new technique and case study from the city of Stockholm
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.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-267995 (URN)10.1190/GEO2014-0527.1 (DOI)000368347800021 ()
Funder
Swedish Research Council Formas
Available from: 2015-12-01 Created: 2015-12-01 Last updated: 2017-12-01Bibliographically approved
2. Analysis of resolution and sensitivity of boat-towed RMT dataover Lake Mälaren, Stockholm
Open this publication in new window or tab >>Analysis of resolution and sensitivity of boat-towed RMT dataover Lake Mälaren, Stockholm
(English)In: Interpretation, ISSN 0020-9635, E-ISSN 2169-4494Article in journal, Editorial material (Refereed) Submitted
Abstract [en]

We examine the resolution and sensitivity of water-borne boat-towed multi-frequencyradio magnetotelluric (RMT) data for delineating weak zones in crystalline environmentunder a lake or a river. 2D modeling of RMT data in joint transverse electric (TE) andtransverse magnetic (TM) and determinant mode of 40 profiles over two water passages fromLake Mälaren near the city of Stockholm was used for this purpose. The 2D modeling revealsthat the estimated models have responses that fit the observed data reasonably well. Themodels show an overall agreement when compared with existing marine seismic results,bathymetry data and other available geological knowledge from the site. A low-resistivityzone observed in the middle of all profiles is suggested to be from a fracture/fault zonestriking in the direction of the water passages. Because RMT is an inductive-based method,the presence of conductive lake sediments shields scanning the underlying fractured bedrock.To reliably interpret the RMT data, synthetic modeling using a three-layered generic modelwith a conductive layer of varying thickness sandwiched between resistive water andfractured crystalline bedrock was performed. The modeling results suggest that lowerfrequency signal is required to have greater depth of penetration that would better resolve thebedrock and fracture zone although some evidence of fractured bedrock can still be inferredfrom the data. The study also shows the advantage of having a water layer that acts as a nearhomogeneous medium that eliminates near surface static shift effects. We propose that theboat-towed RMT acquisition system should be upgraded to enable controlled-source acquisition to improve the depth penetration and to overcome the shortcomings of usingradio-frequencies as signal.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-267996 (URN)
Available from: 2015-12-01 Created: 2015-12-01 Last updated: 2017-12-01

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