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Moving magnetotellurics - experimental setup and case studies from Sweden
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)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The idea of measuring three-component magnetic fields on board an aircraft or on a moving device in a frequency band from 30 Hz to 1000 Hz was tested. A rigid frame was constructed carrying the three magnetic sensors together with accelerometers and angle sensors. Two test measurements with the frame moving were carried out by putting it on a trailer behind a car and on board an aircraft. From both data sets we observed strong engine noise. Three stability test measurements were made by putting the magnetic sensors in parallel and we concluded that uncorrelated signals are generated during movements even when moving the frame at lower walking speed. It was finally decided to measure the three magnetic field components when the frame is at rest on the ground. Additionally two short electric field sensors were set up and five components electromagnetic fields were measured. The new data acquisition technique is based on the traditional magnetotelluric (MT) method but more efficient in several ways. We name it Moving Magnetotellurics (MMT) and did tests in different areas with different earth conditions.  We found that the MMT method suffers from the same noise disturbances as the other MT methods do. Man-made noise such as power lines or railways reduce the data quality to a high degree, but its influence can be reduced using data analysis. Recordings at night time generally are less disturbed from the man-made noise. The data quality is much better when the measurement is carried out in areas with low resistivity. The MMT method is proven to be a applicable method  in shallow depth studies, especially in areas where normal MT measurements are inconvenient and/or too expensive to carry out.

National Category
Geophysics
Identifiers
URN: urn:nbn:se:uu:diva-229916OAI: oai:DiVA.org:uu-229916DiVA: diva2:738250
Available from: 2014-08-16 Created: 2014-08-16 Last updated: 2015-01-22
In thesis
1. Natural and Controlled Source Magnetotelluric Data Processing and Modeling
Open this publication in new window or tab >>Natural and Controlled Source Magnetotelluric Data Processing and Modeling
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, four studies using different geophysical electromagnetic methods are presented. In the first study dealing with airborne measurements, the noise response due to the rotation of the aircraft and the aircraft itself as a metallic conductive body on the Earth's electromagnetic response in very low frequency and low frequency band was investigated. The magnetic fields are independent of the aircraft in the VLF band and part of the LF band. But at higher frequencies (above 100 kHz), the signals are more influenced by the aircraft. The aircraft also generates its own noise frequencies which are mixed with the radio transmitter signals. The second and third studies are applications of radio-, controlled source-magnetotellurics and electrical resistivity tomography methods at a quick-clay landslide site in southwest Sweden. The data are processed and modeled in 2D and 3D, and the models are compared with high-resolution seismic and geotechnical data. The obtained results were further validated and refined by performing synthetic tests in the second study. The third study shows that the 3D models provide larger and more continuous volume of the quick clay structure than traditional 2D models. Both studies have shown that integrated application of geophysical methods for landslides is ideal. Quick clays often overlie the coarse-grained layers showing an increase of resistivity values in the models. In the fourth study, a new audio magnetotelluric data acquisition technique is developed and is named moving magnetotellurics (MMT). In this new technique, the magnetic sensors are placed on the ground and only 15 to 20 minutes data are acquired for each station, which usually is enough to cover the frequency range 30-300 Hz. The new technique is more efficient and convenient than the traditional magnetotelluric method, and test measurements have shown that it is an applicable method in shallow depth studies.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 53 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1165
Keyword
Geophysics, Airborne Electromagnetic Method, Radio Magnetotellurics, Controlled Source Magnetotellurics, Electrical Resistivity Tomography, Moving Magnetotellurics, 2D inversion, 3D inversion
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-229917 (URN)978-91-554-9001-0 (ISBN)
Public defence
2014-10-03, Hambergsalen, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2014-09-10 Created: 2014-08-16 Last updated: 2016-05-13

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