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Natural and Controlled Source Magnetotelluric Data Processing and Modeling
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. (Geophysics)
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 [en]
Geophysics, Airborne Electromagnetic Method, Radio Magnetotellurics, Controlled Source Magnetotellurics, Electrical Resistivity Tomography, Moving Magnetotellurics, 2D inversion, 3D inversion
National Category
Geophysics
Identifiers
URN: urn:nbn:se:uu:diva-229917ISBN: 978-91-554-9001-0 (print)OAI: oai:DiVA.org:uu-229917DiVA: diva2:738379
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
List of papers
1. Interference effects of aircraft on the Earth's electromagnetic response at Very Low Frequency and Low Frequency
Open this publication in new window or tab >>Interference effects of aircraft on the Earth's electromagnetic response at Very Low Frequency and Low Frequency
2015 (English)In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 63, no 1, 211-224 p.Article in journal (Refereed) Published
Abstract [en]

Very Low Frequency (VLF) electromagnetics (EM) has been widely and successfully applied in mineral exploration and ground water exploration in the past decades. Many radio transmitters with strong signal to noise ratios are scattered in the Very Low Frequency (VLF) band and the Low Frequency (LF) band. Based on the experiences gained from ground measurements with the Radio Magnetotelluric (RMT) technique operating in the frequency interval 1-250 kHz, broad band magnetometers have been utilized to cover both the VLF (3-30 kHz) and LF (30-300 kHz) bands to increase the resolution of the near surface structure. The metallic aircraft as a conductive body will distort the magnetic signal to some extent, and thus an investigation of the interference of the aircraft on the electromagnetic signal is important. Noise studies due to the rotation of the aircraft and the aircraft itself as a metallic conductive body has been carried out by three different methods: 3D wave polarization, determination of transmitter direction and full tipper estimation. Both VLF and LF frequency bands were investigated. The results show that the magnetic field is independent of the aircraft at low frequencies in the VLF and part of the LF bands (below 100 kHz). At high frequencies (above 100 Hz), the signals are more influenced by the aircraft and the wave polarization directions are more scattered as seen when the aircraft turns.  Some aircraft generated noise which is mixed with the radio transmitter signals are detected as 'dummy' signals by the 3D wave polarization method. The estimated scalar magnetic transfer functions have dependence on the aircraft flight directions at high frequencies which is caused by the aircraft interference.  The aircraft eigen response in the transfer functions (tippers) between vertical and horizontal magnetic field components can be compensated for in the real part of the estimated tippers, but some unknown effect is still observed in the imaginary parts.

Place, publisher, year, edition, pages
European Association of Geoscientists and Engineers, 2015
Keyword
Airborne Electromagnetics, Noise, Signal processing
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-229914 (URN)10.1111/1365-2478.12149 (DOI)000346898400017 ()
Available from: 2014-08-16 Created: 2014-08-16 Last updated: 2017-12-05Bibliographically approved
2. Integrated 2D modeling and interpretation of geophysical and geotechnical data to delineate quick clays at a landslide site in southwest Sweden
Open this publication in new window or tab >>Integrated 2D modeling and interpretation of geophysical and geotechnical data to delineate quick clays at a landslide site in southwest Sweden
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2014 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 79, no 4, 61-75 p.Article in journal (Refereed) Published
Abstract [en]

Radio Magnetotellurics (RMT), Electrical Resistivity Tomography (ERT) and high-resolution reflection seismic data were collected along four lines to image the geometry and physical properties of geological structures at a quick-clay landslide site in southwest Sweden. The site is situated in the Göta River valley where the normally consolidated materials mainly consist of glacial and post-glacial sediments. Geotechnical data suggest presence of quick clays above coarse-grained layers. These layers play a key role in the formation of quick clays and landslide triggering. The RMT and ERT data were individually and jointly inverted in 2D to study the resolution of resulting models for each data set. The resistivity models from the joint inversions demonstrate superior resolution and accuracy than individual ones. The geometry and location of shallower structures resolved in the 2D resistivity models from joint RMT+ERT inversions correlate well with those imaged in the reflection seismic data and observed in the existing geotechnical boreholes.  The models are poor in resolving deeper resistive bedrock at locations where the thickness of the conductive overburden exceeds a certain limit. However, information from the reflection seismic data could be used to estimate the depth to the top of the bedrock along all the four lines. Comparison between the geotechnical data and the resistivity models suggests that quick clays overlying the coarse-grained layer have higher electrical resistivity than the marine clays. The obtained results were further validated and refined by performing synthetic tests. This study shows that integration of ERT and RMT data with reflection seismic data is ideal for quick-clay landslide studies especially when the clay materials are thick.

National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-229913 (URN)10.1190/GEO2013-0201.1 (DOI)000341985500017 ()
Available from: 2014-08-16 Created: 2014-08-16 Last updated: 2017-12-05Bibliographically approved
3. Integration of controlled-source and radio-magnetotellurics, electrical resistivity tomography and reflection seismics to delineate 3D structures of a quick-clay landslide site in southwest Sweden
Open this publication in new window or tab >>Integration of controlled-source and radio-magnetotellurics, electrical resistivity tomography and reflection seismics to delineate 3D structures of a quick-clay landslide site in southwest Sweden
Show others...
2016 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 81, no 1, B13-B29 p.Article in journal (Other academic) Published
Abstract [en]

Three-dimensional radio magnetotelluric (RMT), controlled-source radio magnetotelluric (CSRMT), electrical resistivity tomography (ERT), and high-resolution reflection seismic data were acquired at a quick-clay landslide site close to the Göta River in southwest Sweden. These data were collected with the main objectives of evaluating the performance of each method and delineating different subsurface materials down to the bedrock, which in turn control a peculiar and hazardous retrogressive-type landslide. These materials consist of mainly marine clays, quick clays, sand to gravel and the crystalline bedrock. The RMT, CSRMT and ERT data were inverted separately using 3D inversion schemes and the independent results from 3D processing of reflection seismic data together with the existing geotechnical data were used for their interpretations. The 3D CSRMT resistivity model shows the highest depth penetration and well resolves the geometry of the bedrock. The RMT and ERT models, however, show better resolution closer to the surface because of their denser data coverage compared with the CSRMT data. Considerable correlations in resolving a coarse-grained layer and the bedrock were observed between the CSRMT model and the 3D reflection seismic data. Because an Occam-type regularization was used in the inversion, the lateral and depth extent of the quick clays cannot accurately be determined just by using the resistivity models. The depth to the top of the resistive crystalline bedrock is to some extent uncertain. Comparisons with the borehole data suggest that the resistivity models are poor in resolving the sharp boundary between the quick clays and their underlying more resistive coarse-grained materials. The upper boundary of the more conductive marine clays underlain by the coarse-grain materials was well modeled by the CSRMT data. This study illustrates the potentials of 3D geophysical data especially when different types are collected and particularly for better site characterizations.

National Category
Geophysics Other Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-229915 (URN)10.1190/GEO2014-0386.1 (DOI)000377880100004 ()
Available from: 2014-08-16 Created: 2014-08-16 Last updated: 2017-12-05Bibliographically approved
4. Moving magnetotellurics - experimental setup and case studies from Sweden
Open this publication in new window or tab >>Moving magnetotellurics - experimental setup and case studies from Sweden
(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:nbn:se:uu:diva-229916 (URN)
Available from: 2014-08-16 Created: 2014-08-16 Last updated: 2015-01-22

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