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Interference effects of aircraft on the Earth's electromagnetic response at Very Low Frequency and Low Frequency
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: 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. Vol. 63, no 1, 211-224 p.
Keyword [en]
Airborne Electromagnetics, Noise, Signal processing
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
URN: urn:nbn:se:uu:diva-229914DOI: 10.1111/1365-2478.12149ISI: 000346898400017OAI: oai:DiVA.org:uu-229914DiVA: diva2:738248
Available from: 2014-08-16 Created: 2014-08-16 Last updated: 2017-12-05Bibliographically approved
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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Shan, ChunlingPedersen, Laust Börsting

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