Airborne Gravity Gradient, Magnetic and VLF 950858/ آ  mأ¤tningar pأ¥bأ¶rjades i 1973 av

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  • ACTA UNIVERSITATIS

    UPSALIENSIS UPPSALA

    2016

    Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1394

    Airborne Gravity Gradient, Magnetic and VLF datasets

    Case studies of modelling, inversion and interpretation

    SAYYED MOHAMMAD ABTAHI

    ISSN 1651-6214 ISBN 978-91-554-9631-9 urn:nbn:se:uu:diva-300126

  • Dissertation presented at Uppsala University to be publicly examined in Hambergsalen, Geocentrum, Villavägen 16, Uppsala, Monday, 19 September 2016 at 10:00 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Professor Thorkild Rasmussen (Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering).

    Abstract Abtahi, S. M. 2016. Airborne Gravity Gradient, Magnetic and VLF datasets. Case studies of modelling, inversion and interpretation. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1394. 59 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-554-9631-9.

    Northern Sweden is one of the largest hosts for mineral resources in Europe and always has been an interesting area for researchers from various disciplines of Earth sciences. This dissertation is a comprehensive summary of three case study papers on airborne VLF, gravity gradient and magnetic data in the area.

    In the first paper, tensor VLF data is extracted from an old data set which contains only the total and the vertical magnetic components. The anomalous part of the horizontal magnetic field components is computed by a Hilbert transform of the vertical magnetic field. The normal part of the horizontal magnetic field component is computed as a function of total, vertical and anomalous part of horizontal magnetic fields. The electric field is also calculated for TE mode and impedance tensor and apparent resistivity are computed. In addition tippers are calculated for two transmitters and inverted by a 3D inversion algorithm. Comparison of the estimated model and geology map of bedrock shows that lower resistivity zones are correlated with mineralizations.

    The second paper deals with the internal consistency of airborne gravity gradient data. The six components of the data are estimated from a common potential function. It is shown that the data is adequately consistent but at shorter land clearances the difference between the estimated data and the original data is larger. The technique is also used for computing the Bouguer anomaly from terrain corrected FTG data. Finally the data is inverted in 3D, which shows that the estimated density model in shallow depth is dominated by short wave length features.

    Inversion of TMI data is the topic of the third paper where a new type of reference model for 3D inversion of magnetic data is proposed by vertically extending the estimated magnetization of a 2D terrain magnetization model. The final estimated 3D result is compared with the magnetization model where no reference model is used. The comparison shows that using the reference model helps the high magnetization zones in the estimated model at shallow depths to be better correlated with measured high remanent magnetization from rock samples. The high magnetization zones are also correlated with gabbros and volcanic metasediments.

    Keywords: VLF, gravity gradient, magnetic, airborne, inversion, non-uniqueness

    Sayyed Mohammad Abtahi, Department of Earth Sciences, Geophysics, Villav. 16, Uppsala University, SE-75236 Uppsala, Sweden.

    © Sayyed Mohammad Abtahi 2016

    ISSN 1651-6214 ISBN 978-91-554-9631-9 urn:nbn:se:uu:diva-300126 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-300126)

  • Dedicated to my parents

  • List of Papers

    This thesis is based on the following papers, which are referred to in the text by their Roman numerals.

    I Abtahi, S.M., Pedersen, L.B., Kamm J. and Kalscheuer, T.

    (2016) Extracting geoelectrical maps from vintage very-low- frequency airborne data, tipper inversion, and interpretation: A case study from northern Sweden. Geophysics, 81(5), B135- B147.

    II Abtahi, S.M., Pedersen, L.B., Kamm J. and Kalscheuer, T. (2016) Consistency investigation, vertical gravity estimation and inversion of airborne gravity gradient data – A case study from northern Sweden. Geophysics, 81(3), B65-B756

    III Abtahi, S.M., Pedersen, L.B., Kamm J. and Kalscheuer, T. (2016) A new reference model for 3D inversion of airborne magnetic data in hilly terrain – a case study from northern Swe- den. submitted to Geophysics

    Reprints were made with permission from the respective publishers.

  • Personal contribution

     In Paper I, I did all of the computations. I also wrote most of the text (which was checked and revised in detail by the co-authors) and I creat- ed all of the figures.

     In Paper II, I developed the code and did most of the computations (ex- cept nonlinear data inversion). I also wrote most of the text (which was revised in detail by the co-authors) and I created all of the figures.

     In paper III, all of the computations were done by me and I also almost wrote almost the entire text of the paper that has been modified and cor- rected in detail by the co-authors. All of the figures are made by me.

  • Contents

    Summary in Swedish .................................................................................... 11 

    1. Introduction ............................................................................................... 15 

    2. VLF method .............................................................................................. 19  2.1 VLF sources ....................................................................................... 19  2.2. VLF data ............................................................................................ 20  2.3. Scalar and tensor VLF measurements ............................................... 21  2.4. Inversion of VLF data ....................................................................... 22  2.5. Major problems with VLF surveying ................................................ 23 

    2.5.1. Variation of normal field ........................................................... 23  2.5.2. The effect of topography ........................................................... 24  2.5.3. Anthropogenic effects ................................................................ 25 

    3. Potential field methods ............................................................................. 27  3.1. Gravity gradient method .................................................................... 27 

    3.1.1. Full tensor gravity gradient data ................................................ 28  3.1.2. Noise in gravity gradient data .................................................... 28 

    3.2. Magnetic method ............................................................................... 29  3.2.1. Magnetic field potential ............................................................. 30  3.2.2. Magnetic and gravity gradient relation ...................................... 30  3.2.2. Major problems with accuracy of magnetic data ....................... 31 

    3.3. Inversion of potential field data ......................................................... 31 

    4. Geophysical inverse theory ....................................................................... 33  4.1. Linear inverse problems .................................................................... 33  4.2. Model structure functions .................................................................. 36 

    4.2.1. Flatness function ........................................................................ 36  4.2.1. Depth weighting ......................................................................... 36 

    4.3. Non-linear inverse problems ............................................................. 37 

    5. Geology of the study area ......................................................................... 39 

    6. Summary of papers ................................................................................... 41  6.1. Paper I ............................................................................................... 41 

    6.1.1. Summary .................................................................................... 41  6.1.2. Discussions ................................................................................ 44 

    6.2. Paper II .............................................................................................. 45 

  • 6.2.1. Summary .................................................................................... 45  6.2.2. Discussion .................................................................................. 48 

    6.3. Paper III ............................................................................................. 49  6.3.1. Summary .................................................................................... 49  6.3.2. Discussion .................................................................................. 52 

    Acknowledgments......................................................................................... 53 

    Bibliography ................................................................................................. 54 

  • Abbreviations

    1D one-dimensional 2D two-dimensional 3D three dimensional ASL above