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Airborne Gravity Gradient, Magnetic and VLF datasets: Case studies of modelling, inversion and interpretation
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. , 59 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1394
Keyword [en]
VLF, gravity gradient, magnetic, airborne, inversion, non-uniqueness
National Category
Geophysics
Identifiers
URN: urn:nbn:se:uu:diva-300126ISBN: 978-91-554-9631-9OAI: oai:DiVA.org:uu-300126DiVA: diva2:950858
Public defence
2016-09-19, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2016-08-29 Created: 2016-08-02 Last updated: 2016-09-05
List of papers
1. Extracting geoelectrical maps from vintage very-low-frequency airborne data, tipper inversion, and interpretation: A case study from northern Sweden
Open this publication in new window or tab >>Extracting geoelectrical maps from vintage very-low-frequency airborne data, tipper inversion, and interpretation: A case study from northern Sweden
2016 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 81, no 5, B135-B147 p.Article in journal (Refereed) Published
Abstract [en]

In 1985, the mining company Luossavaara-Kiirunavaara Aktiebolag collected airborne very-low-frequency (VLF) data in northern Sweden. The operators stored only the vertical component and the total magnetic field, which at that time were believed to be sufficient for qualitative interpretation. Therefore, the data could not be directly used for quantitative tensor VLF processing and inversion. To avoid the costs of resurveying, we have developed a novel technique to estimate the tippers from the measured VLF data by computing anomalous and normal parts of the horizontal components of the magnetic field from two transmitters separately. Retrieval of the normal horizontal components was possible because one component of the horizontal magnetic field was used as the phase reference during the measurements. Additionally, we have determined how the approximate apparent resistivity suitable for data visualization can be computed from the components of the magnetic field assuming an average normal resistivity of the subsurface. Maps of apparent resistivity combined with topography show a clear correlation between high topography and high resistivity, whereas conductive zones are found in valleys in between. More importantly, the 3D model inverted from the calculated tippers shows excellent agreement with a map of the surface geology. Based on this comparison, some less resistive zones can be related to fluids in fractures and others can be related to mineralized contact zones. We suggest to focus further exploration on conductive zones surrounding areas with basaltic composition.

Place, publisher, year, edition, pages
Uppsala: , 2016
Keyword
case history, inversion, interpretation, electromagnetics
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-300086 (URN)10.1190/GEO2015-0296.1 (DOI)
Available from: 2016-08-02 Created: 2016-08-02 Last updated: 2016-09-05Bibliographically approved
2. Consistency investigation, vertical gravity estimation and inversion of airborne gravity gradient data – A case study from northern Sweden
Open this publication in new window or tab >>Consistency investigation, vertical gravity estimation and inversion of airborne gravity gradient data – A case study from northern Sweden
2016 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 81, no 3, B65-B76 p.Article in journal (Refereed) Published
Abstract [en]

For airborne gravity gradient data, it is a challenge to distinguish between high-frequency intrinsic and dynamically produced noise caused by the aircraft and small-scale effects from shallow density variations. To facilitate consistent interpretation, techniques that include all of the measured gravity gradient components are particularly promising. We represented the measurements by a common potential function accounting for lateral and height variations. Thus, it was possible to evaluate the internal consistency between the measured components and to identify components with bias or particularly strong noise. As an extra benefit for data sets that contain terrain-corrected and nonterrain-corrected gravity gradient measurements at flight altitude, we estimated terrain-corrected anomalies on the topographic relief using downward continuation and retrieved nonterrain-corrected gravity gradient data suitable for inversion using upward continuation. For a field data set from northern Sweden, the largest differences (up to 50 eotvos) between the measured and estimated components of the gravity gradient data were found in areas of high topographical relief. But the average residual standard deviations of the individual components were between 3.6 and 7.4 eotvos, indicating that the components were consistent in an average sense. We have determined the successful conversion of terrain-corrected airborne gravity gradient data to Bouguer gravity data on the topographic relief using ground-based vertical gravity data as a reference. A 3D inverse model computed from the nonterrain-corrected data clearly showed the depth extent of the geologic structures observed at the surface, but it only produced a weak representation of the shallow structure. In contrast, a 2D surface density model in which only lateral variations of density in the topographic relief was allowed exhibited more realistic density distributions in fair correlation with geology.

Keyword
gravity, modeling, noise, processing
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-300024 (URN)10.1190/geo2014-0428.1 (DOI)
Available from: 2016-08-02 Created: 2016-08-02 Last updated: 2016-09-05
3. A new reference model for 3D inversion of airborne magnetic data in hilly terrain – a case study from northern Sweden
Open this publication in new window or tab >>A new reference model for 3D inversion of airborne magnetic data in hilly terrain – a case study from northern Sweden
(English)Article in journal (Refereed) Submitted
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-300111 (URN)
Available from: 2016-08-02 Created: 2016-08-02 Last updated: 2016-09-05

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