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A Broad View on the Interpretation of Electromagnetic Data (VLF, RMT, MT, CSTMT)
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
2004 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
En bred syn på Tolkning av Elektromagnetiska Data (VLF, RMT, MT, CSTMT) (Swedish)
Abstract [en]

The resolution power of single Very Low Frequency (VLF) data and multi-frequency Radiomagnetotelluric (RMT) data in delineating conductive structures typical for the sedimentary cover and crystalline basement in Scandinavia is studied with a view to future developments of the technique to increasing the frequency range into the LW radio band. Airborne and ground VLF data are interpreted and correlated with RMT measurements made on the ground to better understand the resolution power of VLF data. To aid in this understanding single and multifrequency VLF and RMT responses for some typical resistivity structures are analyzed. An analytic model is presented for obtaining unique transfer functions from measurements of the electromagnetic components on board an air-plane or on the ground. Examples of 2D inversion of ground and airborne VLF profiles in Sweden are shown to demonstrate the quantitative interpretation of VLF data in terms of both lateral and depth changes of the resistivity in the uppermost crust.

Geothermal resources are ideal targets for Electromagnetic (EM) methods since they produce strong variations in underground electrical resistivity. Modelling of Magnetotelluric (MT) data in SW Iceland indicates an alteration zone beneath the surface, where there are no obvious geothermal manifestations, in between Hengill and Brennisteinsfjoll geothermal systems. It suggests that a hydrothermal fluid circulation exists at depth. It also proves that the MT method, with its ability to map deep conductive features can play a valuable role in the reconnaissance of deep geothermal systems in active rift regimes such as in Iceland.

A damped nonlinear least-squares inversion approach is employed to invert Controlled Source Tensor MT (CSTMT) data for azimuthal anisotropy in a 1D layered earth. Impedance and tipper data are inverted jointly. The effects of near-surface inhomogeneities are parameterized in addition to each layer parameter(s). Application of the inversion algorithm to both synthetic and field data shows that the CSTMT method can be used to detect azimuthal anisotropy under realistic conditions with near surface lateral heterogeneities.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2004. , p. 68
Series
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 959
Keyword [en]
Geophysics, Airborne, azimuthal anisotropy, CSTMT, distortion, EM, geothermal, Hengill, mineralization, MT, peaker, resolution, RMT, tipper, tensor VLF, VLF
Keyword [sv]
Geofysik
National Category
Geophysics
Identifiers
URN: urn:nbn:se:uu:diva-4146ISBN: 91-554-5925-0 (print)OAI: oai:DiVA.org:uu-4146DiVA, id: diva2:164334
Public defence
2004-04-29, Axel Hambergsalen, Geocentrum, Villavägen 16, SE-752 36, Uppsala, 10:00
Opponent
Supervisors
Available from: 2004-03-31 Created: 2004-03-31 Last updated: 2016-05-13Bibliographically approved
List of papers
1. Detection of lateral changes in sedimentary cover by VLF
Open this publication in new window or tab >>Detection of lateral changes in sedimentary cover by VLF
In: Applied GeophysicsArticle in journal (Refereed) Submitted
Identifiers
urn:nbn:se:uu:diva-91556 (URN)
Available from: 2004-03-31 Created: 2004-03-31Bibliographically approved
2. Resolution of airborne VLF data
Open this publication in new window or tab >>Resolution of airborne VLF data
2006 (English)In: Journal of Applied Geophysics, ISSN 0926-9851, E-ISSN 1879-1859, Vol. 58, no 2, p. 158-175Article in journal (Refereed) Published
Abstract [en]

The interpretation of airborne VLF data represents an important aspect of geophysical mapping of the upper few hundred meters of the Earth's crust, especially in areas with crystalline rocks. We have examined the ability of the single frequency VLF method to provide quantitative subsurface resistivity information using two generic models and standard airborne parameters with a flight altitude of 70 m and a frequency of 16 kHz. The models are long thin conductor (10 in thick, 10 Omega in resistivity and 1 km long) and a wider buried conductive dike (100 Omega m resistivity and 500 m wide). Using standard regularized inversion it turned out that for both models the conductivity of the conductors are underestimated and the vertical resolution is rather poor. The lateral positions of the minimum of the resistivity distributions coincide well with the true positions of the shallow conductors. For deeper conductors the position of the minimum resistivity moves from the edges of the conductor into the conductor. The depth to the minimum of the resistivity anomalies correlates well with the true depth to the top of the conductors although the latter is always smaller than the former.

Interpretation of field airborne data collected at 70 in flight height resolved both small scale and large scale near surface conductors (conductance similar to 1 S). Deeper conductors show up in the VLF data as very long wavelength anomalies that are particularly powerful in delineating the lateral boundaries of the conductors. Many of the VLF anomalies in the Stockholm area are dominated by these deep conductor responses with some near surface conductors superimposed. The deep conductors often follow topographic lows coinciding with metasediments. We interpret the frequent absence of near surface responses at 70 in flight height as a result of weak coupling between the primary VLF wave and the small scale (in all three dimensions) near-surface conductors.

Radio magnetotelluric (RMT) ground measurements were carried out along a short profile coinciding with part of an airborne profile. Using data at 9 frequencies (14-250 kHz) small scale conductors in the upper few tens of meters, not identified from the airborne data, could be well resolved. Large scale deeper conductors could be identified by both methods at nearly the same positions.

Keyword
airborne, RMT, modeling, tipper, 2D, VLF
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-91557 (URN)10.1016/j.jappgeo.2005.06.002 (DOI)000241096600006 ()
Available from: 2004-03-31 Created: 2004-03-31 Last updated: 2017-12-14Bibliographically approved
3. Airborne VLF measurements and variations of ground conductivity: a tutorial
Open this publication in new window or tab >>Airborne VLF measurements and variations of ground conductivity: a tutorial
In: Surveys in GeophysicsArticle in journal (Refereed) Submitted
Identifiers
urn:nbn:se:uu:diva-91558 (URN)
Available from: 2004-03-31 Created: 2004-03-31Bibliographically approved
4. The deep geothermal structure of the Mid-Atlantic Ridge deduced from MT data in SW Iceland
Open this publication in new window or tab >>The deep geothermal structure of the Mid-Atlantic Ridge deduced from MT data in SW Iceland
Show others...
2005 (English)In: Physics of the Earth and Planetary Interiors, ISSN 0031-9201, E-ISSN 1872-7395, Vol. 150, no 1-3, p. 183-195Article in journal (Refereed) Published
Abstract [en]

Iceland is very active tectonically as it is crossed by the Mid-Atlantic Ridge and its associated rift zones and transform faults. The high-temperature geothermal systems are located within the neo-volcanic zone. A detailed comparison of the main features of the resistivity models and well data in exploited geothermal fields has shown that the resistivity structure of Iceland is mainly controlled by alteration mineralogy. In areas where the geothermal circulation and related alteration take place at depths of more than 1.5 km, the investigation depth of the DC and TEM methods is inadequate and the MT method appears to be the most suitable survey method. MT soundings were carried out to determine the deep structure between two neighboring Quaternary geothermal fields: the Hengill volcanic complex and the Brennisteinsfjoll geothermal system, both known as high-temperature systems. MT data were analyzed and modeled using 1D and 2D inversion schemes. Our model of electrical conductivity can be related to secondary mineralization from geothermal fluids. At shallow depths, the resistivity model obtained from the MT data is consistent with the general geoelectrical models of high-temperature geothermal systems in Iceland, as revealed by shallow DC and TEM surveys. The current MT results reveal the presence of an outcropping resistive layer, identified as the typical unaltered porous basalt of the upper crust. This layer is underlain by a highly conductive cap resolved as the smectite–zeolite zone. Below this cap a less conductive zone is identified as the epidote–chlorite zone. A highly conductive material has been recognized in the middle of the profile, at about 5 km depth, and has been interpreted as cooling partial melt representing the main heat source of the geothermal system. This conductor may be connected to the shallow structure through a vertical fault zone located close to the southern edge of the profile.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-91559 (URN)10.1016/j.pepi.2004.08.027 (DOI)
Available from: 2004-03-31 Created: 2004-03-31 Last updated: 2017-12-14Bibliographically approved
5. Inversion of controlled-source tensor magnetotelluric data for a layered earth with azimuthal anisotropy
Open this publication in new window or tab >>Inversion of controlled-source tensor magnetotelluric data for a layered earth with azimuthal anisotropy
2000 In: Geophysics, ISSN 0016-8033, Vol. 65, no 2, p. 452-464Article in journal (Refereed) Published
Identifiers
urn:nbn:se:uu:diva-91560 (URN)
Available from: 2004-03-31 Created: 2004-03-31Bibliographically approved

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