Change search
Refine search result
1 - 13 of 13
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1.
    Belay, R.
    et al.
    University of Uppsala, Department of Geophysics.
    Rasmussen, Thorkild Maack
    University of Uppsala, Department of Geophysics.
    One‐dimensional non‐linear inversion of magnetotelluric data: the importance of data errorrs1995In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 43, no 7, p. 905-918Article in journal (Refereed)
    Abstract [en]

    Inversion of noisy magnetotelluric data over a horizontally stratified earth has no unique solution. Instead, the resistivities and thicknesses of the layers can only be specified to lie within certain bounds at a particular confidence level. The significance of having correctly estimated data errors when calculating the parameters and parameter bounds is discussed. Emphasis is put on the very corrunon problem of not being able to obtain models which can be considered acceptable with a reasonable certainty from a statistical point of view. A method is presented to obtain a pragmatic data error description by adjustment of wrongly estimated data errors while keeping the suppression of 1D data features at a minimum. A comparison is made between calculation of the parameter bounds based on approximating the confidence surface by hyperellipsoids and a calculation based on a full non‐linear most‐squares analysis. In general it is found that the approximation works very well when the confidence limit is small. However, significant deviations are found in some cases.

  • 2.
    Brown, R.J.
    et al.
    Luleå tekniska universitet.
    Friesen, G.H.
    Gulf Oil, Canada.
    Hall, D.H.
    Department of Earth Sciences, University of Manitoba.
    Stephenson, O.G.
    Department of Earth Sciences, University of Manitoba.
    Weighted vertical stacking in crustal seismic reflection studies on the Canadian shield1977In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 25, no 2, p. 251-268Article in journal (Refereed)
    Abstract [en]

    Seismic reflection methods are being developed at the University of Manitoba to aid in determining fine crustal structures in the Precambrian of Manitoba and northwestern Ontario. Present-day environmental concern as well as mineshaft conditions necessitate the detonation of several smaller charges repeated, say, I times and followed by ''vertical'' stacking. To obtain the familiar ..sqrt..I improvement in signal-to-noise (S : N) amplitude ratio applying the straight-sum (SS) method, one assumes, among other things, that both S : N ratio and signal variance are the same on all traces. Dropping these assumptions, as we must for our data, it becomes necessary to apply weighting coefficients to optimize the S : N ratio of the stacked trace. We still assume the signal shapes to be the same for repeated shots, so for the jth trace on the record of the ith shot we model the time series as: t/sub ij/ = a/sub i/(s/sub j/ + n/sub ij/); where a/sub i/ is a scaling factor. The proper weights w/sub i/ are then shown to be proportional to sigma/sub si//sigma/sup 2//sub ni/ where sigma/sup 2/ is variance, or to ..gamma../sub i//a/sub i/ where ..gamma../sub i/ is S : N power ratio. Applying the weighted-stack (WS) method gives S : N amplitude ratios which are, on average, 55% of the optimal ratios expected from WS theory compared with only 24% for the SS method. The 45% shortfall in WS performance is ascribed mainly to trace-alignment (or time-delay) errors. Varying noise levels on individual traces, slight dissimilarity of signal shape, and correlated noise may also contribute to a lesser extent (in decreasing order of significance). This WS method appears to strike a good practical balance between S : N improvement and processing efficiency.

  • 3. Das, U.C.
    et al.
    Parasnis, Dattatray
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Resistivity and induced polarization responses of arbitarily shaped 3-D bodies in a two-layered Earth1987In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 35, no 1, p. 98-109Article in journal (Refereed)
  • 4.
    Nissen, Johan
    Luleå tekniska universitet.
    A versatile electromagnetic modeling program for 2-D structures1986In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 34, no 7, p. 1099-1110Article in journal (Refereed)
    Abstract [en]

    A new integral-equation program for calculation of the E-polarization response allows modeling of VLF for plane wave or line source input e.g. magnetotellurics and Turam responses. The anomalous conducting body is modeled by a number of square cells, each of individual size and resistivity, with arbitrary position in the host medium. This provides a high degree of flexibility and allows for simulation of rather complex conductivity structures. The computation time has been drastically reduced by using techniques such as digital filtering and fast Fourier transformation. The interpretation of a measured Turam profile and the influence of galvanically channeled currents in the conducting body is discussed

  • 5.
    Nissen, Johan
    Luleå tekniska universitet.
    Frequency electromagnetic sounding using a vertical magnetic dipole: comment1987In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 35, no 8, p. 934-Article in journal (Other academic)
  • 6.
    Nissen, Johan
    et al.
    Luleå tekniska universitet.
    Enmark, Thomas
    Luleå tekniska universitet.
    An optimized digital filter for the Fourier transform1986In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 34, no 6, p. 897-903Article in journal (Refereed)
    Abstract [en]

    Short filters for calculating Hankel-transformations, with special attention to the d.c.-sounding problem, have been published in recent years. These filters, with a typical length of less than 25 coefficients, have made it possible to implement, e.g. VES-interpretation programs on microcomputers and 3-D electric and electromagnetic modeling programs on minicomputers. Initially the performance of the short filters was rather poor, but with the introduction of short optimized filters there has been a considerable improvement in the accuracy. An optimization procedure is applied to design a 20-point filter for the Fourier sine-transformation. This filter may be useful in electromagnetic prospecting theory

  • 7.
    Olsson, Olle
    Luleå tekniska universitet.
    VLF anomalies from a perfectly conducting half plane below an overburden1980In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 28, no 3, p. 415-434Article in journal (Refereed)
    Abstract [en]

    A theoretical solution to the electromagnetic problem of a perfectly conducting half plane below a conducting overburden has been obtained. The VLF anomalies have been computed for different overburden conductivity and the thickness and also for different dip angles of the half plane. In the computations the contribution to the secondary magnetic field from the electric Hertz potential has been neglected. From the anomaly curves it is possible to define the apparent depth to the top of the conductor as the distance between the peak value and the cross-over of the real component. It is possible to determine the actual depth to the conductor from the relation between the peak-to-peak anomaly and the apparent depth. When the peak-to-peak anomaly is fairly large, it is also possible to make estimates of the dip angle

  • 8. Parasnis, Dattatray
    Geophysical detection of mineral conductors in tropical terrains with target conductors partly embedded in the conductive overburden: comment1988In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 36, no 1, p. 92-93Article in journal (Other academic)
  • 9.
    Parasnis, Dattatray
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    More comments on "A theorem for DC regimes and some of its consequences" by A. Roy1984In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 32, no 1, p. 139-141Article in journal (Other academic)
  • 10.
    Pedersen, Laust Börsting
    et al.
    Section of Solid Earth Physics, University of Uppsala.
    Rasmussen, Thorkild Maack
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. Section of Solid Earth Physics, University of Uppsala.
    Inversion of magnetotelluric data: a nonline least-squares approach1989In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 37, no 6, p. 669-695Article in journal (Refereed)
    Abstract [en]

    The inverse problem of magnetotellurics over a horizontally stratified earth is described, with emphasis on practical application. The inversion is divided into basically two steps. The construction of some best solution, and the analysis of that solution with regard to uncertainty and complexity. For the construction of best solutions a robust non‐linear solver was developed, and for the estimation of parameter errors a modified eigenvalue‐eigenvector analysis is performed to better describe non‐linear effects. The choice of the number of layers is shown to be intimately connected with the structure of data errors and the misfit between model and data. An example from the Siljan impact structure in Sweden illustrates the power of the technique.

  • 11.
    Pedersen, Laust Börsting
    et al.
    Department of Geophysics, Section of Solid Earth Physics, University of Uppsala.
    Rasmussen, Thorkild Maack
    Department of Geophysics, Section of Solid Earth Physics, University of Uppsala.
    Dyrelius, Dan
    Department of Geophysics, Section of Solid Earth Physics, University of Uppsala.
    Construction of component maps from aeromagnetic total field anomaly maps1990In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 38, no 7, p. 795-804Article in journal (Refereed)
    Abstract [en]

    Total field anomalies as defined from normal aeromagnetic surveys give good approximations of the anomalous components along the direction of the main geomagnetic field, which is generally much larger than the anomalous field. Using the relations between vertical and horizontal components of the field, the total field anomaly is related to any vertical or horizontal component and the corresponding horizontal and vertical derivatives. An example from the Siljan impact structure indicates that such directional filters may be applied to extract useful information from magnetic maps.

  • 12.
    Tavakoli, Saman
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Bauer, Tobias
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Rasmussen, Thorkild Maack
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Weihed, Pär
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Deep massive sulphide exploration using 2D and 3D geoelectrical and induced polarization data in Skellefte mining district, northern Sweden2016In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 64, no 6, p. 1602-1619Article in journal (Refereed)
    Abstract [en]

    Geoelectrical and induced polarization data from measurements along three profiles and from one 3D survey are acquired and processed in the central Skellefte District, northern Sweden. The data were collected during two field campaigns in 2009 and 2010 in order to delineate the structures related to volcanogenic massive sulphide deposits and to model lithological contacts down to a maximum depth of 1.5 km. The 2009 data were inverted previously, and their joint interpretation with potential field data indicated several anomalous zones. The 2010 data not only provide additional information from greater depths compared with the 2009 data but also cover a larger surface area. Several high-chargeability low-resistivity zones, interpreted as possible massive sulphide mineralization and associated hydrothermal alteration, are revealed. The 3D survey data provide a detailed high-resolution image of the top ∼450 m of the upper crust around the Maurliden East, North, and Central deposits. Several anomalies are interpreted as new potential prospects in the Maurliden area, which are mainly concentrated in the central conductive zone. In addition, the contact relationship between the major geological units, e.g., the contact between the Skellefte Group and the Jörn Intrusive Complex, is better understood with the help of 2010 deep-resistivity/chargeability data. The bottommost part of the Vargfors basin is imaged using the 2010 geoelectrical and induced polarization data down to ∼1-km depth.

  • 13.
    Wyller, J.
    et al.
    Luleå tekniska universitet.
    Wellander, N.
    Luleå tekniska universitet.
    Larsson, F.
    Parasnis, Dattatray
    Burger's equation as a model for the IP phenomenon1992In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 40, no 3, p. 325-341Article in journal (Refereed)
    Abstract [en]

    The dynamic response characterizing the induced-polarization (IP) phenomenon is modelled by a nonlinear diffusion equation (Burger's equation) supplemented by relevant initial and boundary values. The analysis of the model yields a voltage step response and an impedance curve in the frequency domain which agree qualitatively with experimental measurements. Curve fits based on the model have been made in the case of electrochemical cell measurements. The diffusion coefficients estimated by means of these curves are of the same order of magnitude as those calculated using experimental measurements. The normalized transient with these diffusion coefficients agrees with observations, but probably has a shorter discharge time. We have also carried out a comparison with predictions obtained from a linear, finite diffusion layer model, thus showing that for most practical situations the nonlinear term modelling the migration effect can be neglected

1 - 13 of 13
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf