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  • 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 University of Technology. NTNF/NORSAR, Kjeller, Norway.
    Friesen, G.H.
    Gulf Oil Canada Limited, University of Manitoba, Calgary, Alberta, Canada.
    Hall, D.H.
    Department of Earth Sciences, University of Manitoba, Manitoba, Canada.
    Stephenson, O.G.
    Department of Earth Sciences, University of Manitoba, Manitoba, Canada.
    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.
    Koninklijke/Shell, Exploratie en Produktie Lab., Rijswijk, The Netherlands.
    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)
    Abstract [en]

    Numerical computations using the integral equation method are presented for resistivity and IP responses due to arbitrarily shaped 3‐dimensional bodies in a layered earth. The unknown surface charge density distribution is expressed as the solution of Fredholm's integral equation of the second kind. Use of moment method (with pulse basis function and point‐collocation) yields the matrix equations for the unknowns. The contributions to Green's function are solved (a) analytically for the primary and (b) by convolution for the secondary contributions resulting in a fast algorithm. The further step of computing potential, apparent resistivity, chargeability etc., for any electrode system, is straightforward. Our results show a good agreement with those from finite difference methods and physical tank experiments. The CPU time is only 138 s on a super‐minicomputer for an apparent resistivity pseudo‐section, even with 96 elementary cells as used for discretization. A large number of models for different geological situations were studied; some are presented here.

  • 4.
    Johansen, H. K.
    et al.
    Luleå University of Technology.
    Sørensen, K.
    Laboratory of Geophysics, Finlandsgade 8, DK-8200 Aarhus N, Denmark.
    Fast Hankel-Transforms1979In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 27, no 4, p. 876-901Article in journal (Refereed)
  • 5.
    Nissen, Johan
    Luleå University of Technology.
    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

  • 6.
    Nissen, Johan
    Luleå University of Technology.
    Comments on “Frequency electromagnetic sounding using a vertical magnetic dipole” by e. mundry and E.‐K. Blohm1987In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 35, no 8, p. 934-934Article in journal (Other academic)
  • 7.
    Nissen, Johan
    et al.
    Luleå University of Technology.
    Enmark, Thomas
    Luleå University of Technology.
    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

  • 8.
    Olsson, Olle
    Luleå University of Technology.
    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 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. The anomaly curves which are displayed as EM 16 readings, show a fairly complicated behaviour. This is mainly due to the phase shift and attenuation of the field caused by the conductivity of the overburden and the host rock. 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. The apparent depth is usually larger than the actual depth, but 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. However, a complete set of master curves will be a necessary tool for interpretation of VLF data when there is need to obtain more accurate estimates of the half plane parameters. In a specific case the theoretical calculations are shown to be in good agreement with measured data.

  • 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.
    Parasnis, Dattatray S.
    Luleå University of Technology.
    Comments on ‘Geophysical detection of mineral conductors in tropical terrains with target conductors partly embedded in the conductive overburden’ by J. O. Barongo1988In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 36, no 1, p. 92-93Article in journal (Other academic)
  • 11.
    Pedersen, Laust Börsting
    et al.
    Section of Solid Earth Physics, University of Uppsala, Uppsala, Sweden.
    Rasmussen, Thorkild Maack
    Section of Solid Earth Physics, University of Uppsala, Uppsala, Sweden.
    Inversion Of Magnetotelluric Data: A Non‐Linear 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.

  • 12.
    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.

  • 13.
    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.

  • 14.
    Tavakoli, Saman
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. Section for Natural Hazards, Norwegian Geotechnical Institute, 3930 Ullevaal Stadion, Oslo, N‐0806 Norway.
    Sarlus, Zmar
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. Mining Inspectorate of Sweden, Varvsgatan 41, Luleå, 97232 Sweden.
    Kronsell, Ida
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Mining and Geotechnical Engineering.
    Bauer, Tobias E.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    2.5D geophysical model of the Gällivare mining area: An integrated study to model the top 4km of the subsurface and guide for future exploration activities2021In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 69, no 4, p. 821-841Article in journal (Refereed)
    Abstract [en]

    Potential field and Slingram data alongside rock physical properties, drill‐core information and results from geological field mapping were used to investigate the geometry of the geological structures in the Gällivare area in regional scale. The main purpose of this study was to delineate the vertical and lateral extension of the Malmberget felsic volcanic rocks, the Dundret and Vassaravaara units as well as geological structures related to the crustal‐scale Nautanen deformation zone. Furthermore, we aimed at identifying new magnetite‐hematite and sulphide mineralizations and lithologies related to the mineralizations based on the results from regional‐scale potential field modelling which are delineated with rock physical properties and borehole data. The study result indicated that the dome‐shaped Dundret gabbro extends downwards to ∼4 km depth and has its maximum depth at its centre. Hematite and magnetite assemblages occur within the top 2 km of the Dundret complex. Felsic volcanic rocks in the Malmberget area extend vertically down to a maximum depth of ∼3 km and get considerably thinner towards the west. The model for the known magnetite‐rich mineralizations in Malmberget was inferred from earlier drilling activities and was integrated into the profile models, which indicates a reasonable fit to the measured data, in particular on the magnetic anomaly; whereas the small dimensions of the modelled structures make them invisible on the Bouguer anomaly data. Additional magnetite‐rich mineralizations are suggested within the Malmberget felsic rocks. High real component and low imaginary response of the Slingram data suggests conductive zones within the Nautanen deformation zone towards the NE, which given the geology of the area in the high strain zone can indicate disseminated sulphide mineralizations. Drill‐hole data at the eastern parts of W‐E profiles agreed well with the suggested model inferred from potential field data, suggesting presence of mafic intrusions within the ∼top 1 km of the subsurface. The integrated model based on geophysical, petrophysical and geological data improved earlier understanding about the regional geometry of the key structures in the Gällivare area and indicates new magnetite‐rich and sulphide mineralization prospects which can be used as a guide for future exploration activities in the area.

  • 15.
    Wyller, J.
    et al.
    Luleå University of Technology.
    Wellander, N.
    Luleå University of Technology.
    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

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