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  • 1
    Electronic Resource
    Electronic Resource
    Near-surface distortion effects on controlled source magnetotelluric transfer functions (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 108 (1992), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In this paper, we have studied the distortion effect of 3-D near-surface inhomogeneities within the frame of recently developed concepts of controlled source tensor magnetotellurics (Qian & Pedersen 1991; Li & Pedersen 1991), in which a pair of horizontal dipole (electric or magnetic) sources are used to determine impedance tensor and tipper functions, that are independent of source orientations. The tensor description of the distortion of surface electromagnetic fields generated by horizontal electric and magnetic dipole follows Wannamaker, Hohmann & Ward (1984) and Zhang, Roberts & Pedersen (1987). Reciprocity relations have been derived to relate electromagnetic field distortion matrices when the source and receiver are interchanged. These relations can be used to relate the distortion effect when either the source or the receiver is close to near-surface inhomogeneities. Based on thin sheet modelling we show, contrary to intuitive expectations, that the measurement of magnetic field for electric or magnetic dipole sources will not in general provide an undistorted estimate of the average conductivity below the surface. The characteristics of distortion effects for electric and magnetic dipole sources are discussed in detail both from analytical and numerical modelling points of view.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb03473.x
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  • 2
    Electronic Resource
    Electronic Resource
    CONSTRUCTION OF COMPONENT MAPS FROM AEROMAGNETIC TOTAL FIELD ANOMALY MAPS (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 38 (1990), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1990.tb01875.x
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  • 3
    Electronic Resource
    Electronic Resource
    INVERSION OF MAGNETOTELLURIC DATA: A NON-LINEAR LEAST-SQUARES APPROACH (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 37 (1989), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1989.tb02229.x
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  • 4
    Electronic Resource
    Electronic Resource
    THE MAGNETOTELLURIC IMPEDANCE TENSOR—ITS RANDOM AND BIAS ERRORS (1982)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 30 (1982), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Approaches to the reduction of bias in the computation of the elements of the magnetotelluric impedance tensor have been proposed in the past by several authors. In this paper a clear distinction is made between random errors and bias errors. No effort is made to reduce either, but the emphasis is on their estimation. Both types of errors depend critically upon the polarization of the magnetic field. The random error increases with increasing noise-to-signal ratio in the electrical field, and it is rather insensitive to noise in the magnetic field. The bias error increases with increasing noise-to-signal ratio in the magnetic field. Expressions for random errors and maximum bias errors are developed and discussed using a single station set-up. Random errors with a reference station set-up are also calculated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1982.tb01298.x
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  • 5
    Electronic Resource
    Electronic Resource
    END CORRECTIONS IN POTENTIAL FIELD MODELING (1979)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 27 (1979), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A general treatment of end corrections in gravity and magnetic modeling is presented. The well-known 2-D formulae of Talwani and Talwani and Heirtzler become slightly changed to represent a 2 1/2–D body, i.e. a body of polygonal cross section with the tails In the strike direction cut off.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1979.tb00994.x
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  • 6
    Electronic Resource
    Electronic Resource
    THE GRAVITY AND MAGNETIC FIELDS FROM ELLIPSOIDAL BODIES IN THE WAVENUMBER DOMAIN (1985)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 33 (1985), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Wavenumber domain expressions for bodies with elliptical cross-section and of ellipsoidal shape have been developed both for homogeneous bodies and for certain bodies of density/magnetization varying linearly with depth or, more generally, according to a polynomial with depth. The simple expressions thus obtained lend themselves to an easy analysis, especially for long and short wavelengths. At the long-wavelength end of the spectra their decay is governed by an exponential with a decay “depth” equal to the depth to the center of mass. At the short-wavelength end this depth is replaced by the depth to the upper focus of the ellipsoid (or the elliptic cross-section). For vertically inhomogeneous ellipsoids the decay rate is also dependent on the product of the vertical gradient of density/magnetization and their semi-axes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1985.tb00434.x
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  • 7
    Electronic Resource
    Electronic Resource
    CONSTRAINED INVERSION OF POTENTIAL FIELD DATA (1979)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 27 (1979), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: For constrained inversion of potential field data within the framework of generalized inversion an analysis of data error variances leads to confidence limits for the model parameters. For that purpose Pseudo-hyper-ellipsoids can be used to describe the nonlinear behaviour of the given inverse problem, and upper and lower bounds can be added to those parameters of which some independent knowledge is available. A gravity example is treated to show the application of the method.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1979.tb00993.x
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  • 8
    Electronic Resource
    Electronic Resource
    SOME RELATIONS BETWEEN POTENTIAL FIELDS AND THE STRENGTH AND CENTER OF THEIR SOURCES (1979)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 27 (1979), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: From gravity it is well known how to determine the excess mass and the horizontal center of mass of the disturbing body. We show that a magnetic body—under rather weak assumptions—possesses excess magnetization and centers of magnetization (both horizontal and vertical), which can be uniquely determined from measurements. It also follows that the vertical center of mass can be uniquely determined from the vertical derivative of the gravity field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1979.tb00995.x
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