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A global model of the Earth's crust has been constructed of two-layer blocks of 2° x 2° dimension, with thickness and susceptibility selected from a ten-step classification of the various crustal types. Considering induced magnetization only, the magnetic effect of each block has been approximated by a single dipole in the middle of the block, directed parallel to the field for a given main field model (32,400 dipoles altogether). The magnetic field of this global model of the crust has been calculated for an altitude of 450 km, appropriate for a comparison with anomaly fields from satellite surveys. From field values at 1° x 1° grid points, model anomaly charts for the X, Y, Z-components and for the total intensity can be plotted. The underlying crustal parameters have been prepared for modification towards adjusting the model field to the final Magsat anomaly field. The aim is to construct a truly realistic model of the whole Earth's crust. Based on a global distribution of the Z-component of the model field a spherical harmonic analysis has been made by a direct integral method, up to degree and order 35. The energy density spectrum of the magnetic field, apart from the lowest degree terms, resembles a "white" spectrum in which the level nearly meets that obtained for the crustal part of an actual field model (from n = 15 to 29), except for a factor of less than 3. A supplemental evaluation of the core part of the observed spectrum indicates a source depth of some 100 km below the surface of the Earth's core, supported by a similar result for the secular variation of the core field.
ARK: https://n2t.net/ark:/88439/y060920
Permalink: https://geophysicsjournal.com/article/166
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Acquisition of isothermal remanent magnetization (IRM) and the behaviour of IRM during high and low temperature treatment indicate that magnetite, goethite and minor quantities of hematite determine the magnetic properties of the Upper Jurassic limestones in Southern Germany. The direction of natural remanent magnetization (NRM) of magnetite is of detrital, synsedimentary origin, whereas the goethite, although its NRM is extremely stable against AF cleaning, has been magnetized only partly during the Jurassic. Part of the goethite NRM is interpreted to be of Tertiary (pre-Upper Miocene) age being formed during a period of Karstification in the Lower Tertiary. The apparent polarity sequence described earlier (Heller, 1977) is not of Jurassic age.
ARK: https://n2t.net/ark:/88439/y050321
Permalink: https://geophysicsjournal.com/article/165
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Recent developments in the computation of theoretical seismograms have shown the advantages of numerical integrations in the complex ray parameter plane. We here extend the approach, to calculate the amplitude of body waves interacting with a discontinuous velocity increase. The method incorporates a complex velocity profile to account for attenuation and the uniformly asymptotic method of Langer to account for frequency-dependence of reflection-transmission coefficients near grazing incidence. In the integrand of reflection-transmission coefficients, the Debye ray expansion is not made where it is poorly convergent near critical incidence. The method thereby correctly evaluates all the waves in the high velocity medium that are repeatedly refracted back up to, and reflected from, the low velocity medium. Calculations are equally efficient both for Earth models specified on discrete radii or as analytic functions of radius. The method is used to calculate theoretical seismograms for PKP waves in the PEM Earth model in the distance range 110°–152° and in Earth models 1066B and C2 in the distance range 112°–132°. Calculations of the amplitude near point D of the travel time curve for PKP (which is the critical distance of K waves incident from the fluid core upon the solid inner core) indicate that at finite frequencies the amplitude maximum associated with critical incidence is smaller than that that would be inferred from frequency independent reflection-transmission coefficients. At lower frequencies the maximum is displaced to a longer distance than D. Comparison of observed and calculated seismograms indicates that the PEM model having a P velocity jump of 0.83 km/s at the inner core can fit the observed amplitudes. Consequently the data do not require an anomalous P velocity gradient at the top of the inner core. A low frequency precursor to the DF branch of PKP at distances shorter than the B caustic at 146° agrees with Buchbinder's (1974) explanation of diffraction from the B caustic.
ARK: https://n2t.net/ark:/88439/y040552
Permalink: https://geophysicsjournal.com/article/164
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The usefulness of long-wavelength potential field anomalies in lithospheric interpretation is greatly increased with spherical Earth modeling techniques. Gauss-Legendre quadrature integration is used to calculate the anomalous potential of gravity and magnetic fields and their spatial derivatives on a spherical Earth for an arbitrary body represented by an equivalent point source distribution of gravity poles or magnetic dipoles. The distribution of equivalent point sources is determined directly from the coordinate limits of the source volume. Variable integration limits for an arbitrarily shaped body are derived from interpolation of points which approximate the body's surface envelope. The versatility of the method is enhanced by the ability to treat physical property variations within the source volume and to consider variable magnetic fields over the source and observation surface. A number of examples verify and illustrate the capabilities of the technique, including preliminary modeling of potential field signatures for Mississippi embayment crustal structure at satellite elevations.
ARK: https://n2t.net/ark:/88439/y030173
Permalink: https://geophysicsjournal.com/article/163
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The interpretation of geomagnetic anomalies with deep-seated sources sometimes requires postulating magnetic susceptibilities larger than those measured for common rock types at the Earth's surface. A possible explanation is that rocks buried at depths approaching the Curie point isotherm exhibit enhanced susceptibility due to the Hopkinson effect. In measurements on a sample of single-domain magnetite (0.04 μm particles), the susceptibility increased by a factor 2 between 20 and 500 °C and by a factor 3 at 550 °C. The Hopkinson peak was less pronounced in multidomain magnetites: the relative increase in susceptibility at 550 °C was by a factor 2 in 0.1 μm particles and a factor 1.5 in 0.25 μm particles. Single-domain hematite (0.1–1 μm) gave a spectacular Hopkinson peak, with relative susceptibility enhancement by a factor 5 at 530 °C and a factor 20 at 640 °C. However, rocks containing fine-grained maghemite and magnetite showed an enhancement of 50–70 % at most. The reasons for this variability in the height of the Hopkinson peak are not understood, but the width and shape of the peak are clearly related to the blocking temperature spectrum. Distributed blocking temperatures are associated with a broad peak, while discrete blocking temperatures are accompanied by a sharp susceptibility peak within 50-100 °C of the Curie point. A corollary is that remanent magnetization decreases roughly in inverse proportion to increase in susceptibility, so that the Koenigsberger Qn ratio decreases sharply at high temperature. For this reason, deep-seated anomalies can almost certainly be interpreted in terms of induced magnetization only. Finally, somewhat shallower bodies (temperatures of 200–400 °C) may exhibit thermally enhanced magnetization for two reasons: first, titanomagnetites have widely varying Curie points depending on titanium content, and second, observed anomalies are the result of a geomagnetic field applied over 106 years and viscous magnetization is also known to be enhanced at high temperature.
ARK: https://n2t.net/ark:/88439/y022154
Permalink: https://geophysicsjournal.com/article/162
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A stacking technique is applied to measure phase velocities of the fundamental and several higher Rayleigh modes over an array of long period stations located in Western Europe. The higher mode dispersion has been measured for periods between 25 and 100 s and for phase velocities up to 7.5 km/s. Using Backus-Gilbert inversion, a detailed model for the shear wave velocity in the upper mantle under the array is obtained. The low velocity zone is located between 150 and 230 km depth and is not very pronounced, but it is preceded by a rise in S velocity around 120 km depth. Strong velocity gradients are found at depths of 360 and 520 km. A good fit to the data can only be obtained if a zone of low density is assumed at a depth of 220 km or there about. As yet little can be said about depth, shape and extent of this zone, but the magnitude of the density drop implies a chemical or mineralogical stratification. A mechanism based on eclogite fractionation (Press, 1969) appears to be a likely candidate as the cause for such a gravitationally unstable stratification.
ARK: https://n2t.net/ark:/88439/y010885
Permalink: https://geophysicsjournal.com/article/161
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A two-dimensional magnetometer array was progressively installed in Scandinavia, from 1974 onwards, for operation during the IMS period (1977-1979). The 36 instruments, which are buried in the ground, are of the Gough-Reitzel type, i.e., classical magnetometers, with wire-suspended magnets and optical recording. The time-variation period range observable is from about 50 s to several days. In northern Scandinavia, the spacing between the stations is about 100-150 km in both the north-south and east-west directions. For presentation and analysis of the data a special Cartesian coordinate system has been introduced (the 'Kiruna system'). It is derived by mapping the local Earth's surface onto a tangential plane, with its origin close to Kiruna (geographic coordinates 67.8 N, 20.5 E). First results from some stations show that internal contributions to the recorded horizontal geomagnetic variations are small and possibly negligible at lower frequencies. However, at frequencies above about 2 mHz the variations of the vertical component display a strong amplification near the coast, and indicate the existence of conductivity anomalies at some inland locations. In order to demonstrate the observational capabilities of the array, equivalent overhead current configurations are presented which characterize a substorm recorded on October 7, 1976.
ARK: https://n2t.net/ark:/88439/y001456
Permalink: https://geophysicsjournal.com/article/160
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A method of finding a first approximation to a crustal magnetization distribution from inversion of satellite magnetic anomaly data is described. Magnetization is expressed as a Fourier series in a segment of spherical shell. Input to this procedure is an equivalent source representation of the observed anomaly field. Instability of the inversion occurs when high frequency noise is present in the input data, or when the series is carried to an excessively high wave number. Preliminary results are given for the United States and adjacent areas.
ARK: https://n2t.net/ark:/88439/y090977
Permalink: https://geophysicsjournal.com/article/159
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Formalization of morphostructural zoning of mountain regions, with special emphasis on determination of lineaments, is considered in this paper. The zoning is based on joint analysis of topography, geology, tectonics and geomorphology, represented on corresponding maps and aerial or space photos. Our goal is to make the zoning objective and reproducible. The importance of this goal follows from the fact that morphostructural zoning, especially the scheme of lineaments, is the starting point of our approach to prediction of strong earthquakes; it is important also to location of some mineral deposits. The definition of large elements of relief is formalized in the first place. On the basis of their characteristics a territory is divided into three types of areas: blocks, lineaments and knots. A precise and objective location of knot and lineament positions is the final aim of our formalized scheme. The application of the suggested algorithm to Eastern Tien Shan is described in the conclusion of the paper, as an illustration.
ARK: https://n2t.net/ark:/88439/y085028
Permalink: https://geophysicsjournal.com/article/158
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A theoretical model is proposed which describes the influence of non-uniform ionospheric height-integrated conductivity distributions on VLF-pulsations. The assumption is made that the field-aligned currents carried by the wave-field are closed by polarisation currents in the magnetosphere and by the irrotational part of the height-integrated ionospheric currents. Current continuity at the magnetosphere-ionosphere boundary provides for a differential equation governing the reflected electric field for arbitrary non-uniform conductivity distributions. Model calculations for simple, but realistic conductivity and electric field distributions show that local shifts of the ionospheric field maximum against that of the magnetic field below the ionosphere as well as double-peak distributions of the electric field can occur. Strong electric field anomalies i.e. significant deviations of the electric field distribution as compared with the uniform case occur in conductivity gradient zones and fall off rapidly outside. The previously predicted 90° rotation between the magnetic field below and above the ionosphere does not hold generally because the rotation angle depends strongly on the conductivity gradients.
ARK: https://n2t.net/ark:/88439/y072759
Permalink: https://geophysicsjournal.com/article/157
