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Numerical convection experiments were carried out with the aim of simulating the lithosphere as a strong mechanical boundary layer participating in the circulation, and to study its dynamical role and the governing parameters. The rheological model parameters were successively refined, effective viscosity depending on (1) depth, (2) temperature and pressure, and (3) temperature, pressure, and stress. In all cases a high-viscosity plate rested on a low-viscosity asthenosphere; in the two latter cases it could in principle subduct, but did so only if zones of weakness were built into it. It was possible to model active or inactive plates (moving faster or slower than the asthenosphere below). Because of a lack of numerical resolution it was however, not possible to simulate a narrow sinking slab; rather a broad zone of cooled and highly viscous material developed, often limiting the rate of descent and leading to non-steady convection. The circulation, including subduction, was stabilized by introduction of stress-dependence of viscosity (non-linearity), dissipation, and adiabatic heating. The parameter chiefly responsible for deciding the (active or passive) role of the plate is its decoupling from its neighbours, achieved in the models by assuming weakness zones. Another important result seems to be that the assumption of plausible mantle rheologies and heat input leads to equally plausible effective viscosities, plate velocities, and to upper-mantle temperatures which are relatively low by current ideas, but conforming to earlier estimates based on convection theory. Viscosity distribution and flow pattern are also in reasonable agreement with more detailed boundary layer computations. The main obstacles to our modelling are the numerical limitations, forcing upon us such artificialities as two-dimensionality, rectangular model boxes, coarse grids, and generalized weakness zones.
ARK: https://n2t.net/ark:/88439/y061130
Permalink: https://geophysicsjournal.com/article/176
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The linearization approach to the evaluation of travel-times of seismic body waves propagating in inhomogeneous, slightly anisotropic media is discussed. General linearization equations are specified both for quasi-compressional and quasi-shear waves. Various situations of seismological interest are investigated in detail. This applies, e.g., to the situation where the unperturbed medium is isotropic and to the case where the unperturbed ray is a plane curve. The numerical examples presented suggest that the linearization approach gives travel-times of seismic body waves with an accuracy sufficient to solve direct and inverse kinematic problems for inhomogeneous anisotropic models of the Earth's crust and the uppermost mantle.
ARK: https://n2t.net/ark:/88439/y051821
Permalink: https://geophysicsjournal.com/article/175
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The reflectivity method for the computation of theoretical seismograms is extended to include a dislocation point source of arbitrary orientation buried in a layered medium. The second extension is a change in the numerical integration over the circular frequency in order to avoid time aliasing effects.
ARK: https://n2t.net/ark:/88439/y041182
Permalink: https://geophysicsjournal.com/article/174
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An experiment conducted by using a continuous wave-spectrum high frequency radio wave Doppler sounder array with three sites and nine transmitters (each site with three transmitters) was carried out to observe the coupling of energy between the troposphere and the ionosphere during the period of Hurricane Eloise. The analysis of the Doppler sounder records indicated that gravity waves were detected when the eye wall of Hurricane Eloise was located at the Gulf of Mexico. A group ray tracing has been used in an attempt to locate the sources of these waves. Wave sources are located along the storm track and near the storm center. The wave excitation mechanism is discussed.
ARK: https://n2t.net/ark:/88439/y031553
Permalink: https://geophysicsjournal.com/article/173
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On 19 November 1976 a rather well developed giant pulsation event occurred at about 0600 MLT over northern Scandinavia. Results of observations obtained with a dense network of magnetometer stations are reported. The method of analysis is based on the concept of the analytical signal. The disturbance region was strongly localised. The amplitudes of the horizontal components show a pronounced difference: the amplitude maximum of the N-S component is clearly moving to the west while that of the E-W component is nearly fixed relative to the array. The frequency of the event decreased with time. Both the frequency decrease and the drift of the amplitude maximum are interpreted in terms of bounce - resonance instability. The spatial polarisation pattern found is in agreement with the fieldline-resonance theory in that the sense of rotation of the horizontal disturbance vector is as predicted by the theory. At the region of maximum amplitudes the pulsation is linearly polarised in E-W direction, as observed by other workers. This does not agree with the fieldline-resonance model. Apparent azimuthal wave numbers of the horizontal components show that the event analysed is a westward-travelling wave. The wave numbers are as large as predicted by Olson and Rostoker's (1978) empirically derived relation between the E-W wave number and the frequency of pc 4-5 pulsations.
ARK: https://n2t.net/ark:/88439/y020914
Permalink: https://geophysicsjournal.com/article/172
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A new type of aeromagnetic anomaly map (a long-wavelength anomaly map with anomaly widths in the range 60 km < λ < 4000 km) is presented for the area. It is believed that this group of anomalies represents a physically distinct field. This field shows considerable correlation with the broad features of deep crustal structure as derived from seismic sounding; linear relationships were found between the field and depths to the bottom of the crust and to the boundary between the upper and the lower crustal layers, as well as to the thickness of the lower crustal layer. A theoretical relationship connecting structure on magnetized layers to magnetic anomalies is given showing that the linear relationships are to be expected. It is shown that the lower crustal layer is the most likely source of the anomalies, with an intensity of magnetization of 5.3 X 10-3 emu/cc. It is also indicated that the upper crustal layer could also be the source, but that a shallow plate of magnetization could not explain the anomalies. Thus deep crustal magnetization must (on all interpretations made in the present paper), be responsible for the long-wavelength anomalies. Also, these anomalies are strongly related to major features in surface geology.
ARK: https://n2t.net/ark:/88439/y012135
Permalink: https://geophysicsjournal.com/article/171
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Channel waves serve as a tool for the detection of discontinuities in coal seams caused, for example, by tectonic faulting. In this paper we study Love waves propagating along two-dimensional discontinuous coal seams. Synthetic seismograms, computed with an explicit finite difference method, are presented for three types of discontinuities: the seam end, the horizontal and the vertical offset. In all cases the discontinuity reflects mainly those waves with short wavelength and transmits those waves with large wavelength. An additional damping term is introduced into the finite difference formulation in order to prevent reflections from the edges of the computational grid. The boundary conditions at interfaces are all approximated with a truncation error of second order.
ARK: https://n2t.net/ark:/88439/y000566
Permalink: https://geophysicsjournal.com/article/170
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Data from the Munster IMS Magnetometer Array (Kuppers et al. 1979) have been analysed in the frequency domain to derive the inductive response function, C(ω, 0), from the ratio of the vertical magnetic field to the spatial gradient of the horizontal magnetic field. The response function was best determined by statistical frequency analysis techniques after the spatial gradients had been derived by least-squares fitting of two-dimensional second-order polynomials to the observations, with the constraint imposed that the solutions be curl-free. The derived response function was found to obey two different causality requirements, and most of the inequality constraints imposed on it (Weidelt, 1972). A preliminary model, in which conductivity is a function of depth only and which explains the major details of the observed response, is presented. It has a highly resistive uppermost layer of some 104 Ωm and of the order of 30 km thick, underlain by a layer of about 125 Ωm to a depth of around 140 km, where a transition takes place to a highly conducting (3 Ωm) half space. The effect of various non-uniform sources on the observations of C(ω, k) for this 1D model is illustrated.
ARK: https://n2t.net/ark:/88439/y091007
Permalink: https://geophysicsjournal.com/article/169
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Magnetotelluric measurements were made at 12 sites on a 210 km long profile in northern and eastern Iceland. The profile crosses areas of different geological age ranging from Tertiary flood basalt to the presently active zone of rifting and volcanism. Beneath the investigated area, at a depth of a few km down to about 20 km there exists a 5 km thick layer with low resistivity amounting to 15 Ωm which is imbedded between layers of higher resistivities. The depth to the low resistivity layer increases with increasing distance from the spreading axis. The low resistivity layer is presumably caused by partial melting at the base of the crust and at the top of the anomalous mantle beneath Iceland. Comparison with laboratory measurements confirms a basaltic composition and a temperature of 1,000-1,100° C of the good conductor, and probably partially molten peridotite in the upper mantle beneath. Mean temperature gradients in the crust calculated from the magnetotelluric data are in good agreement to surface temperature gradients measured in drill holes.
ARK: https://n2t.net/ark:/88439/y080108
Permalink: https://geophysicsjournal.com/article/168
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The two-dimensional distribution of the characteristics of Pi2 magnetic pulsations observed by the Scandinavian Magnetometer Array (SMA) during the passage of a westward travelling surge on 11 November 1976 and during three sucessive auroral break-ups around magnetic midnight on 15 February 1977 have been studied in relation to the position of active aurora and the break-up current system. On both days the greatest Pi2 amplitudes were collocated with the region where the brightest auroras were observed. The sense of polarization of the horizontal disturbance vectors changed along longitudinal and meridional lines. The two-dimensional equivalent current system of the Pi2 pulsations resembled a circular current vortex around the location of the localized upward field-aligned currents and changed its direction from counterclockwise to clockwise and back to counterclockwise again within one Pi2 cycle. Our observations indicate that the generation of Pi2 pulsations is not directly connected to periodic fluctuations of the complete current system at substorm onset, but that the upward directed field-aligned currents at the western edge of this system play the most important role for the Pi2 generation.
ARK: https://n2t.net/ark:/88439/y071359
Permalink: https://geophysicsjournal.com/article/167
