-
This paper deals with the computation of wavefields in 3-D inhomogeneous media containing structural elements such as pinch-outs, vertical and oblique contacts, faults, etc. The approach is based on the theory of edge waves. The total wavefield is considered as the superposition of two parts. The first part is described by the ray method. It has discontinuities because of its shadow boundaries. The second part is a superposition of two types of diffracted waves, caused by the edges and vertices of interfaces. This part smooths the above-mentioned discontinuities so that the total wavefield is continuous. Of special importance is the mathematical form of the amplitudes of diffracted waves, described with unified functions of eikonals. In fact, it allows all additional computations to be considered by finding the eikonals of diffracted waves. A modification of the ray method including diffraction by edges and vertices is described. A generalization of the concept of edge waves for caustic situations is given — the method of superposition of edge/tip waves. The result of such a generalization no longer supplements the geometrical seismic description, but completely replaces it by a new description valid for a broader class of wave phenomena (reflection/refraction, diffraction on edges and vertices, formation of caustics, etc.).
ARK: https://n2t.net/ark:/88439/y067930
Permalink: https://geophysicsjournal.com/article/236
-
A complicated system of field-aligned currents is known to exist in the high-latitude region near noon, associated with the cusp. We suggest that the equatorward part of this system, referred to as the Region 1 field-aligned currents, is caused by the leakage of the field-aligned currents associated with the rotational discontinuities at the dayside magnetopause. The poleward part, referred to as the cusp field-aligned currents, is associated with the tail magnetopause. In this situation, it can be shown that the direction of the field-aligned currents at the magnteopause is controlled by the y-component of the interplanetary magnetic field (By). The poleward (equatorward) part of this field-aligned current system is found to flow out of (into) the northern polar ionosphere when By > 0 and into (out of) the northern ionsophere when By < 0. This current pattern reverses systematically in the southern polar ionosphere. Therefore, the suggested mechanism can explain qualitatively the observed changes of the cusp current systems. Further, the latitudinal width of the cusp field-aligned current system at the ionospheric altitude is estimated to be 100-400 km, consistent with observations.
ARK: https://n2t.net/ark:/88439/y057041
Permalink: https://geophysicsjournal.com/article/235
-
We present some results of the theory of high-frequency radiation by seismic sources. The emphasis will be placed on the kinematics of high-frequency waves, especially the stopping phases produced when the rupture encounters barriers of general shape. These results will be obtained from the representation theorem in which we replace the Green function by its asymptotic approximation at high frequencies, i.e. what is usually called the far-field approximation. This yields an expression akin to the Kirchhoff diffraction integral used in the modelling of reflection profiles and in seismic migration. The results obtained by this method are valid at distances from the fault which are longer than the dominant wavelength of the radiation. By a detailed analysis of the asymptotic method we find the wavefront discontinuities produced by rupture velocity jumps (barriers) or slip discontinuities (asperities) on the fault. Some examples of comparison between synthetics calculated with the new methods and those obtained by complete near-field synthesis will be presented. Among the examples we will consider is the circular fault, a model proposed by Bouchon for the Coyote Lake earthquake.
ARK: https://n2t.net/ark:/88439/y047732
Permalink: https://geophysicsjournal.com/article/234
-
A brief review of the reflectivity method is given, including a new analytical solution of the layer matrix equation. The method is extended to allow the computation of complete body waves for different source and receiver structures. Applications of theoretical seismograms to the Grafenberg broadband data are shown. Examples are the detection of depth phases at regional distances in southern Germany, which leads to improved source depth determinations, and the computation of the complete P-wave group for events in ocean-covered areas.
ARK: https://n2t.net/ark:/88439/y037003
Permalink: https://geophysicsjournal.com/article/233
-
Spherical harmonic analysis coefficients of the external and internal parts of the quiet-day geomagnetic field variations (Sq), separated for the North American, European, Central Asian and East Asian regions, were used to determine conductivity profiles to depths of about 600 km by the Schmucker equivalent-substitute conductor method. All three regions showed a roughly exponential increase of conductivity with depth. Distinct discontinuities seemed to be evident near 255-300 km and near 450-600 km. Regional differences in the conductivity profiles were shown by the functional fittings to the data. For depths less than about 275 km, the North American conductivities seemed to be significantly higher than the other regions. For depths greater than about 300 km, the East Asian conductivities were largest.
ARK: https://n2t.net/ark:/88439/y028414
Permalink: https://geophysicsjournal.com/article/232
-
Recent models of Earth structure suggest large horizontal gradients, especially in shear velocities. Some changes in existing methods are required to construct synthetics for broadband signals in many situations, especially when energy can reach the receiver by up-going as well as down-going paths. This can be accomplished by allowing locally dipping structure and making some modifications to generalized ray theory. Local ray parameters are expressed in terms of a global reference which allows a de Hoop contour to be constructed for each generalized ray with the usual application of the Cagniard-de Hoop technique. Several useful approximations of ray expansions and WKBJ theory are presented. Comparisons of the synthetics produced by these two basic techniques alone, or in combination with known solutions, demonstrates their reliability and limitations.
ARK: https://n2t.net/ark:/88439/y018295
Permalink: https://geophysicsjournal.com/article/231
-
The statistical study on the field intensity of daytime whistlers at low (geomag. lat. 25°) and middle (35°) latitudes has been made, based on a lot of data obtained by the field-analysis direction finding system based on the simultaneous measurement of two horizontal magnetic field components and one vertical electric field component. At low latitude, the maximum absolute intensity is estimated to be 250 μV/m, while the corresponding maximum intensity at middle latitude amounts to 600 μV/m, being about 2.4 times that at low latitude. This latitudinal variation of daytime intensity is interpreted in terms of the joint influence of (a) source activity, (b) magnetospheric propagation effect and (c) ionospheric transmission loss. As the result, it is found that whistlers at each station are attributed to ducted propagation in the magnetosphere and have exited the ionosphere close to each observing station, as determined by the direction finding results. Furthermore, the cloud distributions observed by meteorological satellites have yielded that the duct entrance point of whistlers at each station fall within the active thunderstorm region. Hence, we can conclude that daytime whistlers are originated in the active thunderstorms in each conjugate region, are trapped in field-aligned ducts and followed by nearly the vertical exit from the ionosphere at each station. Finally the latitudinal difference of the intensity is satisfactorily interpreted in terms of the difference in the ionospheric transmission loss on the assumption of the same source intensity at each conjugate point and of no amplification in the magnetosphere.
ARK: https://n2t.net/ark:/88439/y007596
Permalink: https://geophysicsjournal.com/article/230
-
The electrical conductivity structure of the Baltic Shield in Finland has been studied by magnetovariational (MV) and magnetotelluric (MT) work. First magnetometer arrays revealed the crustal Oulu conductivity anomaly which consists of a crustal conductive zone and a conductivity boundary. Since 1983 the magnetotelluric technique has been used to study the Oulu anomaly in more detail. The information obtained from the induction vectors of the MV and MT data are compared. 1 D and 2D models of the Oulu anomaly were constructed from MT data using induction vectors as additional information. In the centre of the research area the anomalous body (with a resistivity of 0.5 ohm m) lies below a depth of 4-7 km. Its width is about 25 km and its length is more than 100 km. To the south-west of this anomaly a low-resistive crustal layer exists at a depth of 14 km, whereas to the north-east no crustal layer was identified in the very resistive Karelidic realm.
ARK: https://n2t.net/ark:/88439/y097907
Permalink: https://geophysicsjournal.com/article/229
-
For the one-dimensional magnetotelluric inverse problem the ties between the impedances at neighbouring frequencies, reflecting the analytical properties of the transfer function, are expressed in terms of inequalities between the data. After the derivation of some elementary necessary constraints for data sets with two or three frequencies, a set of necessary and sufficient conditions warranting the existence of a one-dimensional conductivity model in the general M-frequency case is given. This set of constraints characterizes a 1-D data set by the signs of 2M determinants derived from the data.
ARK: https://n2t.net/ark:/88439/y087338
Permalink: https://geophysicsjournal.com/article/228
-
A conceptual model of core dynamics and the Earth's magnetic field is presented. It differs from previous investigations in the use of an estimated core viscosity of 2 x 107 cm2 s- 1. The simplified derivations predict the correct order of magnitude for the external magnetic field and for the westward drift of the nondipole field.
ARK: https://n2t.net/ark:/88439/y077129
Permalink: https://geophysicsjournal.com/article/227
