Journal of Geophysics
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Construction of conductance bounds from magnetotelluric impedances
Get PDFWhereas any finite set of impedance data does not constrain the electrical conductivity σ (z) at a fixed level z in a 1D-model, the conductance function S (z2) as the depth-integrated conductivity from the surface to the depth z2 will be constrained. Assuming only the non-negativity of σ (z), it is shown that for a given depth z2 the models generating the lower and upper bound of S (z2) consist of a sequence of thin sheets. The determination of the positions of the thin sheets and their conductances leads to a system of nonlinear equations. As a limitation the present approach requires the existence of a model, which exactly fits the data. The structure of the extremal models as a function of z2 is discussed in examples with a small number of frequencies. Moreover, it is shown that any set of complex 1D impedances for M frequencies can be represented by a partial fraction expansion involving not more than 2M (positive) constants. For exactly 2M constants there are two complementary representations related to the lower and upper bound of S (z2). For the simple one-frequency case, a more general extremal problem is briefly considered, where the admitted conductivities are constrained by a priori bounds σ – (z) and σ + (z) such that σ – (z) ≤ σ (z) ≤ σ + (z). In this case, the extremal models for S (z2) consist of a sequence of sections with alternating conductivities σ – (z) and σ + (z). The sharpening of conductance bounds by incorporating a priori information is illustrated by an example.
ARK: https://n2t.net/ark:/88439/y078099
Permalink: https://geophysicsjournal.com/article/237
 
Evaluation of the effectiveness of theoretical model calculation in determining the plasmapause structure
No full textThe relative position of the VLF/ELF emission region with respect to the plasmapause is of essential importance in studying their generation and propagation mechanism. On occasions when whistler data are not available, providing extensively the experimental determination of the plasmapause, we are obliged to rely on the theoretical model calculation or, alternatively, on the empirical formulas. The present paper deals with the evaluation of the effectiveness of the use of a theoretical model calculation in estimating the plasmapause location with reference to its comparison with in-situ electron density measurements and empirical formulas, during a specific geomagnetic storm. It is concluded that the temporal evolution with the present theoretical calculation, under a more acceptable convection electric field model, would yield a sufficiently reliable value for the plasmapause configuration rather than the empirical formulas. It can be used in the study of wave-particle interactions when whistler data are not available and also in the study of the erosion of the plasmasphere itself.
ARK: https://n2t.net/ark:/88439/y088608
Permalink: https://geophysicsjournal.com/article/238
 
The reflectivity method: a tutorial
Get PDFAn extended reflectivity method is described by which complete seismograms for a point source in a layered half-space can be calculated. Starting with the differential equations and boundary conditions, the reflection and transmission of plane waves at layered media is treated first, followed by the synthesis of point-source wave fields. The frequency-domain displacements of the half-space surface are expressed as slowness integrals, and the most prominent parts of the integrands are the reflectivities of the layers below and above the point source and a function which is closely related to the transmissivity of the layers above the source. Reflectivities and transmissivities are calculated by recursive methods which are numerically stable for all frequencies and slownesses. Near- and far-field results are given for single-force and moment-tensor point sources. From the general results for the complete medium response, partial responses can easily be extracted, e.g. the original form of the reflectivity method which calculates only the response from the layers below the source. Thus, the extended reflectivity method has a flexibility which is not available if propagator methods are used for the calculation of the integrands. Various other aspects of seismogram calculation are addressed, such as extended sources, an earth-flattening transformation and the inclusion of absorption for constant and frequency-dependent Q. Theoretical seismograms are shown, first for body-wave propagation from explosions in a crustal model and in a model which came from seismic prospecting, and second for surface-wave propagation from a double-couple source. Due to the tutorial nature of this paper the derivations are mostly rather detailed. It is hoped that this will help interested newcomers to the field of theoretical seismograms to get started.
ARK: https://n2t.net/ark:/88439/y040102
Permalink: https://geophysicsjournal.com/article/330
 
Measurement of magnetic susceptibility anisotropy in Buntsandstein deposits from southern Germany
Get PDFThe anisotropy of magnetic low-field susceptibility in the Triassic Plattensandstein formation (Upper Buntsandstein) from northern Bavaria has a typical sedimentary fabric. The anisotropy ellipsoids are strongly oblate with minimum susceptibility axes normal to sedimentary bedding. The directions of the maximum susceptibility axes are consistent with the NNE-NE-trending general sediment transport direction that is derived from geological field observations of cross-bedding structures in the sandstones. However, the very small intensity differences between maximum and intermediate susceptibility require extremely sensitive measurement techniques. Comparative measurements were made with a spinner magnetometer, a cryogenic magnetometer and a susceptibility bridge. Directionally, the most consistent results were obtained with the spinner magnetometer after it was stabilized by means of a low-pass active filter. The directional consistency of the anisotropy principal axes can be improved further by annealing the sandstones at 750° C in air. During this treatment a strongly magnetic, low-coercivity mineral phase - probably magnetite - is formed which enhances the degree of magnetic anisotropy as well as the bulk susceptibility. Low temperature measurements indicate that, in the natural unheated state, paramagnetic minerals contribute substantially to the low-field susceptibility of the sandstones at room temperature.
ARK: https://n2t.net/ark:/88439/y006806
Permalink: https://geophysicsjournal.com/article/220
 
The reflectivity method for different source and receiver structures and comparison with GRF data
Get PDFA 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
 
Observation of kinetic Alfven waves excited at substorm onset
Get PDFGround-based observations of locally confined, very intense, drifting current systems by the EISCAT magnetometer cross in correlation with GEOS-2 measurements will be explained in terms of kinetic Alfven waves. Particle and magnetic flux measurements on GEOS-2 indicate an excitation of the waves at the inner edge of the Earthward-drifting plasma sheet by resonance mode conversion from hydromagnetic surface waves. The collapsing tail-like field configuration itself is identified as the surface wave. The comparison of theoretically deduced quantities with observational results reveals a satisfactory agreement between observations and theory.
ARK: https://n2t.net/ark:/88439/y003986
Permalink: https://geophysicsjournal.com/article/293
 
Upper-mantle cross-section from California to Greenland
Get PDFPure-path upper-mantle models appropriate for tectonic, shield and old ocean have been recently presented by Grand and Helmberger. This was accomplished by modeling a rather restricted data set of S and SS triplication waveforms as well as the beginning portion of the Love waves. A much larger data set of S, SS and SSS, etc. (multibounce S-wave triplications) with a mixture of tectonic paths is available. In particular, events usually occur at tectonic margins and are recorded on stable continents. We present results of modeling these observations for laterally varying structure, essentially along a profile from California to Greenland. The models are allowed to be locally dipping with the lithosphere thickening with age at the expense of a dwindling low-velocity zone. Lateral variation does not appear to be required for depths greater than 400 km along this particular profile. The best-fitting model has a large increase in lithospheric thickness near the Rocky Mountain Front, roughly an increase of 75 km in thickness over a horizontal distance of 400 km or less. The low-velocity zone, with a velocity of 4.4 km/s, is replaced by a much faster upper 300 km with velocities near 4.7 km/s or a 7% overall increase. The one-way travel time jumps by roughly 4 s across this boundary, which compares reasonably well with the direct S residuals obtained from deep earthquake data although the latter data show large scatter.
ARK: https://n2t.net/ark:/88439/y069500
Permalink: https://geophysicsjournal.com/article/12
 
Seismic modelling by methods of the theory of edge waves
Get PDFThis 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
 
Some applications of seismogram synthesis through the summation of modes of Rayleigh waves
Get PDFComplete synthetic seismograms can be computed by the superposition of the fundamental and higher modes of Rayleigh waves. The usefulness of this approach is illustrated by the fact that it is possible to reproduce with sufficient detail experimental signals lasting several tens of seconds and having a high-frequency content (up to 1 Hz). The method has been proven to work even for higher frequencies, up to 10 Hz. To illustrate the source and structure modelling using this method, the whole experimental records from the Carder displacement meter at the station El Centro for the 1968 Borrego Mountain earthquake have been fitted. Seismic profiles (displacement, velocity and acceleration) have been synthesized and they clearly show the expected agreement between ray travel times and arrival times of different phases.
ARK: https://n2t.net/ark:/88439/y068630
Permalink: https://geophysicsjournal.com/article/246
 
Some problems with S, SKS and ScS observations and implications for the structure of the base of the mantle and the outer core
Get PDFComplicated radially symmetric models of the seismic velocity structure at the base of the mantle (Bullen's D" region) and the uppermost outer core have been inferred from analyses of the waveforms and relative amplitudes of S, SKS and ScS phases. Using radially symmetric structure, it has been difficult to construct physically realizable models of the rheology of D" that simultaneously satisfy P and S amplitudes and slownesses in the core shadow. These data are reviewed in the light of an increasing body of evidence that the structure of D" is characterized by heterogeneities having a broad spectrum of scale lengths. Depending on the region and range interval of D" sampled, S waveforms can be found that support either a radially simple or complex model of D". The complex models have one or more first-order discontinuities in velocity. The particle motion measured by three-component recordings of some S + ScS waveforms is consistent with a discontinuous increase in S velocity 250--300 km above the core-mantle boundary. The observed particle motion in these examples cannot readily or alternatively be explained by either general anisotropy or by strong lateral velocity gradients in D". Sufficient variability in S waveforms and travel times exists, however, that any radially symmetric model having a strong degree of complexity should be accepted with caution until all of the competing effects of lateral heterogeneity and possible anisotropy in D" are fully investigated. The distribution and scale lengths of heterogeneities in D" may account for regional differences in the properties of D" inferred from waveform data, including features that mimic intrinsic attenuation and anisotropy.
ARK: https://n2t.net/ark:/88439/y072899
Permalink: https://geophysicsjournal.com/article/13