Journal of Geophysics
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Analytical presentation of statistically estimated magnetotelluric transfer functions by a set of polynomials
Get PDFIn magnetotelluric studies time variations of the horizontal telluric and magnetic field components at the earth's surface are compared to get detailed information of the electrical conductivity structure of the earth's interior. The development of conductivity models from the data demands the thorough estimation of the transfer functions in the frequency domain between the Fourier transforms of the recorded time series. The analytical presentation of the estimated transfer functions allows an individual selection of a number of frequencies for further investigation. Larsen's presentation (Larsen, 1975, 1980) of the transfer functions by single polynomials demands a complicated calculation of confidence limits. Therefore, the transfer functions are presented here by the sum of polynomials which fulfil an orthogonality criterion. The orthogonality criterion allows a rather simple estimation of the frequency-dependentconfidence limits of the transfer functions. The polynomial method is applied to a 100-day record of the magnetic and telluric field variations near Gottingen. As the telluric field is usually partially disturbed during such a long time interval, the polynomial method is extended to treat telluric time series with missing data. The comparison of the smooth polynomial transfer functions with band-averaged estimates yields a good correlation between the estimates as well as between their confidence intervals.
ARK: https://n2t.net/ark:/88439/y026544
Permalink: https://geophysicsjournal.com/article/4
 
Apparent and intrinsic Q: the one-dimensional case
Get PDFThe propagation of plane waves through statistically layered media is investigated both numerically and with single-scattering theory in the one-dimensional case. Exact apparent or stratigraphic Q, Qs, is determined from synthetic seismograms with the spectral-ratio method. Maximum velocity (impedance) fluctuations up to 30% (~40%) are studied; the fluctuations are uniformly distributed with zero mean. In all cases the trend of Qs as a function of frequency is well described by the analytical Qs, as determined from single-scattering theory under the assumption of an exponential autocorrelation function of the impedance fluctuations. The frequency dependence of the analytical Qs-1 follows a Debye-peak function, its maximum is γ2/2 and corresponds to the wavelength 4πa (γ2 = variance of relative impedance fluctuation, a = correlation distance). In further numerical calculations intrinsic or anelastic Q, Qa, is introduced, and it is shown that total attenuation Q -1 agrees very well with the sum of apparent and anelastic attenuation, Qs-1+Qa-1. Finally, a simple, minimum-phase stratigraphic attenuation operator is derived which describes the amplitude decay and the dispersion in a one-dimensional random medium with good accuracy. Stratigraphic attenuation is similar to the anelastic attenuation of a standard linear solid.
ARK: https://n2t.net/ark:/88439/y038753
Permalink: https://geophysicsjournal.com/article/119
 
Seismic response of two-dimensional absorbing structures by the ray method
Get PDFA method of seismic response analysis of 2 — D inhomogeneous structures, based on the ray method and on the application of the Debye procedure to include slight absorption, is presented. Program package RESPO, designed for such an analysis of the seismic response on the free surface of a general 2 — D laterally varying, geological near-surface structure assuming P, SV or SH plane-wave incidence from below, is briefly described. The package is tested on the classical model of a sedimentary basin. The study differs from previous applications of the ray method to the basin model in the following aspects. The frequency-domain approach is used. A comparison of the ray method results with the results of the discrete wavenumber method for long periods is made. More attention is paid to the analysis of the formation of the wave field. This analysis reveals two main types of wave propagation inside the basin: the dominant horizontally propagating local interference waves and less expressive vertically propagating waves in the central part of the basin. Effects of slight absorption (Futterman's causal absorption) are considered. The absorption causes a decrease in amplitudes and time delays at later times. The decrease in the amplitudes is not so expressive because of large periods and relatively short travel times of the waves investigated.
ARK: https://n2t.net/ark:/88439/y070849
Permalink: https://geophysicsjournal.com/article/136
 
The canonical decomposition and its relationship to other forms of magnetotelluric impedance tensor analysis
No full textA technique for magnetotelluric (MT) data analysis, known as the canonical decomposition, is developed from first principles. This analysis is based on the canonical decomposition of the impedance tensor Z and explicitly parametrizes Z in terms of eight physically relevant structural parameters which specify the transfer characteristics of the Earth system (i.e. the maximum and minimum principal apparent resistivities and the associated principal phases) as well as the principal or intrinsic coordinate system for Z (i.e. the two principal orthogonal electric and magnetic field polarization states). It is shown that the formulation of canonical decomposition in which the polarization descriptors are specified in terms of elliptic parameters results in the MT impedance tensor analysis presented by LaTorraca et al. The relationships between canonical decomposition and several other forms of magnetotelluric data analysis are explored. Specifically, we compare the canonical decomposition with the "conventional" analysis, the maximum coherency analysis, the associate and conjugate directions analysis developed by Counil et al., Eggers' eigenstate analysis and Spitz's rotation analysis. It is shown that canonical decomposition is a natural generalization of the conventional analysis in that both the rotation and ellipticity properties of Z are utilized in the definition of a principal coordinate system. A generalization of the maximum coherency analysis is shown to yield the same parameters as those extracted in canonical decomposition. By imposing a specific restriction on the generalized maximum coherency analysis, we next show how to extract the parameters (i.e. the directions of maximum and minimum current and induction and the corresponding electric and magnetic sheet impedances) that were obtained by Counil et al. in their associate and conjugate directions analysis. The relationship between canonical decomposition and Eggers' eigenstate analysis is developed and it is shown that the primary deficiency in the eigenstate formulation resides in the incorporation of an artificial field constraint. Spitz's rotation analysis extracts two analytical rotation angles from the matrix factors in the Cayley factorization of Z. It is shown that the Cayley factorization of Z is nothing more than the repackaging of the information in canonical decomposition and, as a consequence, Spitz's rotation analysis is not required to extract a principal or intrinsic coordinate system of Z.
ARK: https://n2t.net/ark:/88439/y078429
Permalink: https://geophysicsjournal.com/article/92
 
Seismic ground motion of sedimentary valleys - example La Molina, Lima, Peru
Get PDFStrong motion accelerograms recorded at two sites in Lima, Peru, during the earthquake of November 9, 1974, exhibit serious dissimilarities although the sites have nearly the same epicentral distance. The two sites are the Instituto Geofisico del Peru in central Lima and the La Molina sediment-filled valley on the periphery of the city. The anomalously strong and prolonged ground motion at the La Molina site seems to be explained by a combined effect of the complex topography of the bedrock and the presence of low-velocity subsurface sediments. In contrast to an intuitive feeling, a strong velocity contrast along the whole sediment-bedrock interface is not necessary. Because severe earthquake effects in La Molina are of site origin, they should be expected to repeat in the future. As indicated by synthetic accelerograms, the anomaly refers to large areas of the La Molina valley and not only to the immediate vicinity of the recording point. For purposes of seismic microzoning and land-use planning, two microzones in the studied part of the valley will probably be appropriate.
ARK: https://n2t.net/ark:/88439/y042722
Permalink: https://geophysicsjournal.com/article/120
 
Two-dimensional modelling of a towed transient magnetic dipole-dipole sea floor EM system
Get PDFThe discovery of massive sulphide deposits along mid-ocean ridges has prompted the development of towed sea floor electromagnetic mapping tools. One suitable configuration of transmitter and receiver is the in-line, coaxial, magnetic dipole-dipole. The step response of this system to a double half-space model has two distinct parts. The position in time of the initial event is indicative of the conductivity of the sea floor. A reduction in dimensionality greatly simplifies the analytic and numerical computation of more complicated cases. The transmitter is reduced to a pair of horizontal line sources carrying equal but opposite currents and separated by a small vertical distance. The transient responses of the simplified system and the coaxial system to the double half-space model are remarkably similar, even though the electromagnetic mode characterised by vertical current flow is excluded by the simplification. The analytic form of the sensitivity function enables a simple expression for a depth of investigation beneath the sea floor to be derived as a function of time. The magnetic effects of currents impressed in a two-dimensional conductive target embedded in the sea floor by a horizontal magnetic point dipole transmitter may be represented approximately by a system of vortex currents only. Since vortex current flow is the type of current flow induced in a two-dimensional target by a two-dimensional magnetic source, the principal characteristics of the three-dimensional problem can be studied by two-dimensional modelling. The scattered fields from a thin conductive dike and a thin conductive sill are evaluated by setting up and solving a boundary element integral equation in the electric field. Transient response curves are presented for a limited range of models. The sea floor conductivity is fixed at 1/30 that of seawater, a typical value for recent basalt. The array size and plate depth extent are fixed at 100 m and 25 m, respectively, while the depth of burial is allowed to vary from 4 m to 25 m. The ratio of the inductive response parameter of the plate to the response parameter of the crust, which determines the degree of influence of the plate conductor on the combined step response, is varied from 0.4 to 10. Increasing the relative effect of the target delays the onset and decreases the gradient of the initial part of the response. Pronounced minima in the response as a function of array location are observed when either the transmitter or the receiver cross over the target. The minimum field over a wide range of times is close to zero for a shallow dike due to the combined shielding effect of the dike and the seawater. The shallow dike may be distinguished from a shallow sill by the shape of the minima.
ARK: https://n2t.net/ark:/88439/y079019
Permalink: https://geophysicsjournal.com/article/123
 
A paleomagnetic cross-section through the Ardenne and the Brabant Massifs (France-Belgium)
Get PDFIn order to constrain the motions of the allochthonous Ardenne during the Paleozoic, standard paleomagnetic techniques have been applied on acidic and basic sills of the Cambrian Rocroi massif and on Cambrian-to-Visean sedimentary series sampled along the Meuse valley. Ordovician–Silurian volcanics and Devonian–Visean limestones have also been collected in the autochtonous Brabant and Namur basin, in order to get paleomagnetic reference poles. In the Ardenne, two groups of in situ paleomagnetic directions arise. The A components (mean: D = 212°, I = -11°, α95 = 6°, for 10 sites), that are characterized by unblocking temperatures around 330° C, represent Late Variscan (Stephanian–Permian) overprints. The B components (mean: D = 236°, I = 2°, α95 = 11°, for 13 sites) display higher unblocking temperatures, in the range 400°-580° C. They show a large scatter in inclination (–25° < I < 35°) that is not reduced by tectonic correction. The B components are interpreted as post- and partly synfolding overprints of Middle-Late Carboniferous age. In the Brabant, all series display Late Variscan remagnetizations (mean: D = 204°, I = -7°, α95 = 9°, for 5 sites), consistent with the Stephanian- Permian pole of Europe. The existence of Middle-to-Late Variscan B directions in the Armorican Massif, Central Massif, Vosges and Black Forest indicates that in Namurian–Westphalian times the whole investigated Variscan belt, including the Ardenne, was trending N-S. A 45° clockwise rotation relative to the paleomeridian, during the latest Westphalian–Stephanian, has lead the massif to its Permian position.
ARK: https://n2t.net/ark:/88439/y010025
Permalink: https://geophysicsjournal.com/article/140
 
Volcanoes, fountains, earthquakes, and continental motion - What causes them?
No full textAs a common mechanism for the various volcanic phenomena on Earth we point out a severe disobeyance of Poincare's (von Zeipel's) theorem: hot, gas-rich, high-pressure "fingers" (diatremes) can grow out of the boundary layer above a molten domain and thrust their way up from the asthenosphere toward the surface. The isobars of a planet or moon can look like the surface of a bed of nails. Linear arrays of high-pressure diatremes can drive continental motion. Moreover, we hold the tidal torque responsible for magnetic dynamo action.
ARK: https://n2t.net/ark:/88439/y049522
Permalink: https://geophysicsjournal.com/article/7
 
Generation and propagation mechanisms of low-latitude magnetic pulsations - A review
Get PDFContents1. Introduction2. Low-latitude Pc 3 magnetic pulsations 2.1 Introduction 2.2 Upstream waves as a source of low-latitude Pc 3 2.3 Transmission of upstream waves through bow shock and magnetopause 2.4 Propagation mechanism of Pc 3 source waves into deep magnetosphere 2.5 Transmitted Pc 3 source waves in the magnetosphere 2.6 Characteristic frequencies of various Pc 3 oscillations in magnetosphere 2.7 Wave characteristics of low-latitude Pc 3 2.8 Pc 2–3 at very low latitudes (Φ< 22°)3. Low-latitude Pi 2 magnetic pulsations 3.1 Introduction 3.2 Unresolved characteristics of Pi 2 pulsations 3.3 Wave and current fluctuation models for Pi 2 generation 3.4 Model of instantaneous transmission from polar electric field to equator 3.5 A possible model for daytime Pi 24. Summary and conclusions.
ARK: https://n2t.net/ark:/88439/y071009
Permalink: https://geophysicsjournal.com/article/146
 
Deep-seated lateral velocity variations beneath the GRF array inferred from mislocation patterns and P residuals
Get PDFThe analysis of mislocation patterns and the three-dimensional inversion of travel-time residuals for P waves measured at the GRF array reveal the existence of strong lateral velocity variations beneath the array. The most expressive phenomenon consists of an increase of P-wave velocities in the upper mantle from north to south, in addition to a possible thickening of the lithosphere to the south; especially the Moldanubian part of the Bohemian massif located to the southeast of the GRF array is characterized by high P-wave velocities in the upper mantle. The systematic change of the magnitude of the residual variation across the array, depending on the incidence angle for P waves, leads to the conclusion that a low-velocity zone exists in the upper mantle to the northeast of subarray A. The appearance of low-velocity material in the vicinity of the border between the two tectonic units, namely the Saxothuringian zone to the north and the Moldanubian zone to the south, might be connected to the deep structure of the graben area which extends to the northeast into the Egergraben.
ARK: https://n2t.net/ark:/88439/y046952
Permalink: https://geophysicsjournal.com/article/224