Journal of Geophysics
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Estimate of confidence in paleomagnetic directions derived from mixed remagnetization circle and direct observational data
Get PDFA method is presented for obtaining the direction and confidence oval for a paleomagnetic component at a site given a number of independently oriented samples, some of which give an estimate of the remanence direction, while others yield only remagnetization circles. Such mixed remagnetization circle-remanence direction data frequently characterise paleomagnetic sites carrying two remanence components where the component of interest is small and less dispersed compared to a more easily removed one. The method described maximises the amount of usable data per site and thus leads to an improved site direction estimate.
ARK: https://n2t.net/ark:/88439/y000446
Permalink: https://geophysicsjournal.com/article/149
 
Earth's flattening effect on the tidal forcing field
Get PDFA small part of the tidal forcing field whose contribution is omitted in the conventional spherical harmonic development of the tidal potential is caused by the flattening of the Earth. It is a homogeneous tidal field of magnitude of about 1 ngal superposed on the commonly known tidal forcing field. The conventional tidal forcing field can be completely described by the spatial variation of the gravity field of the tide-generating body within the space occupied by the Earth. The advantage of this description is that any reference to the Earth's motion with respect to the tide-generating body (called revolution without rotation) can be avoided.
ARK: https://n2t.net/ark:/88439/y033803
Permalink: https://geophysicsjournal.com/article/193
 
On the thermal state of the Earth's mantle
Get PDFAn apparent paradox is discussed, arising from the contrast between an inferred constant mantle viscosity profile and theoretical and experimental rheological flow laws, which predict a mantle viscosity function varying strongly as a function of both temperature and pressure. One can explain the paradox by a particular choice of material parameters, but then mantle temperatures (computed adiabatically) are too low; increasing the temperature by inserting compensatory thermal boundary layers is considered to be dynamically unfeasible, again because of the flow law. We consider this an impasse, and to resolve it, we suggest that old dogmas concerning boundary layers and adiabats need to be critically re-examined, to understand their basis. When this is done, we find that the observed constant viscosity is, in effect, demanded by the interplay of the rheology with the convective process, the mantle temperature is not necessarily adiabatic, and some form of layering effect may be expected, although the ideas presented here are virtually independent of the precise dynamical style of the convective motion. A consequence of these results is that explanations and extrapolations taken from constant-viscosity convection models are, a priori, unjvstifiable. (Specifically, a constant viscosity mantle is a fundamental consequence of the state of flow together with the fluid parameters and rheology: it is not a passive coincidence, which may then be used to deduce the flow state, etc.)
ARK: https://n2t.net/ark:/88439/y014655
Permalink: https://geophysicsjournal.com/article/17
 
Interplanetary magnetic field power spectra with frequencies from 2.4 x 10^-5 Hz to 470 Hz from HELIOS-observations during solar minimum conditions
Get PDFBy using data from the Technical University of Braunschweig flux-gate and search-coil magnetometer experiments on board of Helios 2 we study the spectral properties of the interplanetary magnetic field over a frequency range from 2.4 x 10-5 Hz up to 470 Hz. Examples of power spectral density estimates at different heliocentric distances are shown as well as the change of the spectra during the progress of a high speed stream. A general feature of the spectra is that in a log-log spectral representation the steepness of the power spectral density estimates varies as a function of frequency. If we relate the spectral densities by a power law P ∼ f –α, the spectral index α increases with increasing frequency. At 1 AU α varies on average from 1.6 to 3.4 and at 0.3 AU from 1.0 to 3.4, the major changes in the spectral index occurring at low frequencies. In addition, just within the frequency gap between the two experiments, between 2Hz and 4.7 Hz, an inflexion point is inferred from the spectrum above and below this frequency range. This spectral feature can at least partly be attributed to the damping of the Alfven-mode waves near the proton and also α-particle cyclotron frequencies. The observed power spectra are compared with models of MHD turbulence and it is found that at least some of the properties of MHD turbulence fit the observations remarkably well.
ARK: https://n2t.net/ark:/88439/y063880
Permalink: https://geophysicsjournal.com/article/70
 
On the equivalence of the "Niblett" and "Bostick" transformations in the magnetotelluric method
Get PDFIn the 1D interpretation of magnetotelluric data, it is often useful to discover a reasonable first-approximation to the true conductivity-depth distribution beneath the recording location. This may be undertaken in the field, in order to ascertain if the station spacing is satisfactory or whether a greater station density of coverage is required, or at the base laboratory, as a prelude to a more sophisticated 1D inversion of the data.
ARK: https://n2t.net/ark:/88439/y020744
Permalink: https://geophysicsjournal.com/article/141
 
Electrical conductivity beneath Iceland - constraints imposed by magnetotelluric results on temperature, partial melt, crust- and mantle structure
Get PDFMagnetotelluric results from 38 sites in North Iceland yield a detailed spatial distribution of conductivity versus depth. A high-conductivity layer, about 10 Ωm, marking the transition between crust and upper mantle is detected throughout the surveyed area, its depth increases from 10 km within the zones of present tectonic activity to 20–30 km at a 50–100 km distance from the rift axis. The high conductivity layer indicates some basaltic melt fraction at temperatures around 1,000°C. The calculated average crustal temperature gradients are 100–150°C/km at the rift axis and 50–70°C/km at about 100 km distance. Below this anomalous zone, down to 150 km depth, the conductivity decreases to 100 Ωm due to the ultrabasic chemical composition of the upper mantle and a possible decrease of melt content with increasing depth. The temperature gradient within the upper mantle cannot be more than a few °C/km. However, a highly conducting asthenosphere exists below 150 km depth. The following petrological model for Iceland is suggested: Diapiric updoming of the asthenosphere leads to partial fusion of parent ultra basic mantle material, resulting in segregation and upward migration of basaltic melt. Lower density, lower viscosity and higher mobility relative to the solid phase causes the melt to rise faster than the ultrabasic material. This melt fraction then forms the basaltic crust of Iceland which is thickening away from the ridge axis with increasing age.
ARK: https://n2t.net/ark:/88439/y023054
Permalink: https://geophysicsjournal.com/article/98
 
Peculiarities of mantle waves from long-range profiles
Get PDFLong-range seismic experiments carried out in different regions of the USSR show strong variations of the mantle wave velocities and attenuation. Relatively high mantle velocities, more than 8.2–8.3 km/s, are observed in the old platform areas. To exclude the crustal influence on the mantle waves and to separate the vertical and horizontal inhomogeneity effect the relations between the overlapping travel-time curves are analyzed. As a result, a mantle stratification with positive average velocity gradient in the strata is revealed up to depths of 200 km. Local inclusions with very high (more than 8.5 km/s) and relatively low (less 8.0 km/s) velocities are typical of the mantle.
ARK: https://n2t.net/ark:/88439/y090547
Permalink: https://geophysicsjournal.com/article/261
 
The intercept-time method – possibilities and limitations
Get PDFIn deep-seismic sounding the reflected and refracted waves recorded form a complex system of traveltime curves. The inverse seismic problem consists in determining the function ν (x, z) along the profile for a series of velocity levely ν (x, z) = const and discontinuities as well. This paper described a method which allows the transformation of observed traveltime curves into an intercept-time section with isolines of velocity. Such a section permits discrimination of the various types of waves and gives a qualitative picture of the crustal structure, and is then converted into a depth section. The application of this method is demonstrated by a number of examples from the Ukrainian Shield, the Tien Shan and the Caucasus showing both possibilities and restrictions.
ARK: https://n2t.net/ark:/88439/y052591
Permalink: https://geophysicsjournal.com/article/288
 
Parameter trade-off in one-dimensional magnetotelluric modelling
Get PDFThe problem of inverting or modelling one-dimensional magnetotelluric data can, today, be considered as largely resolved. Attention now focuses on the class of acceptable models. Viewed in the space of model parameters this class occupies a singly connected volume, bound with a surface where the standard deviation ε between measured and calculated response exceeds the minimum εo of the best-fitting model by a constant factor (typically ε ≈ 1.10 εo). This volume of acceptable models is described by its intersections with the parameter axes, and also by the extreme excursions possible for any of the model parameters when all the other parameters are adjusted accordingly. These extreme excursions therefore represent "trade-off" conditions among the model parameters and are summarized in the "trade-off matrix". In a sense this is a generalization of the parameter correlation matrix, which gives only local information in the vicinity of a proposed model. The trade-off matrix, however, is independent of any initial model. Another important question considered deals with the correct choice of the number of layers with which to model a data set. Whereas a single minimum of ε is found with the correct number no, when this number is too small the information contained in the data is spread among several isolated minima. When n > no the problem becomes "illposed ". There are too many degrees of freedom and it becomes possible, then, to move in model space in directions at right angles to the meaningful dimensions without finding a clear minimum. The problem is analogous to a vanishing determinant in linear algebra. To find a regular problem again it is necessary to specify auxiliary constraints provided, for example, by other soundings or prior geological knowledge, to compensate for the increased number of variables.
ARK: https://n2t.net/ark:/88439/y061860
Permalink: https://geophysicsjournal.com/article/289
 
Source parameters of mine tremors in the eastern part of the Ruhr district (West Germany)
Get PDFAbout 190 mine tremors were recorded during a 120 day period in the eastern part of the Ruhr district (West Germany). Source parameters of 37 tremors (1.3 ≤ ML ≤ 2.3) are calculated. Hypocentral distance was about 2 km. A combined plot of first P-wave motions in an equal area projection of a focal sphere possibly suggests a dip slip mechanism. Displacement P-wave spectra of 13 events show a clear low frequency level and a high frequency asymptotic behaviour as f –3, where f is the frequency. Seismic moment varies from 2.3 x 1018 dyn cm to 2.3 x 1019 dyn cm. Corner frequencies are in the range of 12–16 Hz. Interpretation of spectra with Brune's (1970) model extended by Hanks and Wyss (1972) leads to source dimensions between 87 m and 120 m. Stress drops vary from 0.9 bars to 8.4 bars. Average dislocations range from 0.6 mm to 19 mm. Stress drop, dislocation, and seismic moment have a clear dependence on radiated seismic energy. Seismic energy of the mine tremors is between 1.4 x 1013 ergs and 2.2 x 1015 ergs. The energy magnitude relation log E = 11.8 + 1.5 M given by Gutenberg and Richter is in good agreement with the data. Source dimensions are in the same range as those found for mine tremors in Utah, South Africa, and Poland.
ARK: https://n2t.net/ark:/88439/y073859
Permalink: https://geophysicsjournal.com/article/290