Long period variations of the geomagnetic field and their spatial distribution in Europe

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Simplified theory of series for the gravitational and geomagnetic potential

Studia Geophysica et Geodaetica - Оnuсaн в nрuнцunе nросmоŭ nерехо¶rt; оm...     

Methods of computing the partial derivatives of dispersion curves

Summary Three methods of computing the partial derivatives of dispersion curves are briefly described and compared: numerical differentiation, variational methods and an implicit function method.     

Forces at the Kerr singularity

In a classical-like language introduced in a previous paper, the fact that the ring singularity of the Kerr spacetime cannot be left/reached by timelike geodesic which is not bound to the equatorial plane is interpreted as a result of the interplay of the gravitational and – mainly – the centrifugal forces.     

Effect of small potassium-rich dykes on regional gamma-spectrometry image of a potassium-poor volcanic complex: A case from the Doupovské hory Volcanic Complex, NW Czech Republic

Basaltic rocks with low K, U and Th contents dominate the entire Volcanic Complex of the Doupovské hory Mts. Significant potassium anomaly exceeding 1.5 atomic wt.% of potassium over an area of 4 × 8 km and 2 atomic wt.% of potassium over an area of 2 × 6 km was defined by airborne gamma-ray spectrometry above the central part of the Doupovské hory Volcanic Complex. The following detailed field study, supported by field and laboratory gamma-spectrometry measurements and geochemical analyses of rock samples, resulted in discovery of a swarm of potassium-rich trachytic dykes. The existence of such highly-differentiated rocks in the volcanic complex was unknown till present. These dykes are commonly less than 1 m wide, but their potassium content varies between 4 and 8 atomic wt.%. Owing to this high-K composition and relative abundance of dykes, the dyke rocks significantly modify the regional pattern of gamma-spectrometry data. The potassium anomaly cannot be explained by the presence of Flurbühl intrusive body dominated by ijolites and essexites, as all these rocks are poor in K, with potassium typically not exceeding 1.5 wt.%. On the other hand, much more extensive intermediate trachybasaltic lavas with K content varying within the range 1.8–3 wt.% cause only minor or undetectable anomalies.     

Deep electrical structure under central Europe

Summary The model of the electrical conductivity distribution within the Bohemian Massif to mid-upper mantle depths was derived from magnetotelluric and magnetic continuum long-period data of the Budkov Observatory (Geophysical Institute, Acad. Sci. Czech Republic., Prague) and from the ISL (Induction Scale Lengths) data of the Prhonice Observatory. The ranges of apparent resistivities in the Bohemian Massif (BM) are compared with those obtained at relevant central European observatories. The conductivity profile is estimated from the resistivity/depth graph corresponding to substitute perfect conductor inversion.Dedicated to the Memory of Professor Karel P     

Subdivision of the Lochkovian Stage based on conodont faunas from the stratotype area (Prague Synform,Czech Republic)

Relatively rich conodont faunas from sections in the Prague Synform (Barrandian area, Czech Republic) include a number of indexes and other important guide conodonts that can be correlated with other regions, especially with Nevada and the Spanish Central Pyrenees. The collation and detailed correlation of conodont data from the Lochkovian in two parallel sections in the Požáry quarries, together with biostratigraphic control of additional data from several (incomplete) sections with changing facies development, is the basis for a new detailed regional biozonal scale for the Lochkovian in the Prague Synform. The new subdivision follows, with modification, the global threefold conodont subdivision of the Lochkovian. Data from the Prague Synform enable further detailed subdivision of the lower, middle and upper Lochkovian into small‐scale units. The conodont distribution shows a large proportional discrepancy between the late Lochkovian elsewhere; the conodont record in the latest Lochkovian in the Prague Synform area, which appears to be rather restricted and requires further discussion. Copyright © 2012 John Wiley & Sons, Ltd.     

Timing of the Northern Prince Gustav Ice Stream retreat and the deglaciation of northern James Ross Island,Antarctic Peninsula during the last glacial–interglacial transition

The Northern Prince Gustav Ice Stream located in Prince Gustav Channel, drained the northeastern portion of the Antarctic Peninsula Ice Sheet during the last glacial maximum. Here we present a chronology of its retreat based on in situ produced cosmogenic ¹⁰Be from erratic boulders at Cape Lachman, northern James Ross Island. Schmidt hammer testing was adopted to assess the weathering state of erratic boulders in order to better interpret excess cosmogenic ¹⁰Be from cumulative periods of pre-exposure or earlier release from the glacier. The weighted mean exposure age of five boulders based on Schmidt hammer data is 12.9 ± 1.2 ka representing the beginning of the deglaciation of lower-lying areas (< 60 m a.s.l.) of the northern James Ross Island, when Northern Prince Gustav Ice Stream split from the remaining James Ross Island ice cover. This age represents the minimum age of the transition from grounded ice stream to floating ice shelf in the middle continental shelf areas of the northern Prince Gustav Channel. The remaining ice cover located at higher elevations of northern James Ross Island retreated during the early Holocene due to gradual decay of terrestrial ice and increase of equilibrium line altitude. Schmidt hammer R-values are inversely correlated with ¹⁰Be exposure ages and could be used as a proxy for exposure history of individual granite boulders in this region and favour the hypothesis of earlier release of boulders with excessive ¹⁰Be concentrations from glacier directly at this site. These data provide evidences for an earlier deglaciation of northern James Ross Island when compared with other recently presented cosmogenic nuclide based deglaciation chronologies, but this timing coincides with rapid increase of atmospheric temperature in this marginal part of Antarctica.     

Earthquake location from P-arrival times only: problems and some solutions

Selected problems related to accurate hypocenter locations are discussed in the difficult case that only reliable P-wave readings are available. Near stations are usually only few, and often have a poor azimuthal coverage. As such, they are insufficient because the inversion is highly ill-posed, and the epicenter position strongly trades-off with depth. Thus more distant stations are also needed to obtain the correct epicenter. However, joint use of near and distant stations present another difficulty; it may yield a significantly incorrect depth estimate in case that the crustal model is not fully appropriate. In practice, the erroneous depth often remains unrecognized. An indication of the depth problem can be obtained by analyzing the travel-time residuals at individual stations. It is also useful to check fully independent depth estimates, for example those from the centroid-moment-tensor analysis. If the problematic crustal model is detected, and it is not easy to find a better one, the near- and distant station effects should be decoupled (a two-step location): the epicenter is calculated from all stations, kept fixed, and the source depth is grid-search beneath the epicenter by means of the near stations. The ideas are applied to the M_w 5.2 Efpalio (Western Greece) earthquake of January 18, 2010, and the following aftershock sequence.     

The field of a plane electromagnetic wave at the surface of two-layer anisotropic earth

Резюме На основе анализа электромагнитного поля плоской волны получены формулы (6) и (7) для коэффициентов отражения в случае двухслойной анизотропной Земли, слои которой характеризуются различающимися тензорами электрической проводимости. Коэффициенты отражения, рассчитанные для падающей волны с магнитным полем поляризованным в двух перпендикулярных направлениях, используются для определения главных и дополнительных импедансов в случаях анизотропного пласта на проводящем или непроводящем основаниях (формулы (11) и (12)) и для случая двух анизотропных слоев с совпадающими направлениями главных осей тензора электрической проводимости (формула (13)).

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