Effect of micro‐inhomogeneity on the effective stress coefficients and undrained bulk modulus of a poroelastic medium: a double spherical shell model

Although most rocks are complex multi‐mineralic aggregates, quantitative interpretation workflows usually ignore this complexity and employ Gassmann equation and effective stress laws that assume a micro‐homogeneous (mono‐mineralic) rock. Even though the Gassmann theory and effective stress concepts have been generalized to micro‐inhomogeneous rocks, they are seldom if at all used in practice because they require a greater number of parameters, which are difficult to measure or infer from data. Furthermore, the magnitude of the effect of micro‐heterogeneity on fluid substitution and on effective stress coefficients is poorly understood. In particular, it is an open question whether deviations of the experimentally measurements of the effective stress coefficients for drained and undrained elastic moduli from theoretical predictions can be explained by the effect of micro‐heterogeneity. In an attempt to bridge this gap, we consider an idealized model of a micro‐inhomogeneous medium: a Hashin assemblage of double spherical shells. Each shell consists of a spherical pore surrounded by two concentric spherical layers of two different isotropic minerals. By analyzing the exact solution of this problem, we show that the results are exactly consistent with the equations of Brown and Korringa (which represent an extension of Gassmann's equation to micro‐inhomogeneous media). We also show that the effective stress coefficients for bulk volume α, for porosity n_? and for drained and undrained moduli are quite sensitive to the degree of heterogeneity (contrast between the moduli of the two mineral components). For instance, while for micro‐homogeneous rocks the theory gives n_? = 1, for strongly micro‐inhomogenous rocks, n_? may span a range of values from –∞ to ∞ (depending on the contrast between moduli of inner and outer shells). Furthermore, the effective stress coefficient for pore volume (Biot–Willis coefficient) α can be smaller than the porosity ?. Further studies are required to understand the applicability of the results to realistic rock geometries.     

The ševĚtín astrobleme,southern Bohemia,Czechoslovakia

The geological, geomorphological, petrological, and geophysical data for the 46 km circular structure evtín (4905N, 1435E), southern Bohemia, are consistent with characteristic of moderately eroded structures that have originated by hypervelocity meteorite impact. The astrobleme, dated as late Cretaceous (probably Coniac-Santon, 85-75 m.y.), is dissected into radial fault segments, which exhibit a variable level of erosion. The classification of the evtín astrobleme as a probable impact structure is based on the geological and geophysical evidence of central »scar« about 8 km in diameter, shock metamorphism in the centre of the structure, and relics of silicified impact breccia including globular and suevite types with structural indications of molten material. Shatter coning and shock metamorphism are documented for several formations and lithologies ranging in age from the hercynian granite of the crystalline basement, through Stephanian-Permian, to the Upper Cretaceous-Lower Klikov Formation. The morphological expression of the crater rim is preserved in the less eroded segments. The central uplift (d=21 km, approximately 0.5 of the total diameter), still preserved in part as a positive morphological feature, is partly burried under the post-impact Upper Klikov Formation. The impact event provided the major tectonic, geomorphological, and paleogeographic resetting, which was known for some time in the evolution of the continental Upper Cretaceous of this area and expressed in the splitting of the Klikov Formation into two divisions.

---

Zusammenfassung Geologische, geomorphologische, petrologische und geophysikalische Daten, die über die 46 km im Durchmesser aufweisende Rundstruktur von evtín in Südböhmen (4905N, 1435E) ermittelt wurden, entsprechen der Charakteristik mittelmä\ig erodierten Strukturen, die infolge des Einschlags eines gro\en Meteoriten von Hypergeschwindigkeit entstanden sind. Das Astroblem wird als oberkretazisch (wahrscheinlich von Coniac- bis Santonalter, 85-75 Mill. Jahre) geologisch datiert. Es ist durch Brüche in Radialsegmente aufgegliedert, die verschieden intensiv erodiert sind. Geologische und geophysikalische Belege einer zentralen »Narbe« von 8 km im Durchmesser, eine Sto\wellenmetamorphose im Zentrum der Struktur sowie Relikte silizifierter Impaktbreccien, die globulare und suevitähnliche Typen mit Strukturanzeichen von aufgeschmolzenem Material umfassen, stellen Hauptangaben zur Klassifizierung des Astroblems von evtín als einer wahrscheinlichen Impaktstruktur dar. Die »shatter cones« und die Sto\wellenmetamorphose wurden in mehreren Formationen und verschiedenen Gesteinstypen von dem herzynischen Granit im kristallinen Untergrund, über die stefanischen und permischen Gesteine bis zu der oberkretazischen Klikov-Schichtenfolge (ihrem unteren Teil) ermittelt. Die Morphologie des Kraterrandes ist in weniger erodierten Segmenten durch die Erosion ausgeprägt. Die Zentralerhebung (Durchmesser von 21 km, d.h. ungefähr von einer Hälfte des Kratergesamtdurchmessers) ist morphologisch noch zum Teil erhalten, aber ihr östl. Teil ist von der oberen Abteilung der Klikov-Schichtenfolge überdeckt. In Zusammenhang mit dem Impaktereignis werden bedeutsame tektonische, geomorphologische und paläogeographische änderungen gestellt, die in der Entwicklung der kontmentalen Oberkreide dieses Gebietes bereits früher erkannt wurden und zur Aufgliederung der Klikov-Schichtenfolge in zwei Abteilungen führten.

---

Résumé La structure circulaire de evtin (4905N - 1435E, 46 km de diamètre), examinée à la lumière des données géologiques, géomorphologiques, pétrologiques et géophysiques, présente les caractères d'une structure provoquée par l'impact d'une météorite de très grande vitesse, et modérément érodée. Cet astroblème est daté du Crétacé supérieur, probablement du Coniacien-Santonien (environ 85-75 Ma). Il est subdivisé par des cassures radiales en segments qui présentent divers degrés d'érosion. L'assimilation de l'astroblème de evtin à une structure d'impact se base sur les arguments suivants: cicatrice centrale de 8 km de diamètre, métamorphisme de choc dans la partie centrale, restes de brèche d'impact silicifiée comportant des types globulaires et de suévite avec indices de matériaux fondus. Les »shatter cones« et le métamorphisme de choc s'observent dans plusieurs formations, de lithologies diverses, qui s'échelonnent entre le granite hercynien du socle cristallin, le Stéphano-permien, et le Crétacé supérieur (partie inférieure de la formation de Klikov). La morphologie du rempart du cratère est préservée dans les segments les moins érodés. La zone surélevée centrale (21 km de diamètre, soit la moitié du diamètre total) est partiellement préservée et cachée pour le reste sous la partie supérieure de la formation de Klikov. Le phénomène d'impact a été accompagné de modifications tectoniques, géomorphologiques et paléogéographiques marquantes. Ces modifications étaient connues antérieurement dans l'évolution du Crétacé supérieur de cette région et avaient justifié la subdivision en deux parties de la formation de Klikov.

---

, , , evtin 46 (4905 1435 ) , , . , ( - ; 85–75 ). , . , 8 , , , , , evetin . « », , ( ). . — 21 , . . , , . , , . ; .

     

A model of diffusion produced by a cellular surface flow

A model of diffusion induced by the joint action of random cells of two different sizes (granulation and supergranulation) is developed. The basic properties of the model are defined by the dimensionless ratio = (the life time)/(characteristic size/velocity) constructed for these two types of cells. An analytical expression for the diffusivity as a function of is derived. It is estimated that for the standard parameters used the contribution of granulation to the joint diffusion is small. A comparison between the model and numerical simulations of the kinematic diffusion of magnetic fields on the solar surface is made.     

The Arctic Coastal Dynamics Database: A New Classification Scheme and Statistics on Arctic Permafrost Coastlines

Arctic permafrost coasts are sensitive to changing climate. The lengthening open water season and the increasing open water area are likely to induce greater erosion and threaten community and industry infrastructure as well as dramatically change nutrient pathways in the near-shore zone. The shallow, mediterranean Arctic Ocean is likely to be strongly affected by changes in currently poorly observed arctic coastal dynamics. We present a geomorphological classification scheme for the arctic coast, with 101,447?km of coastline in 1,315 segments. The average rate of erosion for the arctic coast is 0.5?m? year^?1 with high local and regional variability. Highest rates are observed in the Laptev, East Siberian, and Beaufort Seas. Strong spatial variability in associated database bluff height, ground carbon and ice content, and coastline movement highlights the need to estimate the relative importance of shifting coastal fluxes to the Arctic Ocean at multiple spatial scales.     

Automatic Video System for Continues Monitoring of the Meteor Activity

In this paper we present current progress in development of new observational instruments for the double station video experiment. The Meteor Automatic Imager and Analyser (MAIA) system is based on digital monochrome camera JAI CM-040 and well proved image intensifier XX1332. Both the observations as well as the data processing will be fully automatic. We are expecting the recorded data of better quality and both spatial and time resolution in comparison with currently used analogue system. The main goal of the MAIA project is to monitor activity of the meteor showers and sporadic meteor each night for the period of at least 3 years. First version of the system was already assembled and has been intensively tested in the optical laboratory. Optical properties were measured and the result confirmed our expectations according to image quality and resolution. First night sky observation was already carried out.     

Environmental risk assessment of some potentially toxic elements in El-Tabbin region (Cairo, Egypt)

As much as 24 soil samples and 6 stream sediments from the River Nile were studied in El-Tabbin region (Great Cairo, Egypt). Twelve chemicals, potentially toxic elements posing potential environmental risk, were the object of concern in this study. Mean contents of analysed elements (in mg kg^?1) in soils and the River Nile stream sediments were the following: As_s 3.6/As_ss 1.5, Cd_s 0.33/Cd_ss 0.12, Cr_s 87.7/Cr_ss 141.5, Cu_s 40.3/Cu_ss 43.8, Hg_s 0.03/Hg_ss 0.13, Pb_s 33.3/Pb_ss 20.2, Zn_s 150/Zn_ss 109, Se_s 0.24/Se_ss 0.05, Ni_s 37.2/Ni_ss 48, Sb_s 1.25/Sb_ss 1, Ba_s 892/Ba_ss 431, V_s 103.3/V_ss 167.8. Furthermore, geochemical background values were derived for soil and stream sediment samples. The values are as follows (in mg kg^?1): As_s 1.33/As_ss 1, Cd_s 0.48/Cd_ss 0.05, Cr_s 54.7/Cr_ss 106.5, Cu_s 23.8/Cu_ss 23, Hg_s 0.025/Hg_ss 0.095, Pb_s 15.3/Pb_ss 13.5, Zn_s 70/Zn_ss 55, Se_s 0.13/Se_ss 0.05, Ni_s 19.5/Ni_ss 32.5, Sb_s 1/Sb_ss 1, Ba_s 266/Ba_ss 275, V_s 50.7/V_ss 119. More than two-thirds of soil and sediment samples exceeded established (based on literature data) risk limit values for non-polluted environment. Based on environmental risk assessment for potentially toxic elements in soils and sediments in more than 45% of total area disturbed environment (I _ER = 1–3) was documented and more than 13% of territory was characterised with highly disturbed environment (I _ER > 3).     

Improved provenance tracing of Asian dust sources using rare earth elements and selected trace elements for palaeomonsoon studies on the eastern Tibetan Plateau

The Asian Monsoon forms an important part of the earth’s climate system, yet our understanding of the past interactions between its different sub-systems, the East Asian and Indian monsoons, and between monsoonal winds and other prevailing wind currents such as the Westerly jet, is limited, particularly in central Asia. This in turn affects our ability to develop climate models capable of accurately predicting future changes in atmospheric circulation patterns and monsoon intensities in Asia. Provenance studies of mineral dust deposited in terrestrial settings such as peat bogs can address this problem directly, by offering the possibility to examine past deposition rates and wind direction, and hence reconstruct past atmospheric circulation patterns. However, such studies are challenged by several issues, most importantly the identification of proxies that unambiguously distinguish between the different potential dust sources and that are independent of particle size. In addition, a single analytical method that is suitable for sample preparation of both dust source (i.e. desert sand, soil) and receptor (i.e. dust archive such as peat or soil profiles) material is desirable in order to minimize error propagation derived from the experimental and analytical work. Here, an improved geochemical framework of provenance tracers to study atmospheric circulation patterns and palaeomonsoon variability in central Asia is provided, by combining for the first time mineralogical as well as major and trace elemental (Sc, Y, Th and the rare earth elements) information on Chinese (central Chinese loess plateau, northern Qaidam basin and Taklamakan, Badain Juran and Tengger deserts), Indian (Thar desert) and Tibetan (eastern Qinghai-Tibetan Plateau) dust sources.Quartz, feldspars and clay minerals are the major constituents of all studied sources, with highly variable calcite contents reflected in the CaO concentrations. Chinese and Tibetan dust sources are enriched in middle REE relative to the upper continental crust and average shale but the Thar desert has a REE signature distinctly different from all other dust sources. There are significant differences in major, trace and REE compositions between the coarse and fine fractions of the surface sands, with the finest <4 μm fraction enriched in Al₂O₃, Fe₂O₃, MnO, MgO and K₂O and the <32 μm fractions in Sc, Y, Th and the REE relative to the coarse fractions. The <4 μm fraction best represents the bulk REE geochemistry of the samples. The provenance tracers Y/∑REE, La/Er, La/Gd, Gd/Er, La/Yb, Y/Tb, Y/La, Y/Nd and to a certain extent the europium anomaly Eu/Eu^∗ (all REE normalized to post-Archean Australian shale, PAAS) are particle size-independent tracers, of which combinations of Y/∑REE, La/Yb, Y/Tb, Y/La and Eu/Eu^∗ can be used to distinguish the Thar desert, the Chinese deserts, the Chinese loess plateau and the Tibetan soils. Their independence upon grain size means that these tracers can be applied to the long-range provenance tracing of Asian dust even when only bulk samples are available in the source region. Combinations of La/Th, Y/Tb, Y/∑REE, Sc/La and Y/Er distinguish the Tibetan soils from the Chinese loess plateau and the Chinese deserts. La/Th and notably Th/∑REE isolate the signature of the Badain Juran desert and the combination of Sc/La and Y/Er that of the Taklamakan desert. The similarity in all trace and REE-based provenance tracers between the northern Qaidam basin and Tengger desert suggests that these two deposits may have a common aeolian source.