Diffraction of anti-plane SH waves by a semi-circular cylindrical hill with an inside concentric semi-circular tunnel

A closed-form analytic solution of two-dimensional scattering and diffraction of plane SH waves by a semicylindrical hill with a semi-cylindrical concentric tunnel inside an elastic half-space is presented using the cylindrical wave functions expansion method. The solution is reduced to solving a set of infinite linear algebraic equations. Fourier expansion theorem with the form of complex exponential function and cosine function is used. Numerical solutions are obtained by truncation of the infinite equations. The accuracy of the presented numerical results is carefully verified.     

Numerical study of tides in Ontario Lacus,a hydrocarbon lake on the surface of the Saturnian moon Titan

In the context of the emergence of extra-terrestrial oceanography, we adapted an existing oceanographic model, SLIM (www.climate.be/slim), to the conditions of Titan, a moon of Saturn. The tidal response of the largest southern lake at Titan’s surface, namely Ontario Lacus, is simulated. SLIM solves the 2D, depth-averaged shallow water equations on an unstructured mesh using the discontinuous Galerkin finite element method, which allows for high spatial resolution wherever needed. The impact of the wind forcing, the bathymetry, and the bottom friction is also discussed. The predicted maximum tidal range is about 0.56 m in the southern part of the lake, which is more than twice as large as the previous estimates (see Tokano, Ocean Dyn 60:(4) 803–817 10.1007/s10236-010-0285-3 (Tokano 2010)). The patterns and magnitude of the current are also markedly different from those of previous studies: the tidal motion is not aligned with the major axis of the lake and the speed is larger nearshore. Indeed, the main tidal component rotates clockwise in an exact period of one Titan day and the tidal currents can reach 0.046 ms ^?1 close to the shores depending on the geometry and the bathymetry. Except for these specific nearshore regions, the current speed is less than 0.02 ms ^?1. Circular patterns can be observed offshore, their rotational direction and size varying along the day.     

Multiscale mesh generation on the sphere

A method for generating computational meshes for applications in ocean modeling is presented. The method uses a standard engineering approach for describing the geometry of the domain that requires meshing. The underlying sphere is parametrized using stereographic coordinates. Then, coastlines are described with cubic splines drawn in the stereographic parametric space. The mesh generation algorithm builds the mesh in the parametric plane using available techniques. The method enables to import coastlines from different data sets and, consequently, to build meshes of domains with highly variable length scales. The results include meshes together with numerical simulations of various kinds.     

Stereotomography assisted by migration of attributes

Depth velocity model building remains a difficult step within the seismic depth imaging sequence. Stereotomography provides an efficient solution to this problem but was limited until now to a picking of seismic data in the prestack time un-migrated domain. We propose here a method for stereotomographic data picking in the depth migrated domain. Picking in the depth migrated domain exhibits the advantage of a better signal-to-noise ratio and of a more regular distribution of picked events in the model, leading to a better constrained tomographic inverse problem. Moreover, any improvement on the velocity model will improve the migrated results, again leading to improved picking. Our strategy for obtaining a stereotomographic dataset from a prestack depth migration is based on migration of attributes (and not on a kinematic demigration approach!). For any locally coherent event in the migrated image, migration of attributes allows one to compute ray parameter attributes corresponding to the specular reflection angle and dip. For application to stereotomography, the necessary attributes are the source/receiver locations, the traveltime and the data slopes. For the data slope, when the migration velocity model is erroneous, some additional corrections have to be applied to the result of migration of the attributes. Applying these corrections, our picking method is theoretically valid whatever the quality of the migration velocity model. We first present the theoretical aspects of the method and then validate it on 2D synthetic and real seismic reflection data sets.     

Earthquakes produce carbon dioxide in crustal faults

Fourier transform infrared (FTIR) microanalysis of pseudotachylytes (i.e. friction-induced melts produced by seismic slip) from the Nojima fault (Japan) reveals that earthquakes almost instantaneously expel 99 wt.% of the wall rock CO₂ content. Carbon is exsolved because it is supersaturated in the friction melts. By extrapolation to a crustal-scale fault rupture, large events such as the M7.2 Kobe earthquake (1995) may yield a total production of 1.8 to 3.4 × 10³ tons CO₂ within a few seconds. This extraordinary release of CO₂ can cause a flash fluid pressure increase in the fault plane, and therefore enhance earthquake slip or trigger aftershocks; it may also explain the anomalous discharge of carbon monitored in nearby fault springs after large earthquakes. Because carbon saturation in silicate melts is pressure-dependent, FTIR can be used as a new tool to constrain the maximum depth of pseudotachylyte formation in exhumed faults.     

Contour current imprints and contourite drifts in the Bahamian archipelago

New data collected along the slopes of Little and Great Bahama Bank and the abyssal plain of the Bahama Escarpment provides new insights about contour current‐related erosive structures and associated deposits. The Bahamian slope shows abundant evidence of bottom current activity such as furrows, comet‐like structures, sediment waves and drifts. At a seismic scale, large erosion surfaces and main periods of drift growth resulted from current acceleration related to plate tectonic processes and progressive opening and closure of gateways and long‐term palaeoclimate evolution. At present‐day, erosion features and contourite drifts are either related to relatively shallow currents (<1000 m water depth) or to deep currents (>2500 m water depth). It appears that the carbonate nature of the drifts does not impact the drift morphology at the resolution addressed in the present study. Classical drift morphologies defined in siliciclastic environments are found, such as mounded, plastered and separated drifts. In core, contourite sequences show a bi‐gradational trend that resembles classical contourite sequences in siliciclastic deposits showing a direct relationship with a change in current velocity at the sea floor. However, in a carbonate system the peak in grain size is associated with increased winnowing rather than increased sediment supply as in siliciclastic environments. In addition, the carbonate contourite sequence is usually thinner than in siliciclastics because of lower sediment supply rates. Little Bahama Bank and Great Bahama Bank contourites contain open‐ocean input and slope‐derived debris from glacial episodes. Inner platform, platform edge and open ocean pelagic input characterize the classical periplatform ooze during interglacials. In all studied examples, the drift composition depends on the sea floor topography surrounding the drift location and the type of sediment supply. Carbonate particles are derived from either the slope or the platform in slope and toe of slope drifts, very deep contourites have distant siliciclastic sources of sediment supply. The recent discovery of the importance of a large downslope gravitary system along Bahamian slopes suggests frequent interactions between downslope and along‐slope (contour currents) processes. The interlayering of mass flow deposits and contourites at a seismic scale or the presence of surface structures associated with both contour currents and mass flow processes shows that both processes act at the same location. Finally, contour currents have an important impact on the repartition of deep‐water coral mounds. Currents can actively interact with mounds as a nutrient and oxygen supplier or have a passive interaction, with mounds solely being obstacles orienting erosion and deposition.     

Formation of magnesium‐smectite during lacustrine carbonates early diagenesis: Study case of the volcanic crater lake Dziani Dzaha (Mayotte – Indian Ocean)

The volcanic crater lake of Dziani Dzaha in Mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of Mg‐phyllosilicates associated with carbonate rocks. The Dziani Dzaha is characterized by intense primary productivity, volcanic gases bubbling in three locations and a volcanic catchment of phonolitic/alkaline composition. The lake water has an alkalinity of ca 0·2 mol l^?1 and pH values of ca 9·3. Cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon–hydrogen–nitrogen elemental analyzer, X‐ray fluorescence spectrometry and X‐ray powder diffraction. In surface sediments, the content of total organic carbon reaches up to 20 weight %. The mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment. Below 30 cm depth, X‐ray diffraction analyses of the <2 μm clay fraction indicate the presence of a saponite‐like mineral, a Mg‐rich smectite. The saponite‐like mineral accumulates at depth to reach up to ca 30 weight %, concurrent with a decrease of the contents of hydromagnesite and organic matter. Thermodynamic considerations and mineral assemblages suggest that the evolution of the sediment composition resulted from early diagenetic reactions. The formation of the saponite‐like mineral instead of Al‐free Mg‐silicates resulted from high aluminum availability, which is favoured in restricted lacustrine environments hosted in alkaline volcanic terrains commonly emplaced during early stages of continental rifting. Supersaturation of the lake water relative to saponite is especially due to high pH values, themselves derived from high primary productivity. This suggests that a genetic link may exist between saponite and the development of organic‐rich carbonate rocks, which may be fuelled by the input of CO₂‐rich volcanic gases. This provides novel insights into the composition and formation of saponite‐rich deposits under a specific geodynamic context such as the Cretaceous South Atlantic carbonate reservoirs.     

Long-term tendencies in the MLT prevailing winds and tides over Antarctica as observed by radars at Molodezhnaya,Mawson and Davis

Long-term tendencies in horizontal neutral wind parameters in the southern polar mesosphere/lower thermosphere are presented. The wind data analyzed were obtained from meteor and MF radars situated at Molodezhnaya (45.9°E, 67.7°S), Mawson (62.9°E, 67.6°S) and Davis (78.0°E, 68.6°S). The composite dataset covers years from 1970 to 2006. A Bayesian approach in the form proposed by Wang and Zivot [2000. A Bayesian time series model of multiple structural changes in level, trend and variance. Journal of Business and Economic Statistics 8, 374–386] is used for the trend assessment. This approach allows structural breaks to be identified in the trend parameters (slope, mean or variance of residuals) or demonstrates their absence. The results of our analysis have shown persistence through such structural breaks in trends of the winter and summer prevailing winds and in meridional tidal components. It is demonstrated that the wind parameters exhibit different stable states with transitions between the states. Correlations between the southern polar MLT wind and indices of atmospheric variability (Northern annular mode, Southern annular mode, Multivariate El-Niño/Southern Oscillation Index) were then considered. The results show that statistically significant correlations exist during some periods of observations that do not exist during others.     

Geospace imaging using Thomson scattering

We evaluate the potential of imaging for the first time, the near-earth space plasma environment seamlessly from the ionosphere through the magnetosphere by remotely sensing Thomson scattering of solar visible light by geospace electrons. Using state of the art first principles models of the magnetosphere/ionosphere system, we show that the column emission rates are weak, generally less than 10 Rayleighs, but detectable with currently available instrument technology recently deployed for heliospheric imaging. We demonstrate that distinct features such as the bow shock, magnetosheath and magnetopause are detectable in synthetic images simulated using modified solar coronagraphs and white light imagers, providing that the large background signals are properly quantified. The availability of global geospace images of the electron concentration will enable major advances in our understanding of how Earth's near-space environment responds as a coupled system to changing solar forcings. Such images are expected to play a central role in space weather assessment and forecasting, from which significant capabilities will accrue, much as the imaging of the Earth's surface and lower atmosphere has advanced understanding and forecasting of tropospheric weather.