Galileo single frequency ionospheric correction: performances in terms of position

For GPS single frequency users, the ionospheric contribution to the error budget is estimated by the well-known Klobuchar algorithm. For Galileo, it will be mitigated by a global algorithm based on the NeQuick model. This algorithm relies on the adaptation of the model to slant Total Electron Content (sTEC) measurements. Although the performance specifications of these algorithms are expressed in terms of delay and TEC, the users might be more interested in their impact on positioning. Therefore, we assessed the ability of the algorithms to improve the positioning accuracy using globally distributed permanent stations for the year 2002 marked by a high level of solar activity. We present uncorrected and corrected performances, interpret these and identify potential causes for Galileo correction discrepancies. We show vertical errors dropping by 56–64 % due to the analyzed ionospheric corrections, but horizontal errors decreasing by 27 % at most. By means of a fictitious symmetric satellite distribution, we highlight the role of TEC gradients in residual errors. We describe mechanisms permitted by the Galileo correction, which combine sTEC adaptation and topside mismodeling, and limit the horizontal accuracy. Hence, we support further investigation of potential alternative ionospheric corrections. We also provide an interesting insight into the ionospheric effects possibly experienced during the next solar maximum coinciding with Galileo Initial Operation Capability.     

The gold content of volcanogenic massive sulfide deposits

Volcanogenic massive sulfide deposits contain variable amounts of gold, both in terms of average grade and total gold content, with some VMS deposits hosting world-class gold mines with more than 100?t Au. Previous studies have identified gold-rich VMS as having an average gold grade, expressed in g/t, exceeding the total abundance of base metals, expressed in wt.%. However, statistically meaningful criteria for the identification of truly anomalous deposits have not been established. This paper presents a more extensive analysis of gold grades and tonnages of 513 VMS deposits worldwide, revealing a number of important features in the distribution of the data. A large proportion of deposits are characterized by a relatively low gold grade (<2?g/t), with a gradual decrease in frequency towards maximum gold grades, defining a log-normal distribution. In the analysis presented in this paper, the geometric mean and geometric standard deviation appear to be the simplest metric for identifying subclasses of VMS deposits based on gold grade, especially when comparing deposits within individual belts and districts. The geometric mean gold grade of 513 VMS deposits worldwide is 0.76?g/t; the geometric standard deviation is +2.70?g/t Au. In this analysis, deposits with more than 3.46?g/t Au (geometric mean plus one geometric standard deviation) are considered auriferous. The geometric mean gold content is 4.7?t Au, with a geometric standard deviation of +26.3?t Au. Deposits containing 31?t Au or more (geometric mean plus one geometric standard deviation) are also considered to be anomalous in terms of gold content, irrespective of the gold grade. Deposits with more than 3.46?g/t Au and 31?t Au are considered gold-rich VMS. A large proportion of the total gold hosted in VMS worldwide is found in a relatively small number of such deposits. The identification of these truly anomalous systems helps shed light on the geological parameters that control unusual enrichment of gold in VMS. At the district scale, the gold-rich deposits occupy a stratigraphic position and volcanic setting that commonly differs from other deposits of the district possibly due to a step change in the geodynamic and magmatic evolution of local volcanic complexes. The gold-rich VMS are commonly associated with transitional to calc-alkaline intermediate to felsic volcanic rocks, which may reflect a particularly fertile geodynamic setting and/or timing (e.g., early arc rifting or rifting front). At the deposit scale, uncommon alteration assemblages (e.g., advanced argillic, aluminous, strongly siliceous, or potassium feldspar alteration) and trace element signatures may be recognized (e.g., Au?CAg?CAs?CSb ± Bi?CHg?CTe), suggesting a direct magmatic input in some systems.     

L’analyse des surfaces enveloppes appliquée à l’étude morpho-structurale de l’est de la France

Résumén

Dans certaines régions où les sédiments récents (plio-quaternaires) sont rares ou inutilisables comme enregistreurs de déformations, seule l’analyse morpho-strcturale peut mettre en évidence des indices de néotectonique. Deux types d’objets morphologiques se révèlent particulièrement utiles : les surfaces (d’érosion et de comblement) et le réseau hydrographique. Leur prise en compte et leur analyse s’effectuent essentiellement à partir des cartes topographiques à différentes échelles (de 1/50 000 à 1/500 000).

* La démarche est la suivante: 1 – Des traitements appropriés sont développés, mettant en évidence des discontinuités et des anomalies au sein des surfaces enveloppes et du réseau hydrographiques;

2 – Ces discontinuités sont analysées puis interprétées afin de déterminer les causes de leur existence dans le cadre d’un modèle d’évolution du relief qui tient compte de facteurs internes (la lithologie, la structure), et de facteurs externes (le climat, la végétation et les actions humaines);

3 – Des travaux de terrain confirment et précisent les caractéristiques de ces facteurs (nature et quantification de la déformation : âge. amplitude, sens des mouvements).

* Les principaux résultats de cette démarche appliquée à l’Kst du Bassin parisien sont les suivants: 1 – Les cuestas orientales du Bassin parisien apparaissent structurées par des accidents NK-SW (prolongement du fossé de Sarre Nahe) et NNW-SSK ( Argonne-Bar-le-Due);

2 – Les discontinuités morpho-structurales sont localisées et caractérisées par des mouvements récents.

     

Tilting Styx and Nix but not Uranus with a Spin-Precession-Mean-motion resonance

A Hamiltonian model is constructed for the spin axis of a planet perturbed by a nearby planet with both planets in orbit about a star. We expand the planet–planet gravitational potential perturbation to first order in orbital inclinations and eccentricities, finding terms describing spin resonances involving the spin precession rate and the two planetary mean motions. Convergent planetary migration allows the spinning planet to be captured into spin resonance. With initial obliquity near zero, the spin resonance can lift the planet’s obliquity to near 90\(^\circ \) or 180\(^\circ \) depending upon whether the spin resonance is first or zeroth order in inclination. Past capture of Uranus into such a spin resonance could give an alternative non-collisional scenario accounting for Uranus’s high obliquity. However, we find that the time spent in spin resonance must be so long that this scenario cannot be responsible for Uranus’s high obliquity. Our model can be used to study spin resonance in satellite systems. Our Hamiltonian model explains how Styx and Nix can be tilted to high obliquity via outward migration of Charon, a phenomenon previously seen in numerical simulations.     

Petrological evidence for stepwise accretion of metamorphic soles during subduction infancy (Semail ophiolite,Oman and UAE)

Metamorphic soles are tectonic slices welded beneath most large‐scale ophiolites. These slivers of oceanic crust metamorphosed up to granulite facies conditions are interpreted as forming during the first million years of intraoceanic subduction following heat transfer from the incipient mantle wedge towards the top of the subducting plate. This study reappraises the formation of metamorphic soles through detailed field and petrological work on three key sections from the Semail ophiolite (Oman and United Arab Emirates). Based on thermobarometry and thermodynamic modelling, it is shown that metamorphic soles do not record a continuous temperature gradient, as expected from simple heating by the upper plate or by shear heating as proposed in previous studies. The upper, high‐T metamorphic sole is subdivided in at least two units, testifying to the stepwise formation, detachment and accretion of successive slices from the down‐going slab to the mylonitic base of the ophiolite. Estimated peak pressure–temperature conditions through the metamorphic sole, from top to bottom, are 850°C and 1 GPa, 725°C and 0.8 GPa and 530°C and 0.5 GPa. These estimates appear constant within each unit but differing between units by 100–200°C and ~0.2 GPa. Despite being separated by hundreds of kilometres below the Semail ophiolite and having contrasting locations with respect to the ridge axis position, metamorphic soles show no evidence for significant petrological variations along strike. These constraints allow us to refine the tectonic–petrological model for the genesis of metamorphic soles, formed via the stepwise stacking of several homogeneous slivers of oceanic crust and its sedimentary cover. Metamorphic soles result not so much from downward heat transfer (ironing effect) as from progressive metamorphism during strain localization and cooling of the plate interface. The successive thrusts originate from rheological contrasts between the sole, initially the top of the subducting slab, and the peridotite above as the plate interface progressively cools. These findings have implications for the thickness, the scale and the coupling state at the plate interface during the early history of subduction/obduction systems.     

Structure of the low permeable naturally fractured geothermal reservoir at Soultz

The permeability of the granite geothermal reservoir of Soultz is primarily related to major fracture zones, which, in turn, are connected to dense networks of small-scale fractures. The small-scale fractures are nearly vertical and the major direction is about N0°E. This direction differs from that of the Rhine Graben, which is about N20°E to N45°E in northern Alsace. A total of 39 fracture zones, with a general strike of N160°E, have been identified in six wells between 1400 and 5000 m depth. These fracture zones are spatially concentrated in three clusters. The upper cluster at 1800–2000 m True Vertical Depth (TVD) is highly permeable. At 3000–3400 m TVD, the intermediate cluster in composed of a dense network developed in an altered matrix and forms the upper reservoir. In the lower part of the wells, the deeper cluster appears as a fractured reservoir developed within a low permeable matrix. Fracture zones represent a key element to take into account for predicting the geothermal reservoir life time submitted to various thermo-hydromechanical and chemical processes generated by hydraulic or chemical stimulations and by hydraulic circulation tests related to long-term exploitation.     

Ice Sheet And Satellite Altimetry

Since 1991, the altimeters of the ERS European Satellites allow the observation of 80% of the Antarctica ice sheet and the whole Greenland ice sheet: They thus offer for the first time a unique vision of polar ice caps. Indeed, surface topography is an essential data thanks to its capacity to highlight the physical processes which control the surface shape, or to test models. Moreover, the altimeter is also a radar which makes it possible to estimate the snow surface or subsurface characteristics, such as surface roughness induced by the strong katabatic wind or ice grain size. The polar ice caps may not be in a stationary state, they continue to respond to the climatic warming of the beginning of the Holocene, that is 18000 years ago, and possibly start to react to present climatic warming: the altimeter offers the unique means of estimating the variations of volume and thus the contribution of polar ice caps to present sea level change.     

Water table variations in Atacama Desert alluvial fans: discussion of “Evidence of short-term groundwater recharge signal propagation from the Andes to the central Atacama Desert: a singular spectrum analysis approach”*

ABSTRACT

Using a statistical approach, Scheihing attempts to demonstrate the direct influence of recharge events in the Precordillera and the Andean Piedmont on water table variations in downstream alluvial fans. The author “unquestionably” discards the existence of ephemeral recharge processes in alluvial fans although they are frequently impacted by major floods. However, the author does not consider previous publications about recharge processes in arid alluvial fans, the hydraulic setting of the Pampa del Tamarugal Aquifer (PTA) and anthropogenic pressure changes in the PTA. Because the sustainable management of groundwater resources in drylands depends on a good understanding of hydrogeological processes, we propose a thorough reinterpretation of the short- and long-term PTA water table variations addressed by the author. In this comment, we illustrate the limits of the sole use of a statistical approach for characterizing both recharge processes and factors controlling the water table variations in the Atacama Desert.     

Monitoring coastal evolution and associated littoral hazards of French Guiana shoreline with radar images

This paper aims at illustrating different case examples of monitoring active coastal evolutions using remote sensing synthetic aperture radar images (ERS 1–2 and Envisat) in humid tropical areas. Effectively, the radar satellite images may be acquired under most climate conditions, especially in cloudy tropical areas. As case examples, we studied herein French Guiana shoreline and its fast evolution under the combined influences of sea currents, sediments and swells. We focused on two aspects of French Guiana littoral evolution: (1) sedimentation and erosion processes linked to mud banks displacements around Kourou City, and (2) around Iracoubo village. Lastly, we compared this short-term sedimentation balance with long-term data showing the discrepancies of short- and long-term littoral evolutions on whole French Guiana. To conclude, this work demonstrates the importance of SAR imagery to provide high-quality and high-frequency update geographic information for coastal management and littoral hazards especially in such tropical humid and cloudy areas. To cite this article: H. Trebossen et al., C. R. Geoscience 337 (2005).