Correlative and comparative characterization of main ion concentrations in laterite groundwater in semi-arid northern Burkina Faso

About 24 samples from hand-dug wells and boreholes were used to characterize concentrations of the main inorganic ions in a laterite environment under semi-arid climatic conditions in Tikaré, northern Burkina Faso. It was found that the most represented groundwater anion in groundwater was HCO₃ ⁻ with average levels of 49.1 mg/L in the dry season and 33.5 mg/L in the rainy season. The most represented cation was Ca²⁺ with mean concentrations of 13.7 and 9.5 mg/L, respectively. The main processes, which influence the concentrations of these ions, are evaporation (dry season), local enrichment of recharge water in some elements, ion exchange and fixation by clay minerals (in case of K⁺). The best correlations were found between Ca²⁺ and Mg²⁺ (r = 0.95), Cl⁻ and Na⁺ (r = 0.95), HCO₃ ⁻ and Mg²⁺ (r = 0.89), HCO₃ ⁻ and Ca²⁺ (r = 0.89), and between HCO₃ ⁻ and Na⁺ (r = 0.80). In general, the quality of the groundwater from the different wells sampled for this study was good enough to serve as drinking water. However, there were situations where the quality of water was polluted because of anthropogenic contaminants (mainly NO₃ ⁻, K⁺, Cl⁻) from septic tanks and manure pits located in the vicinity of some sampled wells. In addition, application of fertilizers also represents a potential anthropogenic contamination source with regard to SO₄ ²⁻, Ca²⁺, K⁺, Na⁺, and Mg²⁺. Considering the high concentrations of SO₄ ²⁻, Mg²⁺, Na⁺ and Ca²⁺ found in one borehole, the deeper, fractured aquifers were also likely to be enriched in these elements. In contrast, the shallow aquifers are likely to be contaminated with Cl⁻, NO₃ ⁻ and K⁺. Cl⁻ and K⁺ seem to be locally present in recharge water as shown by their relative higher mean concentrations in the rainy season samples.     

Unusual sources of fossil micrometeorites deduced from relict chromite in the small size fraction in ~467 Ma old limestone

Extraterrestrial chrome spinel and chromite extracted from the sedimentary rock record are relicts from coarse micrometeorites and rarely meteorites. They are studied to reconstruct the paleoflux of meteorites to the Earth and the collisional history of the asteroid belt. Minor element concentrations of Ti and V, and oxygen isotopic compositions of these relict minerals were used to classify the meteorite type they stem from, and thus to determine the relative meteorite group abundances through time. While coarse sediment-dispersed extraterrestrial chrome-spinel (SEC) grains from ordinary chondrites dominate through the studied time windows in the Phanerozoic, there are exceptions: We have shown that ~467 Ma ago, 1 Ma before the breakup of the L chondrite parent body (LCPB), more than half of the largest (>63 μm diameter) grains were achondritic and originated from differentiated asteroids in contrast to ordinary chondrites which dominated the meteorite flux throughout most of the past 500 Ma. Here, we present a new data set of oxygen isotopic compositions and elemental compositions of 136 grains of a smaller size fraction (32–63 μm) in ~467 Ma old pre-LCPB limestone from the Lynna River section in western Russia, that was previously studied by elemental analysis. Our study constitutes the most comprehensive oxygen isotopic data set of sediment-dispersed extraterrestrial chrome spinel to date. We also introduce a Raman spectroscopy-based method to identify SEC grains and distinguish them from terrestrial chrome spinel with ~97% reliability. We calibrated the Raman method with the established approach using titanium and vanadium concentrations and oxygen isotopic compositions. We find that ordinary chondrites are approximately three times more abundant in the 32–63 μm fraction than achondrites. While abundances of achondrites compared to ordinary chondrites are lower in the 32–63 μm size fraction than in the >63 μm one, achondrites are approximately three times more abundant in the 32–62 μm fraction than they are in the present flux. We find that the sources of SEC grains vary for different grain sizes, mainly as a result of parent body thermal metamorphism. We conclude that the meteorite flux composition ~467 Ma ago ~1 Ma before the breakup of the LCPB was fundamentally different from today and from other time windows studied in the Phanerozoic, but that in contrast to the large size fraction ordinary chondrites dominated the flux in the small size fraction. The high abundance of ordinary chondrites in the studied samples is consistent with the findings based on coarse extraterrestrial chrome-spinel from other time windows.     

An active wave generating–absorbing boundary condition for VOF type numerical model

The objective of the present work is to discuss the implementation of an active wave generating–absorbing boundary condition for a numerical model based on the Volume Of Fluid (VOF) method for tracking free surfaces. First an overview of the development of VOF type models with special emphasis in the field of coastal engineering is given. A new type of numerical boundary condition for combined wave generation and absorption in the numerical model VOFbreak² is presented. The numerical boundary condition is based on an active wave absorption system that was first developed in the context of physical wave flume experiments, using a wave paddle. The method applies to regular and irregular waves. Velocities are measured at one location inside the computational domain. The reflected wave train is separated from the incident wave field in front of a structure by means of digital filtering and subsequent superposition of the measured velocity signals. The incident wave signal is corrected, so that the reflected wave is effectively absorbed at the boundary. The digital filters are derived theoretically and their practical design is discussed. The practical use of this numerical boundary condition is compared to the use of the absorption system in a physical wave flume. The effectiveness of the active wave generating–absorbing boundary condition finally is proved using analytical tests and numerical simulations with VOFbreak².     

The Integration of Scene Understanding within a Geographic Information System: A Prototype Approach for Agricultural Applications

This paper describes progress towards engineering a combined GIS and image understanding system, designed to analyse remotely-sensed imagery in a task-oriented manner, with the task and contextual parameters being supplied by the GIS. The overall aim is to evaluate some of the more recent developments in the field of computer vision and to show how they may be integrated (along with more traditional methods of image interpretation) to improve system performance in terms of object recognition and extraction. Three aspects are reported: (1) Development of a formal notation as the basis of describing the process of transforming data from low level image representations to high level object representations; (2) Design of collaborating low and high level image processes used in this abstraction process and their control via Process Networks encapsulated into frames ; and (3) Use of Dempster-Shafer Decision Theory as the basis for combining different types of evidence to improve the recognition of objects in the scene. Some sample results are presented to illustrate the issues raised and mechanisms for the co-operative processing of data are suggested.     

Shock history of the fossil ungrouped achondrite Österplana 065: Raman spectroscopy and TEM of relict chrome-spinel grains

Chrome-spinel grains from the fossil ungrouped achondrite Österplana 065 (Öst 065) recovered from Middle Ordovician limestone in Sweden were studied using Raman spectroscopy and TEM. All the studied chrome-spinel grains have a high density of planar fractures and planar features, not seen in chromites from the other L chondritic Ordovician fossil meteorites. Raman spectra of the host chrome-spinel grain and its planar features are similar and no signatures of high-pressure phases of chromite were found. The planar features occur along planar fractures, are enriched in ZnO, and are most probably produced due to enhanced leaching during terrestrial weathering in the marine sediment. Dislocation densities within two FIB sections prepared from two chrome-spinel grains from Öst 065 are similar to the dislocation densities found within chromite grains from the matrix of Tenham L6 chondrite. Using this observation and taking into account the presence of significant fracturing in all the grains, we conclude that the Öst 065 chrome-spinel grains were subjected to moderate to very strong shock corresponding to shock stages of S4–S6. This makes Öst 065 fossil achondrite the highest shocked fossil meteorite studied so far. This is consistent with the hypothesis that Öst 065 is a piece of the impactor that led to the L chondrite parent body breakup.     

Perspectives of a visible instrument on the VLTI

In this paper we present the most promising science cases for a new generation visible instrument on the VLTI and the conceptual idea for the instrumental configuration. We also present a statistical study of the potential targets that may be accessible for the different classes of objects and for the required spectral resolutions.     

Modelling of drainage dynamics influence on sediment routing system in a fold‐and‐thrust belt

Drainage networks link erosional landscapes and sedimentary basins in a source‐to‐sink system, controlling the spatial and temporal distribution of sediment flux at the outlets. Variations of accumulation rates in a sedimentary basin have been classically interpreted as changes in erosion rates driven by tectonics and/or climate. We studied the interactions between deformation, rainfall rate and the intrinsic dynamics of drainage basins in an experimental fold‐and‐thrust belt subjected to erosion and sedimentation under constant rainfall and shortening rates. The emergence of thrust sheets at the front of a prism may divert antecedent transverse channels (perpendicular to the structural grain) leading to the formation of longitudinal reaches, later uplifted and incorporated in the prism by the ongoing deformation. In the experiments, transverse incisions appear in the external slopes of the emerging thrust sheets. Headward erosion in these transverse channels results in divide migration and capture of the uplifted longitudinal channels located in the inner parts of the prism, leading to drainage network reorganization and modification of the sediment routing system. We show that the rate of drainage reorganization increases with the rainfall rate. It also increases in a nonlinear way with the rate of uplift. We explain this behaviour by an exponent > 1 on the slope variable in the framework of the stream power erosion model. Our results confirm the view that early longitudinal‐dominated networks are progressively replaced by transverse‐dominated rivers during mountain building. We show that drainage network dynamics modulate the distribution of sedimentary fluxes at the outlets of experimental wedges. We propose that under constant shortening and rainfall rates the drainage network reorganization can also modulate the composition and the spatial distribution of clastic fluxes in foreland basins.     

Numerical implementation and sensitivity analysis of a wave energy converter in a time-dependent mild-slope equation model

Several Wave Energy Converters (abbreviated as WECs) have intensively been studied and developed during the last decade and currently small farms of WECs are getting installed. WECs in a farm are partly absorbing, partly redistributing the incident wave power. Consequently, the power absorption of each individual WEC in a farm is affected by its neighbouring WECs. The knowledge of the wave climate around the WEC is needed to predict its performance in the farm. In this paper a technique is developed to implement a single and multiple WECs based on the overtopping principle in a time-dependent mild-slope equation model. So far, the mild-slope equations have been widely used to study wave transformations around coastal and offshore structures, such as breakwaters, piles of windmills and offshore platforms. First the limitations of the WEC implementation are discussed through a sensitivity analysis. Next the developed approach is applied to study the wave height reduction behind a single WEC and a farm. The wake behind an isolated WEC is investigated for uni- and multidirectional waves; it is observed that an increase of the directional spread leads to a faster wave redistribution behind the WEC. Further the wake in the lee of multiple WECs is calculated for two different farm lay-outs, i.e. an aligned grid and a staggered grid, by adapting the performance of each WEC to its incident wave power. The evolved technique is a fast tool to find the optimal lay-out of WECs in a farm and to study the possible influence on surrounding activities in the sea.