Estimation of the Variability of Tailings Dams Properties in Order to Perform Probabilistic Assessment

Dams are often used for storing mine tailings. These structures present failure mechanisms that can lead to major risks for surrounding populations. In view to managing these risks, it is necessary to take a probabilistic approach in predicting their behaviour. The application of such approaches is limited by the difficulty of obtaining experimental data to estimate the variability of the parameters and conditioned by the relevance of the probability models chosen to represent this variability. This article proposes (1) a probabilistic modelling of the index properties of the mine tailings constituting these dams based on statistical analyses and (2) a method using dynamic penetration tests to estimate on site the mine tailings friction angle and its variability. This method, applied to chilean tailings dams proposes a single model for all tailings dams in order to associate a probability law to the effective friction angle (ϕ′). The procedure is illustrated on the probabilistic study of slope stability carried out at the global scale of a dam and also at the local scale of each of its constituent layers.     

Groundwater storage variability and annual recharge using well-hydrograph and GRACE satellite data

Most studies using GRACE (Gravity Recovery and Climate Experiment) data for examining water storage anomalies have rich hydrogeological databases. Here, GRACE data are analyzed for southern Mali, Africa, a region with sparse hydrogeological data. GRACE data (2002?C2008) did not overlap with observed groundwater-level data (1982?C2002). Terrestrial water storage from GRACE was corrected for soil moisture using the Global Land Data Assimilation System (GLDAS) model to obtain monthly groundwater storage anomalies and annual net recharge. Historical storage anomalies and net recharge were determined using the water-table fluctuation method for available observation wells. Average annual net recharge averaged 149.1?mm (or 16.4% of annual rainfall) and 149.7?mm (14.8%) from historical water level and GRACE data, respectively. Monthly storage anomaly lows and peaks were observed in May and September, respectively, but have a shift in peak to November using the corrected GRACE data, suggesting that the GLDAS model may poorly predict the timing of soil-water storage in this region. Notwithstanding problems with the GLDAS model, the soil moisture-corrected GRACE data accurately predict the relative timing and magnitude of groundwater-storage changes, suggesting that GRACE data are valuable for identifying long-term regional changes in groundwater storage in areas with sparse hydrogeological data.     

Structure of young East Pacific Rise lithosphere from ambient noise correlation analysis of fundamental- and higher-mode Scholte-Rayleigh waves

Inter-station Green's functions estimated from ambient noise studies have been widely used to investigate crustal structure. However, most studies are restricted to continental areas and use fundamental-mode surface waves only. In this study, we recover inter-station surface (Scholte-Rayleigh) wave empirical Green's function (EGFs) of both the fundamental- and the first-higher mode using one year of continuous seismic noise records on the vertical component from 28 ocean bottom seismographs deployed in the Quebrada/Discovery/Gofar transform faults region on the East Pacific Rise. The average phase-velocity dispersion of the fundamental mode (period band 2–30 s) and the first-higher mode (period band 3–7 s) from all EGFs are used to invert for the 1-D average, shear-velocity structure in the crust and uppermost mantle using a model-space search algorithm. The preferred shear-velocity models reveal low velocities (4.29 km/s) between Moho and 25 km depth below sea-surface, suggesting the absence of a fast uppermost mantle lid in this young (0–2 Myr) oceanic region. An even more pronounced low-velocity zone, with shear velocities ～3.85 km/s, appears at a depth between 25–40 km below sea-surface. Along with previous results, our study indicates that the shear velocity in the uppermost oceanic mantle increases with increasing seafloor age, consistent with age-related lithospheric cooling.     

Groundwater under stress: the importance of management

The most significant water problem facing the United States and the world is the scarcity of water. Scarcity is expected to intensify during the twenty-first century due to global population growth, economic growth and the need to protect environmental assets. In many regions, climate change may cause scarcity to become more acute. Groundwater has the capacity to buffer extreme hydrologic events because recharge is not directly tied to precipitation and run-off events. Groundwater storage offers many opportunities to increase storage and enhance the general water supply. The value of groundwater will increase as scarcity intensifies and effective groundwater management will be required if groundwater values are to be reaped. Effective groundwater management will require attention to certain well-established economic principles. Persistent overdraft is always self terminating. Individualistically competitive modes of extraction are inefficient. It is virtually always cheaper to prevent pollution in the first place than to remediate it. Groundwater management schemes appear to be most effective when they are locally developed and managed. Such schemes can regulate extractions and/or recharge either directly or indirectly. Evidence from the United States suggests that direct regulations can be made to work if they focus on recharge and developing supplementary sources of water. Evidence from South Asia suggests that regulation of extractions and provision of recharge water is most effective when indirect measures are used in which groundwater and ancillary variables such as electricity are co-managed. This evidence suggests that as supplementary sources of supply become scarce, innovative and indirect groundwater management schemes are likely to become more common.     

Multiple-Point Statistics for Modeling Facies Heterogeneities in a Porous Medium: The Komadugu-Yobe Alluvium,Lake Chad Basin

Multiple-point statistics are used to model facies heterogeneities in the vadose zone of the Komadugu-Yobe River valley (southeastern Niger) which is presently submitted to an undergoing intensive agricultural development; therefore, increasing quantitative and qualitative pressures are exerted on groundwater resources. The sand–clay heterogeneities are analyzed by means of a Landsat image acquired during a high flow period over a 160 km stretch in the downstream part of the valley and a set of 50 boreholes drilled near the town of Diffa (4 km×4 km area). The horizontal variograms of heterogeneities are characterized by a noticeably constant length scale of 380 m and clayey objects are shown to be randomly distributed in space according to a Poisson process. A set of two-dimensional vertical images is built based on a Boolean procedure and the Snesim algorithm is used to simulate synthetic three-dimensional media. When the vertical correlation length is fitted, the three-dimensional model satisfactorily reproduces the second order statistics of heterogeneities and the specific facies patterns.     

Hydrothermal faunal assemblages and habitat characterisation at the Eiffel Tower edifice (Lucky Strike,Mid‐Atlantic Ridge)

The Eiffel Tower edifice is situated in the Lucky Strike hydrothermal vent field at a mean depth of 1690 m on the Mid‐Atlantic Ridge (MAR). At this 11‐m‐high hydrothermal structure, different faunal assemblages, varying in visibly dominant species (mussels and shrimp), in mussel size and in density of mussel coverage, were sampled biologically and chemically. Temperature and sulphide (∑S) were measured on the different types of mussel‐based assemblages and on a shrimp‐dominated assemblage. Temperature was used as a proxy for calculating total concentrations of CH₄. Based on the physico‐chemical measurements, two microhabitats were identified, corresponding to (i) a more variable habitat featuring the greatest fluctuations in environmental variables and (ii) a second, more stable, habitat. The highest temperature variability and the highest maximum recorded temperatures were found in the assemblages visibly inhabited by alvinocaridid shrimp and dense mussel beds of large Bathymodiolus azoricus, whereas the less variable habitats were inhabited by smaller‐sized mussels with increasing bare surface in between. Larger mussels appeared to consume more ∑S compared with smaller‐sized (<1 cm) individuals and thus had a greater influence on the local chemistry. In addition, the mussel size was shown to be significantly positively correlated to temperature and negatively to the richness of the associated macrofauna. The presence of microbial mats was not linked to specific environmental conditions, but had a negative effect on the presence and abundance of macro‐fauna, notably gastropods. Whereas some taxa or species are found in only one of the two microhabitats, others, such as polychaetes and Mirocaris shrimp, cross the different microhabitats. Temperature was proposed to be a more limiting factor in species distribution than ∑S.     

Seasonal patterns in zooplankton community structure on the Newfoundland and Labrador Shelf

We report on the patterns in zooplankton community structure on the Newfoundland and Labrador Shelf from seasonal collections along oceanographic sections during 2000–2007. We use a combination of constrained and unconstrained multivariate methods to assess latitudinal and cross shelf patterns in community structure. Both physical and biological features of the region are dominated by the cross-shelf gradient in water mass characteristics although there is evidence of a latitudinal gradient that may be a reflection of the influence of freshwater outflow from the Arctic. All analyses reveal a strong and consistent pattern in species composition among water masses that extends across spring, summer and autumn, although there are some variations that occur among seasons that reflect differences in development state of certain taxa. The strong association between community structure and water mass characteristics in the region may be the result of the Newfoundland and Labrador Shelf being at the intersection of several major oceanographic domains, bounded by strong Labrador and Gulf Stream currents, that allows the formation and persistence of well defined zooplankton communities. Our findings have implications for the region’s potential as a monitoring location for long term changes in ocean state.     

Growth mechanism of snowball garnets from the Lukmanier Pass area (Central Alps,Switzerland): a combined μCT/EPMA/EBSD study

For two decades, considerable efforts have been made to explain the formation of snowball garnets by either the rotational or non‐rotational models. On the basis of morphological, chemical and crystallographic evidence, this paper presents new data on snowball garnets showing that the formation of these microstructures can be explained by the combination of the two previously proposed mechanisms operating consecutively during garnet growth. The crystallization sequence of garnet revealed by Mn contouring and the distribution of crystallographic orientations within the spiral indicate that the final stages of garnet growth are controlled by post‐kinematic crystallization. However, some microstructural arguments plead for a rotational contribution during the first stages of growth. In this view, the overall spiral geometry is thought to overestimate the true amount of rotation experienced by the garnets. Results also reveal the existence of complex snowball garnets consisting of several grains formed from distinct nucleation sites.     

Crustal thickening and lateral flow during compression of hot lithospheres,with particular reference to Precambrian times

Many ancient deformation belts, especially of Archean and Palaeoproterozoic age, show large areas marked by primary flat‐lying fabrics associated with rather monotonous metamorphic conditions of HTLP type and affected by steep transpressive zones involving vertical stretch. These features do not support strain localization along large‐scale thrusts and (or) extensional detachments, as common in modern orogens. Instead, they are consistent with hot and weak lithospheres where gravity‐driven horizontal flow may compete with distributed thickening from early stages of collisional processes. Relevant deformation features are reviewed and highlighted by lithosphere‐scale analogue models involving low‐viscosity lower crust and sub‐Moho mantle. Both nature and models argue that compression of such lithospheres may induce combined distributed thickening and lateral channel flow of the ductile crust accommodated by transpressive zones.     

Titan's 3-micron spectral region from ISO high-resolution spectroscopy

The near-infrared spectrum of Titan, Saturn's largest moon and one of the Cassini/Huygens' space mission primary targets, covers the 0.8 to 5 micron region in which it shows several weak CH₄ absorption regions, and in particular one centered near 2.75 micron. Due to the interference of telluric absorption, only part of this window region (2.9-3.1 μm) has previously been observed from the ground [Noll, K.S., Geballe, T.R., Knacke, R., Pendleton, F., Yvonne, J., 1996. Icarus 124, 625-631; Griffith, C.A., Owen, T., Miller, G.A., Geballe, T., 1998. Nature 395, 575-578; Griffith, C.A., Owen, T., Geballe, T.R., Rayner, J., Rannou, P., 2003. Science 300, 628-630; Geballe, T.R., Kim, S.J., Noll, K.S., Griffith, C.A., 2003. Astrophys. J. 583, L39-L42]. We report here on the first spectroscopic observations of Titan covering the whole 2.4-4.9 μm region by two instruments on board the Infrared Space Observatory (ISO) in 1997. These observations show the 2.75-μm window in its complete extent for the first time. In this study we have also used a high-resolution Titan spectrum in the 2.9-3.6 μm region taken with the Keck [Geballe, T.R., Kim, S.J., Noll, K.S., Griffith, C.A., 2003. Astrophys. J. 583, L39-L42; Kim, S.J., Geballe, T.R., Noll, K.S., Courtin, R., 2005. Icarus 173, 522-532] to infer information on the atmospheric parameters (haze extinction, single scattering albedo, methane abundance, etc.) by fitting the methane bands with a detailed microphysical model of Titan's atmosphere (updated from Rannou, P., McKay, C.P., Lorenz, R.D., 2003. Planet. Space Sci. 51, 963-976). We have included in this study an updated version of a database for the CH₄ absorption coefficients [STDS, Wenger, Ch., Champion, J.-P., 1998. J. Quant. Spectrosc. Radiat. Transfer 59, 471-480. See also http://www.u-bourgogne.fr/LPUB/TSM/sTDS.html for latest updates; Boudon, V., Champion, J.-P., Gabard, T., Loëte, M., Michelot, F., Pierre, G., Rotger, M., Wenger, Ch., Rey, M., 2004. J. Mol. Spectrosc. 228, 620-634]. For the atmosphere we find that (a) the haze extinction profile that best matches the data is one with higher (by 40%) extinction in the atmosphere with respect to Rannou et al. (2003) down to about 30 km where a complete cut-off occurs; (b) the methane mixing ratio at Titan's surface cannot exceed 3% on a disk-average basis, yielding a maximum CH₄ column abundance of 2.27 km-am in Titan's atmosphere. From the derived surface albedo spectrum in the 2.7-3.08 micron region, we bring some constraints on Titan's surface composition. The albedo in the center of the methane window varies from 0.01 to 0.08. These values, compared to others reported in the other methane windows, show a strong compatibility with the water ice spectrum in the near-infrared. Without confirming its existence from this work alone, our data then appear to be compatible with water ice. A variety of other ices, such as CO₂, NH₃, tholin material or hydrocarbon liquid cannot be excluded from our data, but an additional unidentified component with a signature around 2.74 micron is required to satisfy the data.