Centre de Géosciences

Reading, writing, manipulating, analyzing and modeling of spatial data. This package has been superseded by the "terra" package &lt;<a href="https://CRAN.R-project.org/package=terra" target="_top">https://CRAN.R-project.org/package=terra</a>&gt;.

A general description of the mathematical and numerical formulations used in modern numerical reactive transport codes relevant for subsurface environmental simulations is presented. The formulations are followed by short descriptions of commonly used and available subsurface simulators that consider continuum representations of flow, transport, and reactions in porous media. These formulations are applicable to most of the subsurface environmental benchmark problems included in this special issue. The list of codes described briefly here includes PHREEQC, HPx, PHT3D, OpenGeoSys (OGS), HYTEC, ORCHESTRA, TOUGHREACT, eSTOMP, HYDROGEOCHEM, CrunchFlow, MIN3P, and PFLOTRAN. The descriptions include a high-level list of capabilities for each of the codes, along with a selective list of applications that highlight their capabilities and historical development.

Studies of deeply buried, sedimentary microbial communities and associated biogeochemical processes during Ocean Drilling Program Leg 201 showed elevated prokaryotic cell numbers in sediment layers where methane is consumed anaerobically at the expense of sulfate. Here, we show that extractable archaeal rRNA, selecting only for active community members in these ecosystems, is dominated by sequences of uncultivated Archaea affiliated with the Marine Benthic Group B and the Miscellaneous Crenarchaeotal Group, whereas known methanotrophic Archaea are not detectable. Carbon flow reconstructions based on stable isotopic compositions of whole archaeal cells, intact archaeal membrane lipids, and other sedimentary carbon pools indicate that these Archaea assimilate sedimentary organic compounds other than methane even though methanotrophy accounts for a major fraction of carbon cycled in these ecosystems. Oxidation of methane by members of Marine Benthic Group B and the Miscellaneous Crenarchaeotal Group without assimilation of methane-carbon provides a plausible explanation. Maintenance energies of these subsurface communities appear to be orders of magnitude lower than minimum values known from laboratory observations, and ecosystem-level carbon budgets suggest that community turnover times are on the order of 100-2,000 years. Our study provides clues about the metabolic functionality of two cosmopolitan groups of uncultured Archaea.

The hydrometeorological model SIM consists of a meteorological analysis system (SAFRAN), a land surface model (ISBA), and a hydrogeological model (MODCOU). It generates atmospheric forcing at an hourly time step, and it computes water and surface energy budgets, the river flow at more than 900 river‐gauging stations, and the level of several aquifers. SIM was extended over all of France in order to have a homogeneous nationwide monitoring of the water resources: it can therefore be used to forecast flood risk and to monitor drought risk over the entire nation. The hydrometeorological model was applied over a 10‐year period from 1995 to 2005. In this paper the databases used by the SIM model are presented; then the 10‐year simulation is assessed by using the observations of daily streamflow, piezometric head, and snow depth. This assessment shows that SIM is able to reproduce the spatial and temporal variabilities of the water fluxes. The efficiency is above 0.55 (reasonable results) for 66% of the simulated river gauges, and above 0.65 (rather good results) for 36% of them. However, the SIM system produces worse results during the driest years, which is more likely due to the fact that only few aquifers are simulated explicitly. The annual evolution of the snow depth is well reproduced, with a square correlation coefficient around 0.9 over the large altitude range in the domain. The streamflow observations were used to estimate the overall error of the simulated latent heat flux, which was estimated to be less than 4%.

We describe a model for Caribbean plate motion based on GPS velocities of four sites in the plate interior and two azimuths of the Swan Islands transform fault. The data are well fit by a single angular velocity, with average misfits approximately equal to the 1.5–3.0 mm yr −1 velocity uncertainties. The new model predicts Caribbean‐North America motion ∼65% faster than predicted by NUVEL‐1A, averaging 18–20±3 mm yr −1 (2σ) at various locations along the plate boundary. The data are best fit by a rotation pole that predicts obliquely convergent motion along the plate boundary east of Cuba, but are fit poorly by a suite of previously published models that predict strike‐slip motion in this region. The data suggest an approximate upper bound of 4–6 mm yr −1 for internal deformation of the Caribbean plate, although rigorous estimates await more precise and additional velocities from sites in the plate interior.

X‐ray diffraction, electron diffraction, and Raman scattering measurements are presented for dehydrated amorphous ZrO 2 . Although the material lacks sufficient crystallinity to diffract X rays, electron diffraction patterns indicate a microcrystallinity with a grain size of 15 to 30 A. Raman spectra characteristic of the metastable tetragonal polymorph were obtained from these materials. Heating the amorphous materials induces recrystallization into first a metastable tetragonal phase and then the stable monoclinic phase.

This is the first textbook to cover the essential aspects of the topic at a level accessible to students. While focusing on applications in solid earth geophysics, the book also includes excursions into helioseismology, thereby highlighting the strong affinity between the two fields. The book provides a comprehensive introduction to seismic tomography, including the basic theory of wave propagation, the ray and Born approximations required for interpretation of amplitudes, and travel times and phases. It considers observational features while also providing practical recommendations for implementing numerical models. Written by one of the leaders in the field, and containing numerous student exercises, this textbook is appropriate for advanced undergraduate and graduate courses. It is also an invaluable guide for seismology research practitioners in geophysics and astronomy. Solutions to the exercises and accompanying tomographic software and documentation can be accessed online from www.cambridge.org/9780521882446.

The warmest global temperatures of the past 85 million years occurred during a prolonged greenhouse episode known as the Early Eocene Climatic Optimum (52-50 Ma). The Early Eocene Climatic Optimum terminated with a long-term cooling trend that culminated in continental-scale glaciation of Antarctica from 34 Ma onward. Whereas early studies attributed the Eocene transition from greenhouse to icehouse climates to the tectonic opening of Southern Ocean gateways, more recent investigations invoked a dominant role of declining atmospheric greenhouse gas concentrations (e.g., CO2). However, the scarcity of field data has prevented empirical evaluation of these hypotheses. We present marine microfossil and organic geochemical records spanning the early-to-middle Eocene transition from the Wilkes Land Margin, East Antarctica. Dinoflagellate biogeography and sea surface temperature paleothermometry reveal that the earliest throughflow of a westbound Antarctic Counter Current began ~49-50 Ma through a southern opening of the Tasmanian Gateway. This early opening occurs in conjunction with the simultaneous onset of regional surface water and continental cooling (2-4 °C), evidenced by biomarker- and pollen-based paleothermometry. We interpret that the westbound flowing current flow across the Tasmanian Gateway resulted in cooling of Antarctic surface waters and coasts, which was conveyed to global intermediate waters through invigorated deep convection in southern high latitudes. Although atmospheric CO2 forcing alone would provide a more uniform middle Eocene cooling, the opening of the Tasmanian Gateway better explains Southern Ocean surface water and global deep ocean cooling in the apparent absence of (sub-) equatorial cooling.

The kinematics of the East African Rift (EAR) is the least well‐known of all major plate boundaries. Here, we show that present‐day data (a GPS+DORIS geodetic solution and earthquake slip vectors) are consistent with 3.2 Myr‐average spreading rates and transform‐fault azimuths along the Southwest Indian Ridge and support a kinematic model that includes three subplates (Victoria, Rovuma, and Lwandle) between Nubia and Somalia. Continental rifting in the EAR appears to involve localized strain along narrow rift structures that isolate large lithospheric blocks.

ABSTRACT Quantitative methods taking into account the sedimentological characteristics will answer the needs of reservoir engineers. We propose here a geostatistical method for the conditional modelling of the facies of a sedimentary fluvio-deltaic series. This model was elaborated jointly by I.F.P. and the Paris School of Mines, with the aim of modelling reservoir heterogeneities. From the sedimentological study contained in the paper by C. Ravenne et al., we present several simulations, conditioned by "drill-core" taken from the outcrop. The block permeabilities are then calculated from the values given to the facies.

Abstract. Physically-based droughts can be defined as a water deficit in at least one component of the land surface hydrological cycle. The reliance of different activity domains (water supply, irrigation, hydropower, etc.) on specific components of this cycle requires drought monitoring to be based on indices related to meteorological, agricultural, and hydrological droughts. This paper describes a high-resolution retrospective analysis of such droughts in France over the last fifty years, based on the Safran-Isba-Modcou (SIM) hydrometeorological suite. The high-resolution 1958–2008 Safran atmospheric reanalysis was used to force the Isba land surface scheme and the hydrogeological model Modcou. Meteorological droughts are characterized with the Standardized Precipitation Index (SPI) at time scales varying from 1 to 24 months. Similar standardizing methods were applied to soil moisture and streamflow for identifying multiscale agricultural droughts – through the Standardized Soil Wetness Index (SSWI) – and multiscale hydrological droughts, through the Standardized Flow Index (SFI). Based on a common threshold level for all indices, drought event statistics over the 50-yr period – number of events, duration, severity and magnitude – have been derived locally in order to highlight regional differences at multiple time scales and at multiple levels of the hydrological cycle (precipitation, soil moisture, streamflow). Results show a substantial variety of temporal drought patterns over the country that are highly dependent on both the variable and time scale considered. Independent spatio-temporal drought events have then been identified and described by combining local characteristics with the evolution of area under drought. Summary statistics have finally been used to compare past severe drought events, from multi-year precipitation deficits (1989–1990) to short hot and dry periods (2003). Results show that the ranking of drought events depends highly on both the time scale and the variable considered. This multilevel and multiscale drought climatology will serve as a basis for assessing the impacts of climate change on droughts in France.

A long sequence of earthquakes, six with magnitudes between 5 and 6, struck Central Italy starting on September 26, 1997, causing severe damages and loss of human lives. The seismogenic structure consists of a NW‐SE elongated fault zone extending for about 40 km. The focal mechanisms of the largest shocks reveal normal faulting with NE‐SW extension perpendicular to the trend of the Apennines, consistently with the Quaternary tectonic setting of the internal sector of the belt and with previous earthquakes in adjacent regions. Preliminary data on the main shocks and aftershocks show that extension in this region of the Apennines is accomplished by normal faults dipping at low angle (∼40°) to the southwest, and confined in the upper ∼8 km of the crust. These normal faults might have reactivated thrust planes of the Pliocene compressional tectonics. The aftershock distribution and the damage patterns also suggest that the three main shocks ruptured distinct 5 to 15 km‐long fault segments, adjacent and slightly offset from one another.

Through structure design, 3D printing enables the fabrication of mechanically durable superhydrophobic membranes with an ordered porous structure for oil–water separation.

We introduce a new tomographic method for estimating velocity macro-models from seismic reflection data. In addition to traveltimes picked on locally coherent reflected events, the method requires that the associated local slopes of the events be picked simultaneously in the common-shot and common-receiver trace gathers. The data then consist of a discrete collection of traveltimes, positions and slopes for selected reflected events. Unlike traveltime tomography, picked events are only required to be locally coherent. It is not necessary to follow continuous arrivals all over the trace gathers. Indeed, the method does not require the introduction of interfaces in the model description. Several approaches of tomography using the slope have already been proposed. We present a unified formulation for slope tomography methods, in which the model is described by the velocity field and a set of ray-segment pairs associated with the reflected/diffracted events. We propose a new robust slope tomography method, which we call 'stereotomography'. It consists of fitting all observed data (positions, slopes and traveltimes) to data calculated by ray tracing. There are no theoretical limitations in stereotomography for laterally heterogeneous velocity macro-models. Practically, traveltimes and slopes are picked on local slant stack panels. Ray multipathing can be accounted for since paths are discriminated by their associated slopes. The non-linear inverse problem is iteratively resolved by a local optimization. The Fréchet derivatives are estimated by paraxial ray tracing. Validation tests on 1-D and 2-D synthetic data are analysed. In the first 1-D example, we study the sensitivity of the method to model parameters (using a singular-value decomposition). The second 1-D example evaluates picking precision and shows that it is sufficient for constraining the velocity field. The last example is a 2-D application in which data are calculated directly by ray tracing. It shows the performance of the method in the presence of strong lateral velocity variations.

Abstract Is GIS a tool or a science? The question is clearly important in the day-to-day operations of geography departments. Departments need to know if GIS is a tool that should be taught at the undergraduate level, or a science and thus a legitimate research specialty of faculty and graduate students. We summarize the debate on this question that was conducted on GIS-L electronic listserver in late 1993. In evaluating this discussion it became clear that GIS could be understood not by the two distinct positions taken by the GIS-L discussants but as three positions along a continuum ranging from tool to science. These positions attach several meanings to "doing GIS." These are (1) GIS as tool, i.e., the use of a particular class of software, associated hardware tools, and digital geographic data in order to advance some specific purpose; (2) GIS as toolmaking, i.e., the advancement of the tool's capabilities and facilities (ease of use); and (3) the science of GIS, i.e., the analysis of the fundamental issues raised by the use of GIS. Recognizing the importance of understanding what is meant by "doing science " as well as what is meant by "doing GIS," we conclude that only one of these positions—"the science of GIS"—is a sufficient condition for science. The "toolmaker" position is rarely able to meet the test of science; and the "GIS is a tool" position involves "doing science" only if it yields progress on some substantive problem. The debate is certainly problematic in light of the variety of perspectives on science and on GIS. The persistence of the issue suggests, however, that the GIS community should continue to work toward a resolution. Keywords:: GIS-Lsystemsgeographic information sciencegeographic thoughtnature and philosophy of sciencenature of geographic information systems

Abstract. This paper summarizes 4 years (1987‐1990) of weekly net CH 4 flux measurements at permanent sites representing important plant components of Arctic tundra. The data coincide with variations in precipitation and temperature of interest in regional and global modeling efforts and are useful in placing bounds on the role of tundra in the global CH 4 budget. Precipitation in the study area during the summer emission period ranged from twice to half the long‐term mean, and air temperature anomalies were about +2 °C. This data set also permits consideration of temporal (seasonal to interannual) and spatial variability in CH 4 flux. We studied the relationship between the net CH 4 flux and subsurface properties (water table depth, thaw depth, soil temperature, / p CH 4 distributions) at these permanent sites during the 1988 and 1989 emission periods. Net CH 4 emission and subsurface properties are largely unrelated. Relationships between soil temperature (or any single variable) and emission are site specific and are of little value as flux predictors. Parameters that integrate conditions influencing flux appear to be the best flux predictors over the emission period. We estimate that Arctic wet meadow and tussock:shrub tundra presently emit about 42 ± 26 Tg CH 4 yr −1 to the atmosphere. This estimate has a North American bias, but it is supported by measurements in a range of locations, transect studies, and model calculations.

Abstract Oxygen-isotope thermometry played a critical role in the rise of modern geochemistry and remains extensively used in (bio-)geoscience. Its theoretical foundations rest on the assumption that 18 O/ 16 O partitioning among water and carbonate minerals primarily reflects thermodynamic equilibrium. However, after decades of research, there is no consensus on the true equilibrium 18 O/ 16 O fractionation between calcite and water ( 18 α cc/w ). Here, we constrain the equilibrium relations linking temperature, 18 α cc/w , and clumped isotopes ( Δ 47 ) based on the composition of extremely slow-growing calcites from Devils Hole and Laghetto Basso (Corchia Cave). Equilibrium 18 α cc/w values are systematically ~1.5‰ greater than those in biogenic and synthetic calcite traditionally considered to approach oxygen-isotope equilibrium. We further demonstrate that subtle disequilibria also affect Δ 47 in biogenic calcite. These observations provide evidence that most Earth-surface calcites fail to achieve isotopic equilibrium, highlighting the need to improve our quantitative understanding of non-equilibrium isotope fractionation effects instead of relying on phenomenological calibrations.

Abstract Complex velocity models characterized by strong lateral variations are certainly a great motivation, but also a great challenge, for depth imaging. In this context, some unexpected results can occur when using depth imaging algorithms. In general, after a common shot or common offset migration, the resulting depth images are sorted into common-image gathers (CIG), for further processing such as migration-based velocity analysis or amplitude-variation-with-offset analysis. In this paper, we show that CIGs calculated by common-shot or common-offset migration can be strongly affected by artifacts, even when a correct velocity model is used for the migration. The CIGs are simply not flat, due to unexpected curved events (kinematic artifacts) and strong lateral variations of the amplitude (dynamic artifacts). Kinematic artifacts do not depend on the migration algorithm provided it can take into account lateral variations of the velocity model. This can be observed when migrating the 2-D Marmousi dataset either with a wave-equation migration or with a multivalued Kirchhoff migration/inversion. On the contrary, dynamic artifacts are specific to multi-arrival ray-based migration/inversion. This approach, which should provide a quantitative estimation of the reflectivity of the model, provides in this context dramatic results. In this paper, we propose an analysis of these artifacts through the study of the ray-based migration/inversion operator. The artifacts appear when migrating a single-fold subdata set with multivalued ray fields. They are due to the ambiguous focusing of individual reflected events at different locations in the image. No information is a priori available in the single-fold data set for selecting the focusing position, while migration of multifold data would provide this information and remove the artifacts by the stack of the CIGs. Analysis of the migration/inversion operator provides a physical condition, the imaging condition, for insuring artifact free CIGs. The specific cases of common-shot and common-offset single-fold gathers are studied. It appears clearly that the imaging condition generally breaks down in complex velocity models for both these configurations. For artifact free CIGs, we propose a novel strategy: compute CIGs versus the diffracting/reflecting angle. Working in the angle domain seems the natural way for unfolding multivalued ray fields, and it can be demonstrated theoretically and practically that common-angle imaging satisfies the imaging condition in the great majority of cases. Practically, the sorting into angle gathers can not be done a priori over the data set, but is done in the inner depth migration loop. Depth-migrated images are obtained for each angle range. A canonical example is used for illustrating the theoretical derivations. Finally, an application to the Marmousi model is presented, demonstrating the relevance of the approach.

Abstract The Surface Water and Ocean Topography (SWOT) satellite mission planned for launch in 2020 will map river elevations and inundated area globally for rivers &gt;100 m wide. In advance of this launch, we here evaluated the possibility of estimating discharge in ungauged rivers using synthetic, daily “remote sensing” measurements derived from hydraulic models corrupted with minimal observational errors. Five discharge algorithms were evaluated, as well as the median of the five, for 19 rivers spanning a range of hydraulic and geomorphic conditions. Reliance upon a priori information, and thus applicability to truly ungauged reaches, varied among algorithms: one algorithm employed only global limits on velocity and depth, while the other algorithms relied on globally available prior estimates of discharge. We found at least one algorithm able to estimate instantaneous discharge to within 35% relative root‐mean‐squared error (RRMSE) on 14/16 nonbraided rivers despite out‐of‐bank flows, multichannel planforms, and backwater effects. Moreover, we found RRMSE was often dominated by bias; the median standard deviation of relative residuals across the 16 nonbraided rivers was only 12.5%. SWOT discharge algorithm progress is therefore encouraging, yet future efforts should consider incorporating ancillary data or multialgorithm synergy to improve results.

Abstract Classical algorithms used for traveltime tomography are not necessarily well suited for handling very large seismic data sets or for taking advantage of current supercomputers. The classical approach of first-arrival traveltime tomography was revisited with the proposal of a simple gradient-based approach that avoids ray tracing and estimation of the Fréchet derivative matrix. The key point becomes the derivation of the gradient of the misfit function obtained by the adjoint-state technique. The adjoint-state method is very attractive from a numerical point of view because the associated cost is equivalent to the solution of the forward-modeling problem, whatever the size of the input data and the number of unknown velocity parameters. An application on a 2D synthetic data set demonstrated the ability of the algorithm to image near-surface velocities with strong vertical and lateral variations and revealed the potential of the method.