Initial interpretation of Titan plasma interaction as observed by the Cassini plasma spectrometer: Comparisons with Voyager 1

The Cassini plasma spectrometer (CAPS) instrument made measurements of Titan's plasma environment when the Cassini Orbiter flew through the moon's plasma wake October 26, 2004 (flyby TA). Initial CAPS ion and electron measurements from this encounter will be compared with measurements made by the Voyager 1 plasma science instrument (PLS). The comparisons will be used to evaluate previous interpretations and predictions of the Titan plasma environment that have been made using PLS measurements. The plasma wake trajectories of flyby TA and Voyager 1 are similar because they occurred when Titan was near Saturn's local noon. These similarities make possible direct, meaningful comparisons between the various plasma wake measurements. They lead to the following: (A) The light and heavy ions, H⁺and N⁺/O⁺, were observed by PLS in Saturn's magnetosphere in the vicinity of Titan while the higher mass resolution of CAPS yielded H⁺ and H₂⁺as the light constituents and O⁺/CH₄⁺ as the heavy ions. (B) Finite gyroradius effects were apparent in PLS and CAPS measurements of ambient O⁺ ions as a result of their absorption by Titan's extended atmosphere. (C) The principal pickup ions inferred from both PLS and CAPS measurements are H⁺, H₂⁺, N⁺, CH₄⁺ and N₂⁺. (D) The inference that heavy pickup ions, observed by PLS, were in narrow beam distributions was empirically established by the CAPS measurements. (E) Slowing down of the ambient plasma due to pickup ion mass loading was observed by both instruments on the anti-Saturn side of Titan. (F) Strong mass loading just outside the ionotail by a heavy ion such as N₂⁺ is apparent in PLS and CAPS measurements. (G) Except for the expected differences due to the differing trajectories, the magnitudes and structures of the electron densities and temperatures observed by both instruments are similar. The high-energy electron bite-out observed by PLS in the magnetotail is consistent with that observed by CAPS.     

Analysis of the resolution function in seismic prestack depth imaging

We consider the problem of estimating subsurface quantities such as velocity or reflectivity from seismic measurements. Because of a limited aperture and band-limited signals, the output from a seismic prestack reconstruction method is a distorted or blurred image. This distortion can be computed using the concept of resolution function, which is a quantity readily accessible in the Fourier space of the model. The key parameter is the scattering wavenumber, which at a particular image point is defined by the incident and scattered ray directions in a given background model. Any location in any background model can be considered. In general, the resolution function will depend on the following four quantities: the background velocity model, the frequency bandwidth, the wavefield type and the acquisition geometry.
We first establish the resolution function for a general scattering model assuming local reaction. We then adapt this result for two well-known scattering models: Born and Kirchhoff. For each of these approximations the corresponding resolution function is derived and discussed. Finally, by employing a simple synthetic data example we demonstrate the ability of the resolution function to predict the image distortions.     

Improving Sequential Simulation with a Structured Path Guided by Information Content

Multiple-point simulation is a newly developed geostatistical method that aims at combining the strengths of two mainstream geostatistical methods: object-based and pixel-based methods. It maintains the flexibility of pixel-based algorithms in data conditioning, while enhancing its capability of reproducing realistic geological shapes, which is traditionally reserved to object-based algorithms. However, the current snesim program for multiple-point simulation has difficulty in reproducing large-scale structures, which have a significant impact on the flow response. To address this problem, we propose to simulate along a structured path based on an information content measure. This structured path accounts for not only the information from the data, but also some prior structural information provided by geological knowledge. Various case studies show a better reproduction of large-scale structures. This concept of simulating along a structured path guided by information content can be applied to any sequential simulation algorithms, including traditional variogram-based two-point geostatistical algorithms.     

Salinisation des nappes côtières : cas de la nappe nord du Sahel de Sfax,Tunisie

The intensive agricultural and economic activities induce the increase of the risk of groundwater quality degradation through high groundwater pumping rates. The salinization and contamination are the main sources of this pollution, especially in coastal aquifers. The explanation of the origin of salinity for the shallow aquifer of Northern Sahel of Sfax was analysed by a chemical study of the groundwater main compounds. The partitioning of groundwaters into homogenous groups is undertaken by graphical techniques, including a Stiff pattern diagram, an expanded Durov diagram and several binary diagrams. The study indicates the presence of various salinization processes. In the recharge area, salinization is the result of dissolution/precipitation of the aquifer formation material (group I). The irrigation water return and the intensive pumping have been identified as major sources of salinization in the south by direct cation exchange and mixing reactions (groups II and III). The anomaly of high groundwater salinity observed near the Hazeg zone was explained by the presence of a seawater intrusion in this area. This hypothesis is related to the high chloride concentration, to the presence of inverse cation exchange reactions (group IV), and to the piezometric level inferior to sea level. To cite this article: R. Trabelsi et al., C. R. Geoscience 337 (2005).     

Structure de socle,sismostratigraphie et héritage structural au cours du rifting au niveau de la marge d'Ifni/Tan-Tan (Maroc sud-occidental)

Seismic reflection profiles from the Ifni/Tan-Tan Atlantic margin of southern Morocco, interpreted in the light of well data and field geology from the Western Anti-Atlas, allowed us to establish the seismostratigraphic framework of the syn-rift series and to reveal (i) a compressional structural style in the pre-Triassic basement similar to that established in the adjacent outcropping onshore basement but with an opposed western vergence, (ii) the importance of inherited anterior structures in the formation of Triassic-Liassic rift structures and (iii) an east–west propagation of these rift structures. To cite this article: N. AbouAli et al., C. R. Geoscience 337 (2005).     

Impacts of a large Sahelian city on groundwater hydrodynamics and quality: example of Niamey (Niger)

The management of groundwater resources is very important in the semiarid Sahel region, which is experiencing rapid urban development. Impacts of urbanization on groundwater resources were investigated in the unconfined aquifer of the Continental Terminal beneath the city of Niamey, Niger, using water level and chemical data. Hydrodynamic and chemical changes are best described by a combination of factors including the historical development of the city, current land use, water-table depth and topography. Seasonal groundwater recharge occurs with high spatial variability, as indicated by water-level monitoring in all wells, but there was no interannual trend over the 5-year study period. Groundwater salinity shows high spatial variability and a minor rising trend. The highest salinity is in the old city centre, with Na–NO₃ dominant, and it increases seasonally with recharge. Salinity is much lower and more variable in the suburbs (Ca–HCO₃, Ca–NO₃, and Na–NO₃ dominant). Nitrate is the main ionic contaminant and is seasonally or permanently above the international guidelines for drinking water quality in 36 % of sampled wells, with a peak value of 112 mg L^?1 NO₃–N (8 meq L^?1). Comparison of urban and rural sites indicates a long-term increase in groundwater recharge and nitrate enrichment in the urban area with serious implications for groundwater management in the region.     

An Alternative Protocol for Single Zircon Dissolution with Application to (U‐Th‐Sm)/He Thermochronometry

Zircon (U‐Th‐Sm)/He (ZHe) thermochronometry is a powerful tool that has been widely used in geology to constrain the exhumation histories of orogens. In this study, we present an alternative protocol for dissolving zircon grains for determination of parent nuclides. This new alkali fusion procedure developed at the SARM (Service d'Analyse des Roches et des Minéraux) in Nancy, France, is fast (requiring only 2 d, including cleaning steps) and offers several advantages over conventional methods by avoiding: (i) use of HF pressure dissolution and (ii) complete removing of grains from the metal microvials. After dissolution, U, Th and Sm were measured using an ICP‐MS. We tested the new procedure on two different ZHe reference materials, the Fish Canyon Tuff and Buluk Tuff; these provided precision values for ZHe‐age estimations of 9 and 6% (1s), respectively. In addition, using this method, zircons from the Buluk Tuff are shown to be chemically more homogenous and more suitable for assessing the uncertainty of the entire integrated procedure.     

Ramifications of high in-situ temperatures for laboratory testing and inferred stress states of unlithified sediments – A case study from the Nankai margin

The recovery of drill cores involves changes in pressure and temperature conditions, which inevitably alter the mechanical properties of unlithified sediments. While expansion from unloading after core recovery is well studied, the effects from cooling on standard geotechnical tests are commonly neglected. Along the central portion of the Nankai margin sediments were recovered from high in-situ temperatures of up to 110 °C during IODP Leg 190. So far, the interpretation of the consolidation state of the Lower Shikoku Basin facies (LSB) entering the accretionary Nankai margin is ambiguous. Results from laboratory consolidation tests at room temperature show high pre-consolidation stresses. These were interpreted as hardening caused by cementation, while the field-based porosity vs. depth trend points towards normal consolidation. As an explanation for this discrepancy, the change of the mechanical properties by cooling from in-situ to laboratory conditions is proposed. In this paper, the results of a thermo-mechanical model are compared to published field data. This comparison suggests that the observed hardening is at least partially an artefact from cooling during core recovery, and that the strata may be considered normally consolidated to slightly overconsolidated. The latter can be explained by minor cementation or the influence of secondary consolidation. The results suggest that cooling from high in-situ temperatures may be important for the interpretation of the consolidation state of other sedimentary successions elsewhere.     

Preliminary investigation of the surficial sediments in the Cap-Breton Canyon (southwest France) and the surrounding continental shelf

A total of 120 grab samples of the surficial sediments in the Cap-Breton submarine canyon and surrounding continental shelf were collected and analyzed by grain-size sieving. A Q-mode Factor Analysis was made on the grain-size data in order to define the most meaningful facies types. Four distinct lithological facies were found to exist: silt and clay, very fine sand, fine sand, and coarse sand. Comparison with previous work and a ¹⁴C date on the silt and clay facies showed that the facies are not contemporaneous. The sands and coarse sands on the shelf were emplaced during the pre-Würm and Würm regressions, and later probably reworked during the Holocene (Flandrian) transgression. The silty clays found in the canyon and on the shelf to the south are younger and represent sediments brought in as suspended load by the Adour and other nearby rivers during the Holocene (Flandrian) transgression.