Seismicity induced by gas production: II. Lithology correlated events,induced stresses and deformation

A 3D relocation technique permits precise locations of induced earthquakes. Geostatistical processing using the data of 87 boreholes provides the basis of a precise 3D structure, with a dome geometry. Conventional laboratory mechanical tests performed on deep rock samples (1000 m to 5000 m) define the rock properties at depths similar to those of the seismic events (1<M _L<4.2) that range from 1 to 7 km.In the studied period, most (85%), of the events were located above the gas reservoir, with very few located in the reservoir itself. Because the production parameters (50 MPa depletion of the gas pressure reservoir) are homogeneous throughout the gas field, the lateral inhomogeneity of the seismic rupture locations are a consequence of variations in the rheological response of the dome to the deformation induced by gas production.Here a ratio of two is found between the elastic modulus of the seismic rock matrix and the elastic modulus of the aseismic rock matrix. The contrast in strength is at least as great, if not greater. Repeated measured surface deformations involve the whole structure. Spatial and temporal deformations indicate that aseismic deformation is quantitatively the main process of this structural deformation. The heterogeneous stress pattern inferred fromP-axes of induced earthquakes disagrees with the tectonic regional stress field. The radial distribution ofP-axes towards the gas reservoir probably reflects the production induced deformation. The inferred deformation of the dome occurs in response to weak induced stresses.     

Seismicity induced by gas production: I. Correlation of focal mechanisms and dome structure

From August 1976 to June 1979, more than 220 seismic events were recorded by a 7-station network operating over an active gas field in France. 120 earthquakes with magnitudes greater than 1.0 were selected for a detailed study in order to understand the seismic response to a decrease in the gas reservoir pressure of 50 MPa. Composite focal mechanisms were determined for these events. We found that where preexisting fractures were known from well-logs, faults with the most favorable orientation were reactivated. In the part of the gas field where no preexisting faults were known, orientation of the fracture planes was close to the orientations of stratigraphic interfaces.Our kinematic model, constrained by the distribution of the earthquakes and their focal mechanisms, consists of the downward dropping of a block located close to the top of the gas field. The main faults, as defined by the spatial density of the seismic activity, define the limits of this block. The displacements observed along these faults can be either normal or reverse mechanism, dependent on the preexisting orientation of each fault. Another important result is the evidence that the depletion of the gas reservoir exerts the major influence on local seismic displacements, obscuring the effect of regional setting.     

Nutrient dynamics at the sediment-water interface in a Mediterranean lagoon (Thau, France): influence of biodeposition by shellfish farming activities

The Thau Lagoon, a French Mediterranean shallow lagoon, is a site where extensive shellfish farming occurs. The aim of the present work is to evaluate the role of this activity on nutrient exchange at the sediment-water interface in relation to organic matter (OM) sedimentation and degradation. Two stations inside (C5) and outside (C4) of the shellfish farming areas were sampled at three seasons. Porewater chemistry surveys and calculated diffusive fluxes were used to evaluate the trophic status of the Thau lagoon. Quantitative (Particulate Organic Carbon) as well as qualitative OM (Hydrogen Index, Carbohydrates) analyses were performed on sediments to assess OM characteristics. Results emphasized that surficial sediments at C5 are always more enriched in OM. Porewater nutrient concentrations are 10-20 times higher at C5 than at C4. In June 2003, the porewater profiles exhibit a sharp gradient at the bottom waters, indicating a hypereutrophic status, leading to an anoxic crisis.     

Etude expérimentale de la fixation de métaux par un matériau sédimentaire actuel d'origine algaire—I. Isolement,purification et caractérisation de la matière organique

In order to study the ‘in vitro’ fixation of metal ion species on algal and bacterial sedimentary organic matter, two algal mat samples have been studied. The main interest of this organic matter is its specific cyanobacterial origin. After isolation and purification, the organic matter was characterized by elemental and functional analysis, infrared spectroscopy. carbohydrates and amino-acid titration. These analyses indicate the richness of this type of organic matter in amino acids and carbohydrates, its poverty in aromatic structures, and to ascertain the types of bacteriological and chemical transformations undergone by the original biological compounds in the first steps of the diagenesis.     

P–T–X evolution in garnet pyroxenites from Tin Begane (Central Hoggar, Algeria)

Garnet pyroxenite from high pressure granulite facies occurs with different mineral assemblages which involve garnet, clinopyroxene, orthopyroxene, plagioclase, amphibole and quartz with spinel developing as symplectite with orthopyroxene and plagioclase in large cracks. Three successive parageneses have been identified. The primary assemblage is characterised by the presence of quartz. The second assemblage involves orthopyroxene–plagioclase–hornblende symplectite, and the third assemblage is characterised by the development of spinel in symplectites with orthopyroxene and plagioclase. Using THERMOCALC (V2.7), a quantitative pseudosection in the system CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O has been calculated. The assemblage involving quartz developed at high pressure, while the assemblage involving spinel developed at lower pressure. The peak of metamorphism in Tin Begane was calculated at 860 °C and 13.5 kb with a_H2O=0.2. These conditions are followed by a decrease of pressure down to 4.8 kb.     

Geochemical characterization of soil organic matter and variability of a postglacial detrital organic supply (chaillexon lake,france)

Geochemical analysis of sedimentary organic matter in recent lacustrine sediments appears to be a useful tool in providing information concerning past environmental conditions. However, such analysis is often made without knowing the geochemical characteristics of the organic matter derived from the watershed and, more explicitly, its soils. The present work deals with (i) a geochemical investigation (Rock-Eval pyrolysis) of soil organic matter sampled in a lake watershed, and (ii) the study of the sedimentary organic matter trapped in the lake deposits. The research was conducted on Chaillexon Lake which was created by a rock collapse that dammed the palaeovalley of the Doubs River about 12 000 years ago. Since this event, the sediment trap provides a continuous palaeoclimatic record for the Postglacial period. Results obtained lead to two main conclusions. First, the variability of Rock-Eval pyrolysis values observed in soils modifies the common interpretation given to these parameters in the characterization of sedimentary organic matter. Indeed, variations in these parameters point not only to varying proportions of terrestrial and lacustrine organic matter in a lacustrine infilling but also to variations of the terrestrial supply linked with the evolution of vegetal cover in the catchment. The second conclusion is that the story of the Chaillexon lacustrine system is marked by a rather sudden soil and forest development at the Preboreal–Boreal transition (9000 BP ). Copyright © 1998 John Wiley & Sons, Ltd.     

Off-center detonations in evolved stellar cores

Recent work on late stages of evolution has raised the possibility that the stars in the mass range from 4–8M _bd may ignite carbon violently off-center. Hydrodynamic evolution has been carried out for models with various central densities and for temperature profiles both with and without an inversion. It has been found that, provided ignition leads to a detonation, both types of presupernova models have essentially the same final post detonation behavior. In particular, an off-center detonation is unable to prevent a complete disruption of the star if ignition occurs at low density.     

Holocene Climate Variability in Antarctica Based on 11 Ice-Core Isotopic Records

A comparison is made of the Holocene records obtained from water isotope measurements along 11 ice cores from coastal and central sites in east Antarctica (Vostok, Dome B, Plateau Remote, Komsomolskaia, Dome C, Taylor Dome, Dominion Range, D47, KM105, and Law Dome) and west Antarctica (Byrd), with temporal resolution from 20 to 50 yr. The long-term trends possibly reflect local ice sheet elevation fluctuations superimposed on common climatic fluctuations. All the records confirm the widespread Antarctic early Holocene optimum between 11,500 and 9000 yr; in the Ross Sea sector, a secondary optimum is identified between 7000 and 5000 yr, whereas all eastern Antarctic sites show a late optimum between 6000 and 3000 yr. Superimposed on the long time trend, all the records exhibit 9 aperiodic millennial-scale oscillations. Climatic optima show a reduced pacing between warm events (typically 800 yr), whereas cooler periods are associated with less-frequent warm events (pacing >1200 yr).     

La couverture océanique des ultrabasites de Lanzo (Alpes occidentales) : arguments lithostratigraphiques et pétrologiques

Résumé

De nombreuses analogies entre les évolutions pétro-logiques et métamorphiques des ultrabasites de Lanzo et des ophiohtes liguro-piémontaises ont été soulignées récemment. Ces analogies concernent: la nature des ultrabasites (Iherzolites à plagioclase peu appauvries), la présence de gabbros et de filons basaltiques superficiels intrusifs dans les péridotites, l’existence de reliques de paragenèses métamorphiques de haute température attribuables à un épisode océanique, l’empreinte du métamorphisme alpin de haute pression. Ces faits ont conduit certains auteurs à proposer que les péridotites de Lanzo représentent un fragment du plancher océanique téthysien.

Nous avons tenté de vérifier cette hypothèse en recherchant la couverture océanique (volcanique et/ou sédimentaire) des péridotites de Lanzo, dans la partie occidentale du massif où des pincées de métabasites et de schistes lustrés s.l. sont visibles associées aux serpentinites.

Aux environs du village de Richiaglio, à l’Est de la cicatrice de Viù, les successions lithostratigraphiques reconstituées comprennent schémati-quement : les ultrabasites serpentinisées du corps de Lanzo, des métabasites, des quartzites manganésifères (comparables à celles du Mont Viso par exemple), et des calcschistes. Ces successions s’apparentent à celles décrites dans les couvertures d’autres ophiolites piémontaises, ce qui conduit à proposer que ces ensembles métavolcaniques et Dimetaires dimentaires représentent des résidus de la couverture océanique originelle des péridotites. L’analyse des paragenèses éclogitiques des métasédiments et des métabasites permet de montrer que l’évolution P,T alpine de ces lambeaux est compatible avec celle du corps de Lanzo. Cette interprétation et ses implications sont discutées.     

Surface studies of water isotopes in Antarctica for quantitative interpretation of deep ice core data

Polar ice cores are unique climate archives. Indeed, most of them have a continuous stratigraphy and present high temporal resolution of many climate variables in a single archive. While water isotopic records (δD or δ¹⁸O) in ice cores are often taken as references for past atmospheric temperature variations, their relationship to temperature is associated with a large uncertainty. Several reasons are invoked to explain the limitation of such an approach; in particular, post-deposition effects are important in East Antarctica because of the low accumulation rates. The strong influence of post-deposition processes highlights the need for surface polar research programs in addition to deep drilling programs. We present here new results on water isotopes from several recent surface programs, mostly over East Antarctica. Together with previously published data, the new data presented in this study have several implications for the climatic reconstructions based on ice core isotopic data: (1) The spatial relationship between surface mean temperature and mean snow isotopic composition over the first meters in depth can be explained quite straightforwardly using simple isotopic models tuned to d-excess vs. δ¹⁸O evolution in transects on the East Antarctic sector. The observed spatial slopes are significantly higher (～ 0.7–0.8‰·°C^?1 for δ¹⁸O vs. temperature) than seasonal slopes inferred from precipitation data at Vostok and Dome C (0.35 to 0.46‰·°C^?1). We explain these differences by changes in condensation versus surface temperature between summer and winter in the central East Antarctic plateau, where the inversion layer vanishes in summer. (2) Post-deposition effects linked to exchanges between the snow surface and the atmospheric water vapor lead to an evolution of δ¹⁸O in the surface snow, even in the absence of any precipitation event. This evolution preserves the positive correlation between the δ¹⁸O of snow and surface temperature, but is associated with a much slower δ¹⁸O-vs-temperature slope than the slope observed in the seasonal precipitation. (3) Post-deposition effects clearly limit the archiving of high-resolution (seasonal) climatic variability in the polar snow, but we suggest that sites with an accumulation rate of the order of 40 kg.m^?2.yr^?1 may record a seasonal cycle at shallow depths.