Hot Subdwarfs: Magnetic, Oscillatory and Other Physical Properties

Hot subdwarf stars (hot sds) are blue subluminous objects. Only a few determinations areavailable to date regarding the study of such aspects as rotation,microturbulent velocities or the magnetic nature of these objects. Over26 sdBs are known to date to be multiperiodic rapid oscillators. Thisproject presents preliminary results of new observations and models of asample of pulsating and non-pulsating hot sds, includingconsiderations on mass loss and eventual magnetic properties.     

Flow relevant covariance localization during dynamic data assimilation using EnKF

Multiphase dynamic data integration into high resolution subsurface models is an integral aspect of reservoir and groundwater management strategies and uncertainty assessment. Over the past two decades, advances in computing and the development and implementation of robust algorithms for automatic history matching have considerably reduced the time and effort associated with subsurface characterization and reduced the subjectivity associated with manual model calibration. However, reliable and accurate subsurface characterization continues to be challenging due to the large number of model unknowns to be estimated using a relatively smaller set of measurements. For ensemble-based methods in particular, the difficulties are compounded by the need for a large number of model replicates to estimate sample-based statistical measures, specifically the covariances and cross-covariances that directly impact the spread of information from the measurement locations to the model parameters. Statistical noise resulting from modest ensemble sizes can overwhelm and degrade the model updates leading to geologically inconsistent subsurface models. In this work we propose to address the difficulties in the implementation of the ensemble Kalman filter (EnKF) for operational data integration problems. The methods described here use streamline-derived information to identify regions within the reservoir that will have a maximum impact on the dynamic response. This is achieved through spatial localization of the sample-based cross-covariance estimates between the measurements and the model unknowns using streamline trajectories. We illustrate the approach with a synthetic example and a large field-study that demonstrate the difficulties with the traditional EnKF implementation. In both the numerical experiments, it is shown that these challenges are addressed using flow relevant conditioning of the cross-covariance matrix. By mitigating sampling error in the cross-covariance estimates, the proposed approach provides significant computational savings through the use of modest ensemble sizes, and consequently offers the opportunity for use with large field-scale groundwater and reservoir characterization studies.     

Alteration patterns of chromian spinels from La Cabaña peridotite,south-central Chile

La Cabaña peridotite is part of a dismembered ophiolite complex located within the metamorphic basement of the Coastal Cordillera of south-central Chile, and is the only location in Chile were Cr-spinels have been described so far. The La Cabaña peridotite is part of the Western Series unit, which comprises meta-sedimentary rocks, metabasites, and serpentinized ultramafic rocks. This unit has been affected by greenschist-facies metamorphism with reported peak PT conditions of 7.0–9.3 kbar and 380°–420 °C. Within La Cabaña peridotite Cr-spinels are present in two localities: Lavanderos and Centinela Bajo. In Lavanderos, Cr-spinel occurs in small chromitite pods and as accessory/disseminated grains with a porous or spongy texture in serpentinite, whereas in Centinela Bajo Cr-spinel is present as accessory zoned grains in partly serpentinized dunites, and in chromitite blocks. All Cr-spinels display variable degrees of alteration to Fe²⁺-rich chromite with a variation trend of major elements from chromite to Fe²⁺-rich chromite similar to those observed in other locations, i.e., an increase in Fe₂O₃ and FeO, a decrease in Al₂O₃ and MgO. Cr₂O₃ content increases from chromite to Fe²⁺-rich chromite in chromitite pods from Lavanderos and chromitite blocks from Centinela Bajo, but decreases in ferrian chromite zones in accessory grains from Centinela Bajo. The minor element (Ti, V, Zn, Ni) content is mostly low and does not exceed 0.4 wt.%, with the exception of MnO (<0.9 wt.%), which shows a correspondence with increasing degree of alteration. Cr# (Cr/Cr?+?Al) versus Mg# (Mg/Mg?+?Fe²⁺) and Fe³⁺/Fe³⁺+Fe²⁺ versus Mg# plots are used to illustrate the Cr-spinel alteration process. Overall, the Cr-spinels from Lavanderos (chromitite pods and disseminated grains) exhibit Cr# values ranging from 0.6 to 1.0, Mg# (Mg/Mg?+?Fe²⁺) below 0.5, and (Fe³⁺/Fe³⁺+Fe²⁺) <0.4. Cr-spinels from chromitites in Centinela Bajo have Cr# and Mg# values that range from 0.65 to 1.0, and 0.7-0.3, respectively, and (Fe³⁺/Fe³⁺+Fe²⁺)?3+/Fe³⁺+Fe²⁺) ratio is less than 0.4 in chromite cores and Fe²⁺-rich chromite, and >0.5 in ferrian chromite and Cr-magnetite. Interpretation of the data obtained and Cr-spinel textures indicate that the alteration of Cr-spinel is a progressive process that involves in its initial stages the reaction of chromite with olivine under water-saturated conditions to produce clinochlore and Fe²⁺-rich chromite. During this stage the chromite can also incorporate Ni, Mn, and/or Zn from the serpentinization fluids. As alteration progresses, Fe²⁺-rich chromite loses mass resulting in the development of a spongy texture. In a later stage and under more oxidizing conditions Fe³⁺ is incorporated in chromite/Fe²⁺-rich chromite shifting its composition to an Fe³⁺-rich chromite (i.e., ferrian chromite). Depending on the fluid/rock and Cr-spinel/silicate ratios, Cr-magnetite can also form over Fe²⁺-rich chromite and/or ferrian chromite as a secondary overgrowth. The compositional changes observed in Cr-spinels from La Cabaña reflect the initial stages of alteration under serpentinization conditions. Results from this study show that the alteration of Cr-spinels is dependent on temperature. The degree and extent of alteration (formation of Fe²⁺-rich and/or ferrian chromite) are controlled by the redox nature of the fluids, the Cr-spinel/silicate and the fluid/rock ratios.     

Laboratory study of a shear instability of an alongshore sea current

A laboratory study of the alongshore current shear instability in a rotating and nonrotating homogeneous fluid was carried out with special attention paid to the conditions of the coherent eddy structure formation in a shear flow. The cases with both cyclonic and anticyclonic velocity shear between the current core and the coast were reproduced in a wide range of nondimensional velocity shear variations. No coherent eddy-like structure formation was observed in the nonrotating fluid; the flow was always chaotic or turbulent. However, chains of coherent eddy-like structures were formed in the rotating fluid in the case with cyclonic velocity shear in a broad range of its variations. In the case with anticyclonic velocity shear, a chain of eddies was observed only when the velocity shear was quite low. When it was high, the flow was chaotic or turbulent. A physical model that explained the asymmetry in the conditions of the coherent eddy-like structure formation in the rotating fluid with cyclonic and anticyclonic velocity shear was considered. The laboratory results agreed with the observations of coherent submesoscale eddies in the Black Sea coastal zone.