Combining two seismic experiments to attenuate free-surface multiples in OBC data*

The current inverse scattering solution used for multiple attenuation of marine seismic reflection data assumes that sources and receivers are located in the water. To adapt this solution to the ocean-bottom cable (OBC) experiment where receivers are located on the sea-floor, we have proposed combining the conventional marine surface seismic reflection data (streamer data) with OBC data. The streamer data add to the OBC data some of the wave paths needed for multiple attenuation. This combination has allowed us to develop a multiple attenuation method for OBC data which does not require any knowledge of the subsurface and which takes into account all free-surface multiples, including receiver ghosts. A non-linear synthetic data example consisting of pressure and particle velocity fields is used to illustrate the procedure.     

Source and fractionation controls on subduction-related plutons and dike swarms in southern Patagonia (Torres del Paine area) and the low Nb/Ta of upper crustal igneous rocks

The subduction system in southern Patagonia provides direct evidence for the variability of the position of an active continental arc with respect to the subducting plate through time, but the consequences on the arc magmatic record are less well studied. Here we present a geochemical and geochronological study on small plutons and dykes from the upper crust of the southern Patagonian Andes at ~?51°S, which formed as a result of the subduction of the Nazca and Antarctic plates beneath the South American continent. In situ U–Pb geochronology on zircons and bulk rock geochemical data of plutonic and dyke rocks are used to constrain the magmatic evolution of the retro-arc over the last 30 Ma. We demonstrate that these combined U–Pb and geochemical data for magmatic rocks track the temporal and spatial migration of the active arc, and associated retro-arc magmatism. Our dataset indicates that the rear-arc area is characterized by small volumes of alkaline basaltic magmas at 29–30 Ma that are characterized by low La/Nb and Th/Nb ratios with negligible arc signatures. Subsequent progressive eastward migration of the active arc culminated with the emplacement of calc-alkaline plutons and dikes?~?17–16 Ma with elevated La/Nb and Th/Nb ratios and typical subduction signatures constraining the easternmost position of the southern Patagonian batholith at that time. Geochemical data on the post-16 Ma igneous rocks including the Torres del Paine laccolith indicate an evolution to transitional K-rich calc-alkaline magmatism at 12.5?±?0.2 Ma. We show that trace element ratios such as Nb/Ta and Dy/Yb systematically decrease with increasing SiO₂, for both the 17–16 Ma calc-alkaline and the 12–13 Ma K-rich transitional magmatism. In contrast, Th/Nb and La/Nb monitor the changes in the source composition of these magmas. We suggest that the transition from the common calc-alkaline to K-rich transitional magmatism involves a change in the source component, while the trace element ratios, such as Nb/Ta and Dy/Yb, of derivative higher silica content liquids are controlled by similar fractionating mineral assemblages. Analysis of a global compilation of Nb/Ta ratios of arc magmatic rocks and simple geochemical models indicate that amphibole and variable amounts of biotite exert a major control on the low Dy/Yb and Nb/Ta of derivative granitic liquids. Lastly, we suggest that the low Nb/Ta ratio of silica-rich magmas is a natural consequence of biotite fractionation and that alternative models such as amphibolite melting in subduction zones and diffusive fractionation are not required to explain the Nb/Ta ratio of the upper continental crust.     

Reconstruction of the surface-layer vertical structure from measurements of wind,temperature and humidity at two levels

We present a comparison between several methods used to reconstruct fluxes and vertical profiles of wind, temperature and humidity from measurements at two levels in the atmospheric surface layer for different practical applications. An analytical method and an iterative method are tested by evaluating the quality of estimations of surface fluxes from detailed field measurements obtained during a campaign on the site of Lannemezan in the south-west of France. The iterative method yields better results, but the analytical one can give results of the same level of accuracy provided that specific constants in its formulation are modified. Then these techniques are applied to wind and temperature reconstruction for an experiment dedicated to wind power estimates over flat terrain. If turbulent fluxes are not needed, a simple power law appears to be sufficient, as the method based on Monin–Obukhov theory does not improve the accuracy of the vertical profile reconstruction.     

A gamma‐ray spectroscopy survey of Omani meteorites

The gamma‐ray activities of 33 meteorite samples (30 ordinary chondrites, 1 Mars meteorite, 1 iron, 1 howardite) collected during Omani‐Swiss meteorite search campaigns 2001–2008 were nondestructively measured using an ultralow background gamma‐ray detector. The results provide several types of information: Potassium and thorium concentrations were found to range within typical values for the meteorite types. Similar mean ²⁶Al activities in groups of ordinary chondrites with (1) weathering degrees W0‐1 and low ¹⁴C terrestrial age and (2) weathering degree W3‐4 and high ¹⁴C terrestrial age are mostly consistent with activities observed in recent falls. The older group shows no significant depletion in ²⁶Al. Among the least weathered samples, one meteorite (SaU 424) was found to contain detectable ²²Na identifying it as a recent fall close to the year 2000. Based on an estimate of the surface area searched, the corresponding fall rate is ~120 events/10⁶ km²*a, consistent with other estimations. Twelve samples from the large JaH 091 strewn field (total mass ~4.5 t) show significant variations of ²⁶Al activities, including the highest values measured, consistent with a meteoroid radius of ~115 cm. Activities of ²³⁸U daughter elements demonstrate terrestrial contamination with ²²⁶Ra and possible loss of ²²²Rn. Recent contamination with small amounts of ¹³⁷Cs is ubiquitous. We conclude that gamma‐ray spectroscopy of a selection of meteorites with low degrees of weathering is particularly useful to detect recent falls among meteorites collected in hot deserts.     

ISO SWS Spectroscopy of WR 146 (WC6+O)

We have obtained ISO SWS spectroscopy of WR 146 (WC6+O) covering the wavelength range 2.6-20 μm. WC6 wind emission is observed in numerous lines of He II and C IV, as well as in the [Ne III] 15.5 μm line, but not in [Ne II] 12.81 or [Ne V] 14.32 μm. An analysis of these spectra (and complementary radio and optical data) yields for the WC6 star: v∞ = 2700 km s-1; M=2.6×10-5M⊙yr-1; C/He = 0.15; and a neon abundance bound of 3.4×10-3≤Ne/He≤6.8×10-3. The neon abundance is close to that predicted in stellar evolution models of WC stars. This revised version was published online in August 2006 with corrections to the Cover Date.     

Opaline concretions in Weichselian Late-glacial lake marl from Flanders, northern Belgium

Macroscopic opal-A concretions were observed in lake marl deposited in a small Flemish lake (Belgium) during the Allerød biozone of the Weichselian Late-glacial (ca. 12–11 ka BP). The silica from these concretions was derived within the profile, by the leaching of siliceous microfossils – mainly diatom frustules. Formation of the concretions probably resulted from pH- and/or evaporation related precipitation of the silica at a lower stratigraphic level, presumably corresponding more or less to a former low position of the groundwater table. The presence of these concretions is probably related to alternatingly wet and dry local conditions during the middle and later part of the Allerød.     

Ionizing Power of Massive Stars in the Cores of Two Giant HII Regions: R136A and HD 97950

We evaluate the integrated ionizing fluxes in cores of giant HII regions below the important continuum edges, considering both O and WR stars. We find that they contribute in equivalent proportions to the HI and HeI continua, but that only the hottest O stars emit in the HeH continuum. This revised version was published online in July 2006 with corrections to the Cover Date.