Rising methane gas bubbles form massive hydrate layers at the seafloor

Extensive methane hydrate layers are formed in the near-surface sediments of the Cascadia margin. An undissociated section of such a layer was recovered at the base of a gravity core (i.e. at a sediment depth of 120 cm) at the southern summit of Hydrate Ridge. As a result of salt exclusion during methane hydrate formation, the associated pore waters show a highly elevated chloride concentration of 809 mM. In comparison, the average background value is 543 mM. A simple transport-reaction model was developed to reproduce the Cl- observations and quantify processes such as hydrate formation, methane demand, and fluid flow. From this first field observation of a positive Cl- anomaly, high hydrate formation rates (0.15–1.08 mol cm-2 a-1) were calculated. Our model results also suggest that the fluid flow rate at the Cascadia accretionary margin is constrained to 45–300 cm a-1. The amount of methane needed to build up enough methane hydrate to produce the observed chloride enrichment exceeds the methane solubility in pore water. Thus, most of the gas hydrate was most likely formed from ascending methane gas bubbles rather than solely from CH4 dissolved in the pore water

Effect of pulsed laser irradiation on the SiC surface

The effect of a pulsed laser irradiation (Nd:YVO4, 1064 nm) in air on the Surface morphology and chemical composition of silicon carbide and on the adhesion with an epoxy adhesive was investigated. Scanning and transmission electron microscopies, atomic force microscopy, and X-ray photoelectron spectroscopy revealed that the laser treatment reduced the contamination level of the Surface and induced the formation of a silica-based nanostructured colum nar layer on the SiC surface. The mechanism for the formation of five different microstructural regions is described in this paper. In addition, the formation of a 5-10-nm-thick graphite layer between the oxide layer and SiC was observed. The joining test with Hysol EA9321 showed that the nanostructured columnar silica layer was completely infiltrated by the adhesive, thus leading to a significant increase in the joined specific area and a mecha nical interlocking at the adhesive/substrate interface. Nevertheless, the apparent shear strength of the joined SiC samples slightly decreased after the laser processing of the surfaces from about 42 MPa for lapped SiC to about 35 MPa for laser-nanostructured SiC. The formation of the graphite layer was found to be responsible of the poor adhesion properties of the SiC surfaces modified by the laser radiation

Zukünftige kommerzielle Nutzung von Methanhydratvorkommen im Meeresboden

Commercial utilization of methane hydrate deposits in the seabed: The vast amount of natural gas bound in methane hydrates is considered as future energy resource by a growing number of states and companies in South-East Asia and North America. Successful field production tests showed that gas hydrates can be dissociated in the sub-surface by heat addition and pressure reduction while the released gas is produced via conventional drill wells. Laboratory studies demonstrate that CO2 from coal power plants can be applied to liberate methane from the hydrate structure and produce natural gas while the injected CO2 is safely stored as hydrate in the sub-surface. The commercial exploitation of sub-seabed gas hydrates may start in the next decade pending on the success of field production tests off Japan scheduled for 2012 and 2014. Specific environmental risks are associated with the future utilization of gas hydrates. These include the extinction of special benthic ecosystems relying on methane from hydrates as energy source, the triggering of slope failure, and leakage of greenhouse gases into the marine environment. Suitable measures have to be taken to avoid these risks. An appropriate legal framework should be established at the international level to meet the specific challenges and risks associated with the commercial use of gas hydrates in the marine environment

TriHex: combining formation flying, general circular orbits and alias-free imaging, for high resolution L-band aperture synthesis

The Soil Moisture and Ocean Salinity (SMOS) mission of the European Space Agency (ESA), together with NASA’s Soil Moisture Active Passive (SMAP) mission, is providing a wealth of information to the user community for a wide range of applications. Although both missions are still operational, they have significantly exceeded their design life time. For this reason, ESA is looking at future mission concepts, which would adequately address the requirements of the passive L-band community beyond SMOS and SMAP. This article proposes one mission concept, TriHex, which has been found capable of achieving high spatial resolution, radiometric resolution, and accuracy, approaching the user needs. This is possible by the combination of aperture synthesis, formation flying, the use of general circular orbits, and alias-free imaging.Peer ReviewedPostprint (author's final draft

Lessons from a one-year hospital-based surveillance of acute respiratory infections in Berlin- comparing case definitions to monitor influenza

<p>Abstract</p> <p>Background</p> <p>Surveillance of severe acute respiratory infections (SARI) in sentinel hospitals is recommended to estimate the burden of severe influenza-cases. Therefore, we monitored patients admitted with respiratory infections (RI) in 9 Berlin hospitals from 7.12.2009 to 12.12.2010 according to different case definitions (CD) and determined the proportion of cases with influenza A(H1N1)pdm09 (pH1N1). We compared the sensitivity and specificity of CD for capturing pandemic pH1N1 cases.</p> <p>Methods</p> <p>We established an RI-surveillance restricted to adults aged ≤ 65 years within the framework of a pH1N1 vaccine effectiveness study, which required active identification of RI-cases. The hospital information-system was screened daily for newly admitted RI-patients. Nasopharyngeal swabs from consenting patients were tested by PCR for influenza-virus subtypes. Four clinical CD were compared in terms of capturing pH1N1-positives among hospitalized RI-patients by applying sensitivity and specificity analyses. The broadest case definition (CD1) was used for inclusion of RI-cases; the narrowest case definition (CD4) was identical to the SARI case definition recommended by ECDC/WHO.</p> <p>Results</p> <p>Over the study period, we identified 1,025 RI-cases, of which 283 (28%) met the ECDC/WHO SARI case definition. The percentage of SARI-cases among internal medicine admissions decreased from 3.2% (calendar-week 50-2009) to 0.2% (week 25-2010). Of 354 patients tested by PCR, 20 (6%) were pH1N1-positive. Two case definitions narrower than CD1 but -in contrast to SARI- not requiring shortness of breath yielded the largest areas under the Receiver-Operator-Curve. Heterogeneity of proportions of patients admitted with RI between hospitals was significant.</p> <p>Conclusions</p> <p>Comprehensive surveillance of RI cases was feasible in a network of community hospitals. In most settings, several hospitals should be included to ensure representativeness. Although misclassification resulting from failure to obtain symptoms in the hospital information-system cannot be ruled out, a high proportion of hospitalized PCR-positive pH1N1-patients (45%) did not fulfil the SARI case-definition that included shortness of breath or difficulty breathing. Thus, to assess influenza-related disease burden in hospitals, broader, alternative case definitions should be considered.</p

Simulation of the Effect of Improved Ground Clocks on GPS Timing Performance

The GPS system is simulated in three scenarios, so as to estimate how its performance would be improved by more stable ground clocks. One scenario is an approximation to the current system. The second replaces the cesium frequency standards at the monitor sites with masers. The third replaces the masers at the two USNO-controlled monitor sites with atomic fountains. The simplifications exaggerate the differences between the scenarios, and the results suggest that the three scenarios are similar in the timing information delivered to the user with broadcast parameters for positioning, but that GPS Time could be more stable with improved clocks. Results are tentative, pending more realistic simulations

Usign sign patterns to distinguish feared clock events

The first and second difference of clock measurements is investigated to detect and identify clock events. A hypothesis test of the difference functions is described which is calibrated by given Allan deviations of the observed clock differences. The impact of four defined clock events, namely time step, frequency step, drift step and outlier on the first and second differences is calculated. The analysis outlines that the sign of these calculations generate a pattern of “1”,”0” and “-1” which are called event patterns. In order to identify the underlying event type, the identification method interprets the detection process again as a sequence of “1”, “0” and “-1” and maps the event patterns to the detection patterns. The identification and detection method are assessed by analyzing the time offset measurements between a commercial Rubidium frequency standard and an active Hydrogen maser. Both clocks are operated at the DLR timing laboratory

Prototype of the DLR operational composite clock: methods and test cases

The operational implementation of the ensemble Kalman filter Composite Clock (CC developed by K. R. Brown), is subject of the paper. Although the mathematical background of the CC is well-known, its autonomous and robust operation in a clock laboratory requires further modifications of the CC. The suitable initialization of the first ensemble estimate is discussed. Due to the fact, that only difference measurements are available, there is a dimensional degree of freedom in the first as in the following ensemble estimates. Furthermore, to guarantee a robust, consistent, and autonomous estimation of the ensemble clocks, a consistency check is described. It outputs a so-called consistency matrix which describes the active and nonactive clocks. Since the identified active clocks can change from measurement to measurement, the corresponding Kalman filter (KF) parameters are adapted in a consistent way. Finally, the non-active clocks are estimated based on the KF output. The performance of the OCC using an ensemble of five DLR laboratory clocks, measured over a period of one year, is outlined. The clock ensemble consists of three high-performance Cesiumclocks (HP5071A), an active H-Maser (CH1-75) and a GPS disciplined Rb clock. Although miscellaneous anomalies like phase steps, outliers and clock exclusions arise, the OCC autonomously processes the five ensemble clocks and establishes a robust system time. Analysis results of this ensemble with different types of clocks when applying the OCC are presented

Simulating Future GPS Clock Scenarios With Two Composite Clock Algorithms

Using the GPS Toolkit, the GPS constellation is simulated using thirty-one satellites (SV) and a ground network of seventeen monitor stations (MS). At every 15 minute measurement epoch, the monitor stations measure the time signals of all satellites above a parameterized elevation angle. Once a day, the satellite clock estimates are uploaded to the satellites. Two composite clock algorithms are applied to estimate the station and satellite clocks. The first composite clock (B) is based on the Brown algorithm [1], and is now used by GPS. The second one (G) is based on the Greenhall algorithm [2]. The composite clock of G and B performances are investigated using three ground clock models. Model C simulates the current GPS configuration, in which all stations are equipped with cesium clocks, except for masers at USNO and Alternate Master Clock (AMC) sites. Model M is an improved situation in which every station is equipped with active hydrogen masers. Finally, Model F is a future scenario in which the USNO and AMC stations are equipped with fountain clocks instead of masers. Each model is evaluated using three performance metrics. The timing related user range error having all satellites available is the first performance index (PI1). The second performance index (PI2) relates to the stability of the broadcast GPS system time itself. The third performance index (PI3) evaluates the stability of the time scales computed by the two composite clocks