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Le Pichon A.,CEA DAM Ile-de-France | Ceranna L.,BGR | Pilger C.,BGR | Mialle P.,Provisional Technical Secretariat | And 3 more authors.
Geophysical Research Letters | Year: 2013

On 15 February 2013, a large Earth-impacting fireball disintegrated over the Ural Mountains. This extraordinary event is, together with the 1908 Tunguska fireball, among the most energetic events ever instrumentally recorded. It generated infrasound returns, after circling the globe, at distances up to ~85,000 km, and was detected at 20 infrasonic stations of the global International Monitoring System (IMS). For the first time since the establishment of the IMS infrasound network, multiple arrivals involving waves that traveled twice round the globe have been clearly identified. A preliminary estimate of the explosive energy using empirical period-yield scaling relations gives a value of 460 kt of TNT equivalent. In the context of the future verification of the Comprehensive Nuclear-Test-Ban Treaty, this event provides a prominent milestone for studying in detail infrasound propagation around the globe for almost 3 days as well as for calibrating the performance of the IMS network. © 2013. American Geophysical Union. All Rights Reserved.

Matoza R.S.,CEA DAM Ile-de-France | Matoza R.S.,University of California at San Diego | Landes M.,CEA DAM Ile-de-France | Le Pichon A.,CEA DAM Ile-de-France | And 2 more authors.
Geophysical Research Letters | Year: 2013

The ability of the InternationalMonitoring System (IMS) infrasound network to detect atmospheric nuclear explosions and other signals of interest is strongly dependent on stationspecific ambient noise. This ambient noise includes both incoherent wind noise and real coherent infrasonic waves. Previous ambient infrasound noise models have not distinguished between incoherent and coherent components. We present a first attempt at statistically and systematically characterizing coherent infrasound recorded by the IMS. We perform broadband (0.01-5Hz) array processing with the IMS continuous waveform archive (39 stations from 1 April 2005 to 31 December 2010) using an implementation of the Progressive Multi-Channel Correlation algorithm in logfrequency space. From these results, we estimate multi-year 5th, 50th, and 95th percentiles of the RMS pressure of coherent signals in 15 frequency bands for each station. We compare the resulting coherent infrasound models with raw power spectral density noise models, which inherently include both incoherent and coherent components. Our results indicate that IMS arrays consistently record coherent ambient infrasound across the broad frequency range from 0.01 to 5Hz when wind noise levels permit. The multi-year averaging emphasizes continuous signals such as oceanic microbaroms, as well as persistent transient signals such as repetitive volcanic, surf, thunder, or anthropogenic activity. Systematic characterization of coherent infrasound detection is important for quantifying a station's recording environment, signal-to-noise ratio as a function of frequency and direction, and overall performance, which all influence the detection probability of specific signals of interest. © 2013. American Geophysical Union. All Rights Reserved.

Green D.N.,Atomic Weapons Establishment | Vergoz J.,CEA DAM Ile-de-France | Gibson R.,BBN Technologies | Le Pichon A.,CEA DAM Ile-de-France | Ceranna L.,BGR
Geophysical Journal International | Year: 2011

Infrasound propagation paths through the atmosphere are controlled by the temporally and spatially varying sound speed and wind speed amplitudes. Because of the complexity of atmospheric acoustic propagation it is often difficult to reconcile observed infrasonic arrivals with the sound speed profiles predicted by meteorological specifications. This paper provides analyses of unexpected arrivals recorded in Europe and north Africa from two series of accidental munitions dump explosions, recorded at ranges greater than 1000km: two explosions at Gerdec, Albania, on 2008 March 15 and four explosions at Chelopechene, Bulgaria, on 2008 July 3. The recorded signal characteristics include multiple pulsed arrivals, celerities between 0.24 and 0.34kms-1 and some signal frequency content above 1Hz. Often such characteristics are associated with waves that have propagated within a ground-to-stratosphere waveguide, although the observed celerities extend both above and below the conventional range for stratospheric arrivals. However, state-of-the-art meteorological specifications indicate that either weak, or no, ground-to-stratosphere waveguides are present along the source-to-receiver paths. By incorporating realistic gravity-wave induced horizontal velocity fluctuations into time-domain Parabolic Equation models the pulsed nature of the signals is simulated, and arrival times are predicted to within 30s of the observed values (<1 per cent of the source-to-receiver transit time). Modelling amplitudes is highly dependent upon estimates of the unknown acoustic source strength (or equivalent chemical explosive yield). Current empirical explosive yield relationships, derived from infrasonic amplitude measurements from point-source chemical explosions, suggest that the equivalent chemical yield of the largest Gerdec explosion was of the order of 1kt and the largest Chelopechene explosion was of the order of 100t. When incorporating these assumed yields, the Parabolic Equation simulations predict peak signal amplitudes to within an order of magnitude of the observed values. As gravity wave velocity perturbations can significantly influence both infrasonic arrival times and signal amplitudes they need to be accounted for in source location and yield estimation routines, both of which are important for explosion monitoring, especially in the context of the Comprehensive Nuclear-Test-Ban Treaty. © 2011 Crown copyright Geophysical Journal International © 2011 RAS.

Le Pichon A.,CEA DAM Ile-de-France | Ceranna L.,BGR | Vergoz J.,CEA DAM Ile-de-France
Journal of Geophysical Research: Atmospheres | Year: 2012

To monitor compliance with the Comprehensive Nuclear-Test ban Treaty (CTBT), a dedicated International Monitoring System (IMS) is being deployed. Recent global scale observations recorded by this network confirm that its detection capability is highly variable in space and time. Previous studies estimated the radiated source energy from remote observations using empirical yield-scaling relations which account for the along-path stratospheric winds. Although the empirical wind correction reduces the variance in the explosive energy versus pressure relationship, strong variability remains in the yield estimate. Today, numerical modeling techniques provide a basis to better understand the role of different factors describing the source and the atmosphere that influence propagation predictions. In this study, the effects of the source frequency and the stratospheric wind speed are simulated. In order to characterize fine-scale atmospheric structures which are excluded from the current atmospheric specifications, model predictions are further enhanced by the addition of perturbation terms. A theoretical attenuation relation is thus developed from massive numerical simulations using the Parabolic Equation method. Compared with previous studies, our approach provides a more realistic physical description of long-range infrasound propagation. We obtain a new relation combining a near-field and a far-field term, which account for the effects of both geometrical spreading and absorption. In the context of the future verification of the CTBT, the derived attenuation relation quantifies the spatial and temporal variability of the IMS infrasound network performance in higher resolution, and will be helpful for the design and prioritizing maintenance of any arbitrary infrasound monitoring network. Copyright 2012 by the American Geophysical Union.

Kaufhold S.,BGR | Dohrmann R.,BGR
Clay Minerals | Year: 2010

Extensive drying of smectites can cause the interlayer space to break down (collapse). This can affect the properties of bentonites as geotechnical barriers of HLRW (highly radioactive waste) repositories. If and to what extent the collapse occurs depends strongly on the type of interlayer cation. In particular K is known to lead to ready dehydration, in contrast to Ca and Mg. In the present study, various bentonites and one illite/smectite clay were dried/heated at 90°C for 1.5 a and in a different experiment at 120°C for 4.5 a and investigated with respect to mineralogical and geochemical changes of the smectite. Smectite alteration after extensive drying was restricted to changes of the exchangeable cations. The CEC decreased by 9% (90°C test) and 14% (120°C test). A slight decrease of exchangeable Na+ was observed following the 90°C test. No significant further decrease was observed after the 120°C test. In contrast, the larger cation exchange capacity (CEC) decrease after the 120°C test could be explained by increased Ca/Mg fixation. A possible mechanism for the observations is presented. © 2010 Mineralogical Society.

Bentonites are supposed to be suitable materials for the production of geotechnical barriers in high level radioactive waste (HLRW) repositories if the bentonites' swelling capacity is maintained at the conditions expected; e.g. in contact with different solutions which may occur during the long term storage. Among all other common cations, K+ is believed to play a special role in contact with smectites because it can be adsorbed irreversibly and may cause collapse of the interlayer space.In the present study 36 different bentonites were reacted with KCl-solutions at 60°C with and without wetting - drying cycles and extensive drying. The study was conducted mainly to study the mechanism of irreversible K+ fixation and the resulting loss of swelling capacity (LOS).The K+ exchange for the originally present cations was to some extend buffered by carbonates (minor components of some bentonites). Such buffer reactions are supposed to be relevant for real HLRW repository situations.The expected decrease of the swelling of the bentonites depended on the adsorption energy of the probe molecules (water, EG=ethyleneglycol, Cu-triene). The results of different methods (XRDEG, water adsorption, CECCu-triene, soda-soluble silica) indicated that the smectite fraction of the products (on average) was composed of approximately 50% swelling K+ smectite, 45% non-swelling collapsed K+ smectite (50% of which with fixed K+), and 5% illite which probably formed by dissolution and precipitation. This illite was detected by the increase of soda-soluble silica. In contrast to the solvation with ethyleneglycol, which only measures the collapsed layers rather than illite particles, the determination of soluble-silica is regarded as optimum for differentiating between collapsed layer domains and real illitization.The number of collapsed layers of the K-smectites depended on the layer charge density (LCD) but not on the tetrahedral charge. A model is proposed which explains the correlation of collapsed interlayers and LCD. © 2010 Elsevier B.V.

One of today's big challenges is to store safely the increasing amount of high-level radioactive waste (HLRW) in the world. In some of the concepts devised for this challenge, bentonite, a natural swelling clay, plays a key role in encasing the canisters containing the waste. The use of bentonite as a geotechnical barrier in HLRW repositories is a new venture; specifications to ensure either optimum performance or that a minimum standard is reached at least do not exist yet. The present study summarizes relevant research and discusses possible HLRW-bentonite specifications. The importance of these specifications for any given repositories has to be assessed on a case by case basis, depending on the concept being employed and any special circumstances for the individual repositories. Ten key issues were identified which were used to discuss bentonite specifications. In some of these key issues the optimum bentonite performance depended more on processing and production (compaction) than on the bentonite type (e.g. swelling pressure and thermal conductivity). In contrast, in some of the other key issues, the type of bentonite was found to influence possible specifications: the bentonite should not alter its mineral composition or its geotechnical parameters such as the swellability. Therefore, the bentonite should contain neither soluble nor reactive phases (e.g. organic matter, pyrite, gypsum). The structural Fe content of the smectites should be small because of the lesser stability and greater reactivity of the Fe-rich bentonites. Also, a large layer-charge density of the swelling clay minerals leads to less corrosion at the iron-bentonite interface (relevant if iron canisters are used). The hydraulic conductivity and swelling pressure can be tailored by compaction of the bentonite resulting in different dry densities. From an engineering point of view, a bentonite with least dependence of the hydraulic conductivity/swelling pressure on the dry density would be best. Using a bentonite which has been investigated extensively over many years means less uncertainty compared to unknown materials. © 2016 Mineralogical Society 2016.

Barckhausen U.,BGR | Engels M.,BGR | Franke D.,BGR | Ladage S.,BGR | Pubellier M.,Laboratoire Of Geologie
Marine and Petroleum Geology | Year: 2014

The continental breakup which gave way to the formation of the oceanic South China Sea (SCS) basin began in the latest Cretaceous in the northeastern SCS and propagated in southern and western direction over a long period of time, possibly more than 40m.y. The seafloor spreading history of the South China Sea has been interpreted in different ways in the past and the debate over the correct timing of the major tectonic events continues. We review the different models that have been published and present a revised interpretation of seafloor spreading anomalies based on three datasets with documented high quality which cover all of the SCS but the northernmost and southernmost parts. We can precisely date the onset of seafloor spreading in the central part of the SCS at 32Ma. After a ridge jump at 25Ma spreading also began in the southwestern sub-basin and spreading ended at 20.5Ma in the entire basin, followed by a phase of magmatic seamount formation mainly along the abandoned spreading ridge. Spreading rates vary from 56mm/yr in the early stages to 72mm/yr after the ridge jump to 80mm/yr in the southwestern sub-basin. We find indications for a stepwise propagation of the seafloor spreading from northeast to southwest in segments bounded by major fracture zones. Seafloor spreading ended abruptly probably because the subduction zone along the eastern and southern boundary of the SCS (of which today the Manila Trench remains) was blocked by collision with a continental fragment, possibly the northern part of Palawan or a part of the Dangerous Grounds. © 2014 Elsevier Ltd.

Kaufhold S.,BGR | Dohrmann R.,BGR
Applied Clay Science | Year: 2011

The prerequisite for the usage of bentonites as a geotechnical barrier in HLRW repositories is the stability of the bentonite under the conditions expected. In addition to high temperature and exposure to radiation, different types of aqueous solutions could also affect the bentonite. In this respect hyperalkaline solutions from Portland cements are considered, because aluminosilicates are known to be soluble under these conditions. The aim of the present study was to compare the 'alkaline-reactivity' of a set of different well characterized bentonites and to identify the reasons for the differences. Theoretically, this would allow for the selection of an optimum bentonite at least with respect to the stability at the cement-bentonite interface. The 'alkaline-reactivity' was characterized both by measuring the dissolved structural elements and analyzing changes of the solid material (e.g. the cation exchange capacity). However, the concentration of the dissolved structural elements could not be compared systematically probably because of the unknown nature and the different amounts of precipitates. As an example, the Si concentration in the alkaline solutions strongly varied from bentonite to bentonite because it depended on the type and amount of SiO2 phases present in the different natural bentonites. Possibly, the associated SiO2 phases governed the Si/(Al+Mg+Fe) ratio in the alkaline solution. Amongst other parameters, this ratio determines which phases precipitate. This ratio obviously depends on the type and amount of SiO2 accessories. Results obtained in the present study were in accordance with published literature. More pronounced dissolution/precipitation processes than found in the present study (60-90°C, 3-5months, saturated solution-excess Ca(OH)2) are expected at even larger pH values, which - on the other hand - do not reflect conditions expected in a bentonite barrier in contact with cement. The most important conclusion from this study is that bentonites are surprisingly resistant against solutions at pH around 12 and up to 90°C. Above this temperature and at larger pH values dissolution is expected to be much faster. Therefore, for a real HLRW repository it seems to be favorable to use low pH cements and keep temperature well below 100°C. © 2010 Elsevier B.V.

Kaufhold S.,BGR | Dohrmann R.,BGR
Journal of Colloid and Interface Science | Year: 2013

One of the most important properties of smectites is the cation exchange capacity resulting both from the structural permanent charge and the pH depending variable charge. The variable charge, in turn, mainly results from the edge aluminol groups (in the case of dioctahedral smectites). The permanent charges can be calculated using the layer charge density; the variable charges, however, were rarely quantified accurately. The present study was conducted to (i) test and compare different methods for the measurement of the variable charge of dioctahedral smectites (mainly montmorillonites), (ii) characterize the range of differences of the variable charge of smectites from different deposits, and (iii) identify the reason for different variable charge values. Considering a 0.1×0.1μm montmorillonite particle, a variable charge of 8meq/100g (about 10% of the CEC) was calculated. Two CEC (Cu trien) based methods provided slightly larger values. These values ranged from 2% to 14% between pH 4 and 6 and from approximately 10% to 30% between pH 4 and 9. With the potentiometric titration method, even larger values for the variable charge were determined (15-35%). Interestingly, both the CEC based methods and the titration method provided comparable trends. Accordingly, any of these methods is suitable to distinguish materials with larger or lower variable charge. One of the reasons for the differences of the variable charge of different bentonites is the variable chemical composition of the smectites, particularly the structural Mg content. Further reasons may be the roughness of the edge surface (not considered in the calculation) and the submicron particle size distribution, which are both difficult to determine. © 2012 Elsevier Inc.

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