Geodetic techniques applied to the study of the Etna volcano area (Italy)     

Santo La Delfa  Monia Negusini  Sabrina Di Martino  Giuseppe Patanè 《International Journal of Earth Sciences》2012,101(4):1065-1075

Volcanic behaviour of Mt. Etna is due to the complex interaction between both the local and the regional stress field involving the eastern Sicily. Eruptions could trigger (be triggered?) during crust extension and/or compression, which are strictly linked with the dynamics of the lower mantle. In this study, very long baseline interferometry (VLBI) space geodesy technique has been used in order to study Etna volcano’s activity by means of the crustal deformations between Noto and Matera (located on the African and the Eurasian Plates, respectively). By analysing VLBI data, we obtained the behaviour of the baseline which crosses the Etnean area, from 1990 December to 2003 March, representing the time variations of the distance between the two geodetic stations; the linear trend of the baseline shows a general increasing, pointing out an extension of the crust between them. A detailed analysis of the Noto-Matera baseline allows the identification of three parts of the VLBI curve in the considered period. In the first part of the curve (from 20/12/90 to 09/02/94), VLBI data are rather poor and therefore no reliable consideration about correlation between crust movements and volcanic and seismicity activity has been made. In the second part of the curve (from 09/02/94 to 04/09/00), VLBI data are more frequent and show slightly fluctuations in the distance. Increasing in the extension and compression were observed in the central and in the final part of this period. In the third period (from 04/09/00 to 25/03/03), VLBI data are very sparse even if the time series was quite long; therefore, to fill gaps in the information, we analysed global positioning system (GPS) data. GPS technique performs continuous observations, and we were able to highlight both extensions and compressions in detail. Comparisons between the trend of Noto-Matera baseline length variations, volcanic activity and seismicity in the Etna area show the complexity of the development over time and space of the phenomenology determined by a deep cause which can be traced, in our opinion, to the interaction between the asthenospheric mantle, deep crust and surface crust. Therefore, we state that crustal distension and compression are determined by the lower pulsating mantle.  相似文献   

Preliminary report on a dinosaur tracksite from Lower Cretaceous strata in Mount Lebanon     

Raymond Gèze  Isabelle Veltz  Jean-Claude Paicheler  Bruno Granier  Roland Habchi  Dany Azar  Sibelle Maksoud 《Arabian Journal of Geosciences》2016,9(19):730

A dinosaur tracksite was discovered in Batha on the side of the road from Harissa to Ghosta (Keserwan, Lebanon). About ten trackways are exposed at the top of two beds of Lower Cretaceous limestones over an area of approximately 1000 m². These footprints were probably made by at least two dinosaur species, Sauropoda and either Theropoda or Ornithopoda. The site, which is the first record from Lebanon, should be protected to allow further scientific investigations.  相似文献   

Porosity and mineralogy evolution during the decay process involved in the Chellah monument stones     

Abderrahim Samaouali  Larbi Laânab  Mohamed Boukalouch  Yves Geraud 《Environmental Earth Sciences》2010,59(6):1171-1181

The objective of this work is to study the decay process involved in the historical Roman Chellah located in the Rabat city (Morocco). This monument is made up of porous calcarenite stone. Several samples, taken from altered and unaltered blocks, were analyzed by the water saturation, the mercury intrusion porosimetry techniques and using the scanning electron microscopy coupled to energy dispersive X-ray spectrometer (SEM–EDX). To perform a reliable chemical analysis, some samples were also analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The mercury porosimetry results show a bimodal porous network for this porous material, the deterioration process of these stones involved an increase in porous volume of about 2%. The lowest porosity observed in the unaltered block is connected to the presence of sparitic cement which causes a partial inter-granular porosity clogging. The highest porosity of the deteriorated block without crust is due to the increase in inter-granular space. SEM photographs of the unaltered sample show the presence of the porous primary grains, of ovoid forms and millimeter-length sizes, and of the secondary grains, of rhombohedric forms and micro size. Porosity is essentially located between the primary grains and can be completely clogged by secondary precipitations. Various forms of deterioration are observed on the altered samples such as the dissolution of the secondary grains edges, wells of dissolution and also the presence of argillaceous residues on the surface. This last was also detected by the ICP-AES and EDX analysis which show an increase of the silicon and aluminum contents toward the surface.  相似文献   

A double seismic antenna experiment at teide Volcano: existence of local seismicity and lack of evidences of Volcanic tremor     

J. Almendros  J. M. Ibez  G. Alguacil  J. Morales  E. Del Pezzo  M. La Rocca  R. Ortiz  V. Araa  M. J. Blanco 《Journal of Volcanology and Geothermal Research》2000,103(1-4)

Data analyzed in the present work correspond to a 40 days field experiment carried out in Teide Volcano (Canary Islands, Spain) with two short-period small-aperture dense seismic antennas in 1994. The objective of this experiment was to detect, analyze and locate the local seismicity. We analyzed also the background seismic noise to investigate the possible presence of volcanic tremor. From a set of 76 events, we selected 21 of them in base of their good signal-to-noise ratio and their possibility to locate their seismic source by using the seismic antennas. A visual classification based on the S–P time and seismogram shape has permitted to establish three groups of events: local seismicity (S–P time between 3 and 5 s), very local earthquakes (S–P time smaller than 3 s) and artificial explosions. These earthquakes have been located by applying the Zero Lag Cross-Correlation technique and the inverse ray-tracing procedure. Those earthquakes that were recorded simultaneously by both seismic antennas were also located by intersecting both back-azimuths. The analysis of the seismicity has revealed that the amount of seismicity in Teide Volcano is moderate. This seismicity could be distributed in three main areas: inside the Caldera Edifice (below the Teide–Pico Viejo complex), in the eastern border of the Caldera Edifice and offshore of the island. At present, this activity is the only indicator of the volcano dynamics. The analysis of the back-ground seismic noise has revealed that at frequencies lower than 2 Hz, the Oceanic Load signal is predominant over other signals, even over local earthquakes with a magnitude of 2.0. Due to this, although if in the Teide area were present a weak volcanic tremor, or other volcanic signals with predominant peaks below 2 Hz, to observe them would be a very difficult task.  相似文献   

西藏高原汛期降水类型的研究   总被引：18，自引：0，他引：18  

周顺武  普布卓玛  假拉 《气象》2000,26(5):39-43

利用西藏高原２６个测站２６年（１９７３￣１９９８年）汛期（５￣９月）降水量资料,采用主成分分析和旋转主成分分析方法,对高原汛期降水空间分布型进行了分析。结果表明,主成分分解得到的降水空间分布形式较为集中,前３个特征向量场的分布型具有十分明确的物理意义,可表示降水场部方差的６３．１４％。旋转主成分分解生前６个载荷向量的累积方差贡献达７６．６７％,可较好反映西藏高原汛期降水６个异常敏感区：东南部、东北  相似文献   

The contribution of faint active galactic nuclei to the hard X-ray background     

F. Fiore  F. La Franca  P. Giommi  M. Elvis  G. Matt  A. Comastri  S. Molendi  I. Gioia 《Monthly notices of the Royal Astronomical Society》1999,306(4):L55-L60

Hard X-ray selection is the most efficient way to discriminate between accretion-powered sources, such as active galactic nuclei (AGN), and sources dominated by starlight. Hard X-rays are also less affected than other bands by obscuration. We have therefore carried out the BeppoSAX High Energy Large Area Survey (HELLAS) in the largely unexplored 5–10 keV band, finding 180 sources in ∼50 deg² of sky with flux≳5×10⁻¹⁴ erg cm⁻² s⁻¹. After correction for the non-uniform sky coverage this corresponds to resolving about 30 per cent of the hard cosmic X-ray background (XRB). Here we report on a first optical spectroscopic identification campaign, finding 12 AGN out of 14 X-ray error boxes studied. Seven AGN show evidence for obscuration in X-ray and optical bands, a fraction higher than in previous ROSAT or ASCA – ROSAT surveys (at 95–99 and 90 per cent confidence levels respectively), thus supporting the scenario in which a significant fraction of the XRB is created by obscured AGN.  相似文献   

贵州普定后寨地下河流域岩溶水特征研究   总被引：23，自引：1，他引：22  

王腊春  许有鹏  张立峰  史运良  汪文富 《地理科学》2000,20(6):557-562

岩溶地区由于独特的地貌地质结构,其岩溶水特也具有特殊性和复杂性,以贵州普定后寨地下河流域为例,从岩溶水形成机制出发,通过脉冲试验和降雨天然脉冲分析,论述了流域岩溶水水文特征、水化学特征以及其与流域贮水结构、地貌地质结构的相线关系以及隙流水和管流水的转化关系,这对非均一含水介质岩溶水运动研究、岩溶水文模拟及岩水资源开发利用均有一定的参考意义。  相似文献   

A single vs. double spike approach to improve the accuracy of Th measurements in small-volume seawater samples     

Alessia M. Rodriguez y Baena  Juan Carlos Miquel  Pere Masqu  Pavel P. Povinec  Jerome La Rosa 《Marine Chemistry》2006,100(3-4):269

²³⁴Th is a particle-reactive radionuclide widely used to trace biogeochemical oceanic processes occurring over short timescales. During the last few years, small-volume techniques based on the co-precipitation of ²³⁴Th with MnO₂ coupled with beta-counting have been developed as an alternative to large volume gamma-spectrometric techniques. Here a procedure has been developed to enhance quantitative measurement of ²³⁴Th in MnO₂ precipitates. The main objectives were to obtain a purified Th fraction for beta-counting and to determine the chemical recovery of ²³⁴Th using Th spikes and alpha-spectrometry as an alternative to ICP-MS based methods. Two variations of the procedure are presented. In the first “1 spike” method a ²³⁰Th tracer is added to the sample prior to precipitation of MnO₂, and UTEVA® extraction chromatography is used to obtain a NdF₃(Th) purified source that can be used for both beta-counting of ²³⁴Th and alpha-spectrometry of ²³⁰Th. In the “2 spike” method a ²³⁰Th spike is added and the MnO₂(Th) precipitate is directly beta-counted for ²³⁴Th and subsequently spiked with ²²⁸Th or ²²⁹Th prior to UTEVA® purification and alpha-spectrometry. The results confirm the need to process small-volume seawater samples for ²³⁴Th measurement in presence of a yield tracer, and show that both the 1 spike and 2 spike methods allow an accurate and precise determination of ²³⁴Th (relative percent difference, RPD, between expected and mean measured value < 1%; CV between replicate samples < 3%). Our work also suggests that, although the combined analytical uncertainty on total ²³⁴Th measurements accomplished with both versions of the NdF₃ procedure is promising (6% for 2-L samples), the precision of the ²³⁴Th flux estimation will ultimately depend on the degree of disequilibrium between ²³⁴Th and ²³⁸U.  相似文献   

Local and controlled prestack depth migration in complex areas     

Isabelle Lecomte 《Geophysical Prospecting》1999,47(6):799-818

Prestack depth migrations based on wavefield extrapolation may be computationally expensive, especially in 3D. They are also very dependent on the acquisition geometry and are not flexible regarding the geometry of the imaging zone. Moreover, they do not deal with all types of wave, considering only primary reflection events through the model. Integral approaches using precalculated Green's functions, such as Kirchhoff migration and Born-based imaging, may overcome these problems. In the present paper, both finite-difference traveltimes and wavefront construction are used to obtain asymptotic Green's functions, and a generalized diffraction tomography is applied as an example of Born-based acoustic imaging. Target-orientated imaging is easy to perform, from any type of survey and subselection of shot/receiver pairs. Multifield imaging is possible using Green's functions that take into account, for instance, reflections at model boundaries. This may help to recover parts of complex structures which would be missing using a paraxial wave equation approach. Finally, a numerical evaluation of the resolution, or point-spread, function at any point of the depth-migrated section provides valuable information, either at the survey planning stage or for the interpretation.  相似文献   

Site response obtained from array techniques applied to the seismic noise: Two examples in Italy     

R. Maresca  E. Del Pezzo  M. La Rocca  G. Liguori  G. Milana  C. Sabbarese 《Journal of Seismology》1999,3(1):31-43

The vertical component of the seismic noise has been recorded in two different sites near the towns of Mercato S. Severino and Benevento in Southern Italy by a small aperture array, in order to investigate the characteristics of the noise propagation and to study the site response. Three different array techniques have been applied in the two investigated sites: Beam Forming, High Resolution and Spatial Correlation methods. We used two simple array geometry for localising possible noise sources and estimating local shallow structure using ambient noise. The cross shaped array results effective for determining the phase velocity of waves in the case when the noise is from a single localised source; the circular array, on the other hand, is successfully used when the noise sources are distributed. The main results are: the analysis of a coherent component of the noise recorded in the two sites, interpreted as Rayleigh waves, results in reasonable velocity models; the noise recorded in the M.S.S. Plain is a space stationary signal, while the noise at the Benevento site is possibly produced by a stable noise source located close to the array. Due to this evidence, the correlation method does not yield satisfactory results when applied to the Benevento site. The 2–6 Hz spectral peaks of the noise recorded in the M.S.S. Plain can be interpreted as due to a site effect, considering the satisfactory agreement of the noise spectrum at those frequencies with the theoretical transfer function computed on the basis of the velocity model deduced from the Rayleigh waves dispersion analysis.  相似文献