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排序方式: 共有597条查询结果,搜索用时 15 毫秒
471.
472.
Matt A. King Zuheir Altamimi Johannes Boehm Machiel Bos Rolf Dach Pedro Elosegui François Fund Manuel Hernández-Pajares David Lavallee Paulo Jorge Mendes Cerveira Nigel Penna Riccardo E. M. Riva Peter Steigenberger Tonie van Dam Luca Vittuari Simon Williams Pascal Willis 《Surveys in Geophysics》2010,31(5):465-507
The provision of accurate models of Glacial Isostatic Adjustment (GIA) is presently a priority need in climate studies, largely due to the potential of the Gravity Recovery and Climate Experiment (GRACE) data to be used to determine accurate and continent-wide assessments of ice mass change and hydrology. However, modelled GIA is uncertain due to insufficient constraints on our knowledge of past glacial changes and to large simplifications in the underlying Earth models. Consequently, we show differences between models that exceed several mm/year in terms of surface displacement for the two major ice sheets: Greenland and Antarctica. Geodetic measurements of surface displacement offer the potential for new constraints to be made on GIA models, especially when they are used to improve structural features of the Earth’s interior as to allow for a more realistic reconstruction of the glaciation history. We present the distribution of presently available campaign and continuous geodetic measurements in Greenland and Antarctica and summarise surface velocities published to date, showing substantial disagreement between techniques and GIA models alike. We review the current state-of-the-art in ground-based geodesy (GPS, VLBI, DORIS, SLR) in determining accurate and precise surface velocities. In particular, we focus on known areas of need in GPS observation level models and the terrestrial reference frame in order to advance geodetic observation precision/accuracy toward 0.1 mm/year and therefore further constrain models of GIA and subsequent present-day ice mass change estimates. 相似文献
473.
Zhongqiang Liu Graham Gilbert Jose Mauricio Cepeda Asgeir Olaf Kydland Lysdahl Luca Piciullo Heidi Hefre Suzanne Lacasse 《地学前缘(英文版)》2021,12(1):385-393
This paper introduces three machine learning(ML)algorithms,the‘ensemble'Random Forest(RF),the‘ensemble'Gradient Boosted Regression Tree(GBRT)and the Multi Layer Perceptron neural network(MLP)and applies them to the spatial modelling of shallow landslides near Kvam in Norway.In the development of the ML models,a total of 11 significant landslide controlling factors were selected.The controlling factors relate to the geomorphology,geology,geo-environment and anthropogenic effects:slope angle,aspect,plan curvature,profile curvature,flow accumulation,flow direction,distance to rivers,water content,saturation,rainfall and distance to roads.It is observed that slope angle was the most significant controlling factor in the ML analyses.The performance of the three ML models was evaluated quantitatively based on the Receiver Operating Characteristic(ROC)analysis.The results show that the‘ensemble'GBRT machine learning model yielded the most promising results for the spatial prediction of shallow landslides,with a 95%probability of landslide detection and 87%prediction efficiency. 相似文献
474.
Ksenia Tlatova Andrey Tlatov Alexei Pevtsov Kalevi Mursula Valeria Vasil’eva Elina Heikkinen Luca Bertello Alexander Pevtsov Ilpo Virtanen Nina Karachik 《Solar physics》2018,293(8):118
We use recently digitized sunspot drawings from Mount Wilson Observatory to investigate the latitudinal dependence of tilt angles of active regions and its change with solar cycle. The drawings cover the period from 1917 to present and contain information as regards polarity and strength of magnetic field in sunspots. We identified clusters of sunspots of same polarity, and used these clusters to form “bipole pairs”. The orientation of these bipole pairs was used to measure their tilts. We find that the latitudinal profile of tilts does not monotonically increase with latitude as most previous studies assumed, but instead, it shows a clear maximum at about 25?–?30 degree latitudes. Functional dependence of tilt (\(\gamma\)) on latitude (\(\varphi\)) was found to be \(\gamma= (0.20\pm0.08) \sin(2.80 \varphi) + (-0.00\pm0.06)\). We also find that latitudinal dependence of tilts varies from one solar cycle to another, but larger tilts do not seem to result in stronger solar cycles. Finally, we find the presence of a systematic offset in tilt of active regions (non-zero tilts at the equator), with odd cycles exhibiting negative offset and even cycles showing the positive offset. 相似文献
475.
Pietro Bolli Luca Cresci Frederick Huang Sergio Mariotti Dario Panella 《Experimental Astronomy》2018,45(2):165-176
A High Temperature Superconductor filter operating in the C-band between 4200 and 5600 MHz has been developed for one of the radio astronomical receivers of the Sardinia Radio Telescope. The motivation was to attenuate an interference from a weather radar at 5640 MHz, whose power level exceeds the linear region of the first active stages of the receiver. A very sharp transition after the nominal maximum passband frequency is reached by combining a 6th order band-pass filter with a 6th order stop-band. This solution is competitive with an alternative layout based on a cascaded triplet filter. Three units of the filter have been measured with two different calibration approaches to investigate pros and cons of each, and data repeatability. The final performance figures of the filters are: ohmic losses of the order of 0.15–0.25 dB, matching better than ?15 dB, and ?30 dB attenuation at 5640 MHz. Finally, a more accurate model of the connection between external connector and microstrip shows a better agreement between simulations and experimental data. 相似文献
476.
Nicolò Incerpi Luca Martire Gianreto Manatschal Stefano M. Bernasconi Axel Gerdes György Czuppon László Palcsu Garry D. Karner Christopher A. Johnson Patricio H. Figueredo 《Basin Research》2020,32(1):91-115
This paper investigates the hydrothermal fluid circulation that was linked to the extensional evolution of the Adriatic rifted margin during the Jurassic opening of the Alpine Tethys. Remnants of this rifted margin are spectacularly preserved in SE Switzerland and N Italy. Five study areas were chosen ranging from the former proximal to the most distal part of the margin. We demonstrate an intimate link between Jurassic extensional tectonics and fluid activity affecting the pre- to early post-rift sedimentary sequences. Nature, composition and origin of fluids are constrained by a multidisciplinary approach based on field observations and including geochemical (O-C, Sr, He isotopes, U-Pb datings, fluid inclusion microthermometry) and petrological methods. Several fluid-related diagenetic products and processes such as dolomitization, veining, hydraulic brecciation, and silicification can be recognized. It appears that different paleogeographic settings and different stratigraphic levels document distinct phases of fluid activity. The fluids reached temperatures of up to 150°C near paleo-seafloor. They were enriched in 18O, had high 87Sr/86Sr and low 3He/4He ratios, documenting a strong interaction between seawater and a granitic basement. Many lines of evidence point to the occurrence of over-pressured fluids and long-lasting fluid circulation due to fault-valve mechanisms and high thermal gradients. Two main stages with different fluid chemistry can be recognized: (1) a carbonate-rich stage that developed during the stretching phases and was linked to high-angle normal faulting, and (2) a silica-rich stage occurring during late rift exhumation of crustal and mantle rocks in the distal domains in the presence of detachment faults and high thermal gradients. This paper provides, for the first time, a large and robust characterization of fluid–rock interactions occurring during rifting along an almost complete section across a magma-poor rifted margin. 相似文献
477.
Christian Ferrarin John Rapaglia Luca Zaggia Georg Umgiesser Giovanni Maria Zuppi 《Marine Chemistry》2008,112(3-4):179-188
A mass balance of the naturally occurring short-lived radium isotopes (223,224Ra) in the Venice Lagoon was conducted by an integrated approach combining the directly estimated individual Ra contributions and hydrodynamic model results. Hydrodynamic data allows for the calculation of the Ra mass balance in sub-sections of the Venice Lagoon (boxes), which are characterized by physically homogeneous properties, instead of investigating the entire lagoon. Utilizing this method, both the seasonal and the spatial variability of the submarine groundwater discharge in the Venice Lagoon have been estimated. Between 14–83 × 109 L d− 1 of water were calculated to flow across the sediment–sea interface, corresponding to 5–28 times the mean annual river input. The submarine groundwater discharge estimates were correlated with the residence time calculation to better understand spatial and seasonal variation. 相似文献
478.
This paper presents a conceptual model for the prediction of energy transmission in presence of emergent permeable low crested structures. The transmitted wave is reconstructed from the superposition of perturbations generated leeward the structure by filtration and overtopped volumes. The model requires only structure geometry and incident wave conditions (wave height, period). A fair agreement is obtained by comparing the predicted transmission coefficients and wave spectra with measurements performed in wave flume and wave basin. 相似文献
479.
Luca Barale Carlo Bertok Anna d’Atri Gabriele Domini Luca Martire Fabrizio Piana 《Comptes Rendus Geoscience》2013,345(1):47-53
The present article illustrates a straightforward case of hydrothermal dolomitization, affecting Jurassic platform limestones of the Provençal and Subbriançonnais Domains (Maritime Alps, North-Western Italy). Dolomitized bodies are randomly distributed within the host limestone, and are commonly associated with dolomite vein networks and tabular bodies of dolomite-cemented breccias discordant with respect to bedding. Main dolomite types are a finely to medium-crystalline replacive dolomite and a coarsely-crystalline saddle dolomite occurring both as replacive and as cement. Stratigraphic constraints indicate that dolomitization occurred during the Cretaceous, in a shallow burial context, and was due to the circulation of hot fluids (temperature about 200 °C, as indicated by fluid inclusion microthermometry) through faults and related fracture networks. Hydrothermal dolomitization therefore indirectly documents a Cretaceous fault activity in the Maritime Alps segment of the European Tethyan passive margin. 相似文献
480.
Correlation between nitrate concentration in groundwater and parameters affecting aquifer intrinsic vulnerability 总被引:3,自引:0,他引:3
This paper is the result of a study which was carried out in order to verify if the traditional methods to evaluate the intrinsic
vulnerability or vulnerability related parameters, are able to clarify the problem of nitrate pollution in groundwater. In
particular, the aim was to evaluate limitations and problems connected to aquifer vulnerability methods applied to nitrate
contamination prevision in groundwater. The investigation was carried out by comparing NO3
− concentrations, measured in March and November 2004 in the shallow aquifer, and the vulnerability classes, obtained by using
GOD and TOT methods. Moreover, it deals with a comparison between NO3
− concentrations and single parameters (depth to water table, land use and nitrogen input). The study area is the plain sector
of Piemonte (Northern Italy), where an unconfined aquifer nitrate contamination exists. In this area the anthropogenic presence
is remarkable and the input of N-fertilizers and zootechnical effluents to the soil cause a growing amount of nitrates in
groundwater. This approach, used in a large area (about 10,000 km2) and in several monitoring wells (about 500), allowed to compare the efficiency of different vulnerability methods and to
verify the importance of every parameter on the nitrate concentrations in the aquifer. Furthermore it allowed to obtain interesting
correlations in different hydrogeological situations. Correlations between depth to water table, land use and nitrogen input
to the soil with nitrate concentrations in groundwater show unclear situations: in fact these comparisons describe the phenomenon
trend and highlight the maximum nitrate concentrations for each circumstance but often show wide ranges of possible nitrate
concentrations. The same situation could be observed by comparing vulnerability indexes and nitrate concentrations in groundwater.
These results suggest that neither single parameters nor vulnerability methods (GOD and TOT) are able to describe individually
the complex phenomena affecting nitrate concentrations in soil, subsoil and groundwater. In particular, the traditional methods
for vulnerability analysis do not analyze physical processes in aquifers, such as denitrification and nitrate dilution. According
to a recent study in the shallow unconfined aquifer of the Piemonte plain, dilution can be considered as the main cause for
nitrate attenuation in groundwater. 相似文献