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81.
Ivan I. Shevchenko 《Monthly notices of the Royal Astronomical Society》2008,384(3):1211-1220
The chaotic orbital motion of Prometheus and Pandora, the 16th and 17th satellites of Saturn, is studied. Chaos in their orbital motion, as found by Goldreich & Rappaport and Renner & Sicardy, is due to interaction of resonances in the resonance multiplet corresponding to the 121:118 commensurability of the mean motions of the satellites. It is shown rigorously that the system moves in adiabatic regime. The Lyapunov time (the 'time horizon of predictability' of the motion) is calculated analytically and compared to the available numerical–experimental estimates. For this purpose, a method of analytical estimation of the maximum Lyapunov exponent in the perturbed pendulum model of non-linear resonance is applied. The method is based on the separatrix map theory. An analytical estimate of the width of the chaotic layer is made as well, based on the same theory. The ranges of chaotic diffusion in the mean motion are shown to be almost twice as big compared to previous estimates for both satellites. 相似文献
82.
83.
A focussing function is a specially constructed field that focusses on to a purely downgoing pulse at a specified subsurface position upon injection into the medium. Such focussing functions are key ingredients in the Marchenko method and in its applications such as retrieving Green's functions, redatuming, imaging with multiples and synthesizing the response of virtual sources/receiver arrays at depth. In this study, we show how the focussing function and its corresponding focussed response at a specified subsurface position are heavily influenced by the aperture of the source/receiver array at the surface. We describe such effects by considering focussing functions in the context of time-domain imaging, offering explicit connections between time processing and Marchenko focussing. In particular, we show that the focussed response radiates in the direction perpendicular to the line drawn from the centre of the surface data array aperture to the focussed position in the time-imaging domain, that is, in time-migration coordinates. The corresponding direction in the Cartesian domain follows from the sum (superposition) of the time-domain direction and the directional change due to time-to-depth conversion. Therefore, the result from this study provides a better understanding of focussing functions and has implications in applications such as the construction of amplitude-preserving redatuming and imaging, where the directional dependence of the focussed response plays a key role in controlling amplitude distortions. 相似文献
84.
Stefano De Santis Paolo Casadei Gerardo De Canio Gianmarco de Felice Marialaura Malena Marialuisa Mongelli Ivan Roselli 《地震工程与结构动力学》2016,45(2):229-251
An innovative solution for the seismic protection of existing masonry structures is proposed and investigated through shake table tests on a natural scale wall assemblage. After a former test series carried out without reinforcement, the specimen was retrofitted using Steel Reinforced Grout. The strengthening system comprises horizontal strips of ultra‐high strength steel cords, externally bonded to the masonry with hydraulic lime mortar, and connectors to transversal walls, applied within the thickness of the plaster layer. In order to assess the seismic performance of the retrofitted wall, natural accelerograms were applied with increasing intensity up to failure. Test results provide a deep understanding of the effectiveness of mortar‐based composites for improving the out‐of‐plane seismic capacity of masonry walls, in comparison with traditional reinforcements with steel tie‐bars. The structural implications of the proposed solution in terms of dynamic properties and damage development under earthquake loads are also discussed.Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
85.
The standard ray theory (RT) for inhomogeneous anisotropic media does not work properly or even fails when applied to S-wave
propagation in inhomogeneous weakly anisotropic media or in the vicinity of shear-wave singularities. In both cases, the two
shear waves propagate with similar phase velocities. The coupling ray theory was proposed to avoid this problem. In it, amplitudes
of the two S waves are computed by solving two coupled, frequency-dependent differential equations along a common S-wave ray.
In this paper, we test the recently developed approximation of coupling ray theory (CRT) based on the common S-wave rays obtained
by first-order ray tracing (FORT). As a reference, we use the Fourier pseudospectral method (FM), which does not suffer from
the limitations of the ray method and yields very accurate results. We study the behaviour of shear waves in weakly anisotropic
media as well as in the vicinity of intersection, kiss or conical singularities. By comparing CRT and RT results with results
of the FM, we demonstrate the clear superiority of CRT over RT in the mentioned regions as well as the dangers of using RT
there. 相似文献
86.
Dietmar Grimm Ivan Barkhorn David Festa Kate Bonzon Judd Boomhower Valerie Hovland Jason Blau 《Marine Policy》2012,36(3):644-657
What are the effects of transitioning traditionally managed fisheries to incentive-based catch shares fisheries? In a study of all major United States federal catch share fisheries and associated shared stock fisheries in British Columbia, catch shares result in environmental improvements, economic improvements, and a mixture of changes in social performance, relative to the race for fish under traditional management. Environmentally, compliance with total allowable catch increases and discards decrease. Economically, vessel yields rise, total revenues grow, and long-term stock increases are encouraged. Socially, safety increases, some port areas modestly consolidate, needed processing capacity often reduces, and labor markets shift from part time jobs to full time jobs with similar total employment. Newer catch shares address many social concerns through careful design. 相似文献
87.
In the high mountain oligotrophic Seven Rila Lakes (2 095–2 535 m a.s.l.) bacterial abundance was recorded in July and September
2006. In July, bacterial abundance (average (3.32±2.5)×105 cell/ml) in the lakes correlated negatively with elevation, while in September (average (26.39±7.1)×105 cell/ml) this pattern was altered and the greatest abundance of bacteria occurred at the highest ((39.05±1.5)×105 cell/ml) and the lowest ((30.63±6.0)×105 cell/ml) elevated lakes. Regression analyses suggested that the principal factor controlling bacterial abundance, in both
July and September, was inorganic nitrogen (NO3-N and NH4-N; R
2=0.70, P<0.05). Other major regulating factors were temperature (R
2=0.66, P<0.03) in July and filter-feeding zooplankton (R
2=0.95, P<0.01) in September. All factors, except NO3-N, had a positive effect on bacterial abundance. 相似文献
88.
Laura Gaggero Laura Buzzi Ivan Haydoutov Luciano Cortesogno 《International Journal of Earth Sciences》2009,98(8):1853-1877
Eclogitic rocks were sampled from two zones in the basement of the Sredna Gora terrane (central western Bulgaria): (1) partially
retrogressed eclogites and amphibolites embedded in sillimanite-bearing garnet-micaschists with kyanite relics and migmatites
and (2) banded amphibolites associated with muscovite-bearing metagranites within two-mica paragneisses. Rutile relics and
oligoclase + green hornblende + epidote ± biotite pseudomorphs after garnet suggest an eclogite facies event. A tholeiitic,
transitional affinity was determined for the protoliths, suggesting a continental rift environment, consistent with several
eclogite-bearing complexes in the eastern segments of the Variscan belt that arose from the Cambro-Ordovician Gondwana break-up.
Decreasing pressure after the eclogite overprint was demonstrated by (a) diopside-albite symplectite, and (b) plagioclase
+ red–brown to green amphibole kelyphite. The early static re-equilibration, dated to 398 ± 5.2 Ma by 40Ar–39Ar technique, was followed by an amphibolite facies foliation, which was pervasive in amphibolites, gneisses and micaschists,
and poorly developed in eclogites. The lithospheric PT paths corresponding to higher and lower metamorphic gradients reflect
the juxtaposition of crustal and lithospheric mantle units, respectively. In the build-up of the basement of the Balkan orogen,
the physical properties of the lithological complexes might have influenced the collisional pattern of involved microplates. 相似文献
89.
Meiobenthic fauna is complex and essential part of the stream benthos and it is known as intermediaries from bacteria to higher trophic levels. However, the dynamics and mechanisms regulating this community at small-scale largely have been neglected. This study was carried out to determine meiofaunal dynamics in a small scale-patterns influenced by flow velocity and other abiotic and biotic factors. We examined meiofauna within bryophytes on a tufa barrier in Plitvice Lakes National Park using a detailed taxonomic approach of various meiofaunal groups and their feeding guilds. We choose three microhabitats (slow, medium, fast) differing significantly in flow velocity above bryophytes. Bdelloid rotifers were the most abundant group in microhabitat with highest flow velocity, while in other two microhabitats nematodes and monogonont rotifers prevailed in abundance. Data on environmental variables and main meiofaunal taxa and feeding guilds were analyzed using redundancy analysis. This analysis indicated that microfilter feeding guild (e.g. bdelloid rotifers) was strongly affected by interaction of flow velocity and POM fractions. Other feeding guilds were influenced by temperature, oxygen and/or pH and did not prefer high flow velocity. Suction-feeder nematodes and microfilter-feeder rotifers were dominant on temporal and spatial scale, indicating their good adaptations on frequently disturbed conditions that prevailed on bryophyte covered tufa barrier. Our results provide comprehensive survey of diversity, density as well as trophic structure of meiofauna in aquatic bryophytes. Differences in meiofaunal composition and density between three microhabitats suggest that the meiofauna is relevant indicator of environmental changes even at small-scale pattern. 相似文献
90.