212 research outputs found
Sismique réflexion monotrace à Nouméa (Nouvelle Calédonie) : préliminaires : Octobre 1986 à Juillet 1987
Vibration control in plates by uniformly distributed PZT actuators interconnected via electric networks
In this paper a novel device aimed at controlling the mechanical vibrations
of plates by means of a set of electrically-interconnected piezoelectric
actuators is described. The actuators are embedded uniformly in the plate
wherein they connect every node of an electric network to ground, thus playing
the two-fold role of capacitive element in the electric network and of couple
suppliers. A mathematical model is introduced to describe the propagation of
electro-mechanical waves in the device; its validity is restricted to the case
of wave-forms with wave-length greater than the dimension of the piezoelectric
actuators used. A self-resonance criterion is established which assures the
possibility of electro-mechanical energy exchange. Finally the problem of
vibration control in simply supported and clamped plates is addressed; the
optimal net-impedance is determined. The results indicate that the proposed
device can improve the performances of piezoelectric actuationComment: 22 page
The 1999-2000 seismic experiment of Macas swarm (Ecuador) in relation with rift inversion in Subandean foothills
International audienceWe propose to explain the origin of the double trend in seismicity of the Macas swarm in the Subandean Cordillera of CutucĂş (Ecuador) and characterize the corresponding active deformation of that region. For that purpose, seismological and geological data have been used, with the deployment of a temporary seismological array, with geological field observations and image processing. We found that some earthquakes are aligned on a well known NNE SSW trend corresponding to the orientation of the nodal planes of the reverse focal mechanism of the Mw=7.0 1995 Macas earthquake as for its aftershocks. Nevertheless, many smaller events are aligned on an unexpected NNW SSE trend inside the CutucĂş Cordillera. We interpret these two orientations of the Macas swarm as linked to Subandean basement thrusts inherited from the inversion tectonics of a NNE SSW trending Triassic Jurassic rift, which has been uplifted and partly extruded in the CutucĂş Cordillera. The present partitioning of this part of the Subandean deformation is controlled by pre-existing NNE SSW to NNW SSE Triassic Jurassic normal faults that have been subsequently compressed transpressed and reactivated into reverse faults. Major boundary faults of the rift were NNE SSW oriented and correspond now to some main Subandean thrusts as confirms the focal mechanism of the 1995 main shock located on the eastern border (Morona frontal thrust) and the orientation of its aftershocks. In the CutucĂş Cordillera, the double orientation of present swarm can be interpreted as the result of accommodation of deformation along NNW SSE pre-existing faults inside the inverted rift system, linked to the motion of the Morona frontal NNE SSW thrust
Dome C site testing: surface layer, free atmosphere seeing and isoplanatic angle statistics
This paper analyses 3.5 years of site testing data obtained at Dome C,
Antarctica, based on measurements obtained with three DIMMs located at three
different elevations. Basic statistics of the seeing and the isoplanatic angle
are given, as well as the characteristic time of temporal fluctuations of these
two parameters, which we found to around 30 minutes at 8 m. The 3 DIMMs are
exploited as a profiler of the surface layer, and provide a robust estimation
of its statistical properties. It appears to have a very sharp upper limit
(less than 1 m). The fraction of time spent by each telescope above the top of
the surface layer permits us to deduce a median height of between 23 m and 27
m. The comparison of the different data sets led us to infer the statistical
properties of the free atmosphere seeing, with a median value of 0.36 arcsec.
The C_n^2 profile inside the surface layer is also deduced from the seeing data
obtained during the fraction of time spent by the 3 telescopes inside this
turbulence. Statistically, the surface layer, except during the 3-month summer
season, contributes to 95 percent of the total turbulence from the surface
level, thus confirming the exceptional quality of the site above it
Thermalizing a telescope in Antarctica: Analysis of ASTEP observations
The installation and operation of a telescope in Antarctica represent
particular challenges, in particular the requirement to operate at extremely
cold temperatures, to cope with rapid temperature fluctuations and to prevent
frosting. Heating of electronic subsystems is a necessity, but solutions must
be found to avoid the turbulence induced by temperature fluctua- tions on the
optical paths. ASTEP 400 is a 40 cm Newton telescope installed at the Concordia
station, Dome C since 2010 for photometric observations of fields of stars and
their exoplanets. While the telescope is designed to spread star light on
several pixels to maximize photometric stability, we show that it is
nonetheless sensitive to the extreme variations of the seeing at the ground
level (between about 0.1 and 5 arcsec) and to temperature fluctuations between
--30 degrees C and --80 degrees C. We analyze both day-time and night-time
observations and obtain the magnitude of the seeing caused by the mirrors, dome
and camera. The most important effect arises from the heating of the primary
mirror which gives rise to a mirror seeing of 0.23 arcsec K--1 . We propose
solutions to mitigate these effects.Comment: Appears in Astronomical Notes / Astronomische Nachrichten, Wiley-VCH
Verlag, 2015, pp.1-2
Experimental study of a liquid Xenon PET prototype module
A detector using liquid Xenon in the scintillation mode is studied for
Positron Emission Tomography (PET). The specific design aims at taking full
advantage of the liquid Xenon properties. It does feature a promising
insensitive to any parallax effect. This work reports on the performances of
the first LXe prototype module, equipped with a position sensitive PMT
operating in the VUV range (178 nm).Comment: Proc. of the 7th International Workshops on Radiation Imaging
Detectors (IWORID-7), Grenoble, France 4-7 July 200
Photometric quality of Dome C for the winter 2008 from ASTEP South
ASTEP South is an Antarctic Search for Transiting Exo- Planets in the South
pole field, from the Concordia station, Dome C, Antarctica. The instrument
consists of a thermalized 10 cm refractor observing a fixed 3.88\degree x
3.88\degree field of view to perform photometry of several thousand stars at
visible wavelengths (700-900 nm). The first winter campaign in 2008 led to the
retrieval of nearly 1600 hours of data. We derive the fraction of photometric
nights by measuring the number of detectable stars in the field. The method is
sensitive to the presence of small cirrus clouds which are invisible to the
naked eye. The fraction of night-time for which at least 50% of the stars are
detected is 74% from June to September 2008. Most of the lost time (18.5% out
of 26%) is due to periods of bad weather conditions lasting for a few days
("white outs"). Extended periods of clear weather exist. For example, between
July 10 and August 10, 2008, the total fraction of time (day+night) for which
photometric observations were possible was 60%. This confirms the very high
quality of Dome C for nearly continuous photometric observations during the
Antarctic winter
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