1,721,052 research outputs found
HIGH-RESOLUTION SPECTROMETRY WITH A MASKED FABRY-PEROT-INTERFEROMETER
No full textFeatures of a real Fabry-Perot interferometer are severely affected by surface irregularities. A new configuration, based on appropriately masking the plates, is proposed. Theoretical results show an increase in the finesse of as much as a factor of 10 as well as an increase in the contrast factor of as much as a factor of 100, while the luminosity-finesse product is preserved. The case of nonnormal incidence is also considered, and the differences arising from that case are discussed
FRINGE PATTERN-ANALYSIS IN LOW-QUALITY INTERFEROGRAMS
No full textMechanical vibrations in the frame of an interferometer can introduce a space-dependent factor throughout an interferogram, making its analysis quite difficult. This factor is formally equivalent to the modulus of the complex degree of coherence in the case of quasimonochromatic light. A simple method for retrieving the underlying phase distribution and detecting small perturbations is presented. Introducing an appropriate correction for the above-mentioned factor, good sensitivity is also achievable in noisy environments
Source fault parameters and slip distribution: the importance of crustal layering for the 1908 Messina Straits earthquake.
No full textWhat is behind Campi Flegrei inflations and deflations? Clues from 35 years of geodetic monitoring.
No full textVolcanic risk in the explosive Campi Flegrei (CF) caldera is extremely high, because of its location in a densely populated area about 15 km west of Naples inside the Campanian Plain. CF is renowned as a site of continual slow vertical movements. Since the last eruption in 1538, the caldera generally subsided until 1969 then uplift began in the early 1970s (about 1.5 m max), a further large uplift episode occurred from 1982 to 1984 (about 1.8 m max), and a few smaller uplift episodes occurred in 1989, 1994, 2000, and 2006. From 2006, CF is mostly uplifting.
Ground deformation monitoring of CF started in 1905, when the Istituto Geografico Militare established a first leveling line from Naples to Serapeum; since then the leveling line has been much extended and now the leveling network consists of 350 altimetric benchmarks. Geodetic precise-traversing surveys-distance (EDM) and angular measurements were carried out from time to time, starting from June 1970. A Continuous Global Positioning System (CGPS) network began operating at the end of the 1990s. Ground displacement data from Synthetic Aperture Radar (SAR) Interferometry are available since 1992.
Such an abundance of data has allowed detailed studies of the ground deformation time history. In particular, we have recently found that CF ground deformation, within data uncertainties, is always given by the sum of two "stationary contributions" at least since 1980. Large-scale deformation can be explained by a quasi-horizontal source, oriented NW/SE and mathematically represented by a pressurized finite thin triaxial ellipsoid (PTE) embedded in a layered medium at about 4 km; residual deformation not accounted for by PTE can be mathematically explained by a small (point) pressurized oblate spheroid about 2 km below the Solfatara fumarolic field. The location and geometry of the two sources are constant with the exception of volume changes (potency); potency time histories are somewhat similar but not identical.
A long-standing controversy characterizes the interpretation of CF inflation and deflation phases, which are generally ascribed to fluid injections at the basis of the hydrothermal system or to shallow magma injections. As a matter of fact, all published models of both processes predict complex spatial and/or temporal patterns of CF ground deformation.
We will give a review of our knowledge about CF ground deformation and present a new model of magma intrusion, which is consistent with the observed constancy of the ground deformation pattern
The 1982-1984 unrest of the Campi Flegrei caldera (Italy): clues on the resolving power of the available geodetic data and the robust features of the deformation source
No full textLimits on earthquake nucleation and other pre-seismic phenomena from continuous strain in the near field of the 2009 L'Aquila earthquake
No full textTwo laser strainmeters that operate at 1400‐m depth, about 20 km NE of the epicenter of the 2009/04/06 magnitude‐6.3 L’Aquila, Italy, earthquake, have produced records of strain since 1995. During the two years before the event, no anomalous signal larger than a few tens of nanostrains is visible, limiting the volume of the possible earthquake preparation zone to less than 100 km 3 ;
moreover, earth tidal response is stable within 0.5% in amplitude and 0.5° in phase. Thus, reality of large‐scale precursory phenomena seems unlikely. During the last few days, there is some evidence of dilatancy of satured rock
over the earthquake causative fault, maybe related to the foreshocks. Seconds before the event, strain is stable at the 10^−12 level and prerupture nucleation slip in the hypocentral region is constrained to have a moment less than 2 × 10^12 Nm, i. e. 0.00005% of the main shock seismic moment
INTERFEROMETRIC INVESTIGATIONS OF CONVECTION AROUND CYLINDERS AT SMALL GRASHOF NUMBERS
No full textExperiments with a Twyman-Green interferometer have been carried out to determine the temperature distribution around a horizontal cylinder in the presence of natural convection for Grashof numbers around 10-4 and a Prandtl number of 0.7, a region of parameters poorly covered by previous investigations. Temperature differences are recovered by digital analysis of the fringe pattern perturbation. The results indicate significant discrepancies in the temperature profile, when compared with numerical models
AUTOMATIC FRINGE RECOGNITION AND DETECTION OF SUBWAVELENGTH PHASE PERTURBATIONS WITH A MICHELSON INTERFEROMETER
No full textSimple systems and procedures based on personal computer hardware to analyze the fringe pattern in a Michelson interferometer, retrieve information on the irregularities of the combined phase front and use it to obtain a correct interferogram are presented. In one system, where one of the mirrors can be moved piezoelectrically, good sensitivity (λ/30) is obtained while using an economical array of 1-bit image sensing elements. In another system, based on a TV camera and a 6-bit frame grabber, images of the two separate laser beams and of the fringe patterns are stored; by adequate numerical analysis, higher sensitivity (λ/100) is achieved
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