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The European VLBI Network has been performing a regular series of geodetic observations (EUROPE) whose main aim is to detect and monitor deformations in the European part of the Eurasian plate with high accuracy. The interaction between the African and Eurasian plates drives the tectonic motions detected in the Southern part of Europe that, with the spreading of the Middle Atlantic ridge and other local geophysical (e.g., post-glacial rebound) and geological phenomena, determine the overall crustal deformation of the region. We present the latest results obtained from the analysis of the EUROPE campaigns of observations; for certain stations, we have integrated other data acquired during different campaigns that involved at least three antennae of the network. In particular, for Ny Aalesund, we had to increase further the amount of data by using 144 experiments providing a more continuous and redundant data span to ensure a stronger statistical solution and a network with better spatial distribution. We present the velocities estimated for the antennas in the European geodetic network, along with an interpretation within their local geophysical frame. 相似文献
13.
This paper sets the rules for an optimal definition of precise signal path variation (SPV) models, revising and highlighting
the deficiencies in the calculations adopted in previous studies and improving the computational approach. Hence, the linear
coefficients that define the SPV model are rigorously determined. The equations that are presented depend on the dimensions
and the focal lengths of the telescopes as well as on the feed illumination taper. They hold for any primary focus or Cassegrainian
very long baseline interferometry (VLBI) telescope. Earlier investigations usually determined the SPV models assuming a uniform
illumination of the telescope mirrors. We prove this hypothesis to be over-simplistic by comparing results derived adopting
(a) uniform, (b) Gaussian and (c) binomial illumination functions. Numerical computations are developed for AZ-EL mount, 32
m Medicina and Noto (Italy) VLBI telescopes, these latter being the only telescopes which possess thorough information on
gravity-dependent deformation patterns. Particularly, assuming a Gaussian illumination function, the SPV in primary focus
over the elevation range [0°, 90°] is 10.1 and 7.2 mm, for Medicina and Noto, respectively. With uniform illumination function
the maximal path variation for Medicina is 17.6 and 12.7 mm for Noto, thus highlighting the strong dependency on the choice
of the illumination function. According to our findings, a revised SPV model is released for Medicina and a model for Noto
is presented here for the first time. Currently, no other VLBI telescope possesses SPV models capable of correcting gravity-dependent
observation biases. 相似文献
14.
Height bias and scale effect induced by antenna gravitational deformations in geodetic VLBI data analysis 总被引：2，自引：2，他引：0
The impact of signal path variations (SPVs) caused by antenna gravitational deformations on geodetic very long baseline interferometry
(VLBI) results is evaluated for the first time. Elevation-dependent models of SPV for Medicina and Noto (Italy) telescopes
were derived from a combination of terrestrial surveying methods to account for gravitational deformations. After applying
these models in geodetic VLBI data analysis, estimates of the antenna reference point positions are shifted upward by 8.9
and 6.7 mm, respectively. The impact on other parameters is negligible. To simulate the impact of antenna gravitational deformations
on the entire VLBI network, lacking measurements for other telescopes, we rescaled the SPV models of Medicina and Noto for
other antennas according to their size. The effects of the simulations are changes in VLBI heights in the range [−3, 73] mm
and a net scale increase of 0.3–0.8 ppb. The height bias is larger than random errors of VLBI position estimates, implying
the possibility of significant scale distortions related to antenna gravitational deformations. This demonstrates the need
to precisely measure gravitational deformations of other VLBI telescopes, to derive their precise SPV models and to apply
them in routine geodetic data analysis. 相似文献
15.
Alessandro Amorosi Irene Sammartino Giovanni Sarti 《Environmental Earth Sciences》2013,69(5):1661-1671
Identification of reliable background values of potentially toxic metals in sediments requires detailed integration of geochemical data with accurate sedimentological studies. Through analysis of 60 soil samples from the Pisa coastal plain, this study shows to what extent sediment provenance and facies characteristics may influence the natural distribution of potentially toxic metals (Cr, Ni, Cu, Zn, Pb) within alluvial and coastal sediments. Metals supplied to the alluvial plain are mostly concentrated within the finest sediment fraction (floodplain clays), while coarser crevasse and overbank deposits exhibit invariably lower metal contents. Beach-ridge sands display the lowest metal concentrations. Transport of ophiolitic detritus by the longshore drift may account for locally high Cr concentrations within beach deposits. Geochemical fingerprinting of individual facies associations in terms of natural metal contents results in the construction of a geologically-based geochemical map. This map offers a more reliable depiction of spatial distribution of background levels than interpolation techniques based uniquely upon statistical methods. Matching background values against metal concentrations from topsoil samples leads to the reliable assessment of the pollution status of Pisa coastal plain. Metal contents exceeding the threshold values designated for contaminated areas (Cr) simply reflect catchment geology, and are not the product of artificial contamination. On the other hand, anthropogenic disturbance may be detected even where metal contents (Pb, Cu) lie below the threshold values. The use of sedimentological criteria is presented here as a pragmatic tool to enhance predictability of natural metal contents in sediments, with obvious positive feedbacks for legislative purposes and environmental protection. 相似文献
16.
Claudio Abbondanza Zuheir Altamimi Pierguido Sarti Monia Negusini Luca Vittuari 《Journal of Geodesy》2009,83(11):1031-1040
Tie vectors (TVs) between co-located space geodetic instruments are essential for combining terrestrial reference frames (TRFs)
realised using different techniques. They provide relative positioning between instrumental reference points (RPs) which are
part of a global geodetic network such as the international terrestrial reference frame (ITRF). This paper gathers the set
of very long baseline interferometry (VLBI)–global positioning system (GPS) local ties performed at the observatory of Medicina
(Northern Italy) during the years 2001–2006 and discusses some important aspects related to the usage of co-location ties
in the combinations of TRFs. Two measurement approaches of local survey are considered here: a GPS-based approach and a classical
approach based on terrestrial observations (i.e. angles, distances and height differences). The behaviour of terrestrial local
ties, which routinely join combinations of space geodetic solutions, is compared to that of GPS-based local ties. In particular,
we have performed and analysed different combinations of satellite laser ranging (SLR), VLBI and GPS long term solutions in
order to (i) evaluate the local effects of the insertion of the series of TVs computed at Medicina, (ii) investigate the consistency
of GPS-based TVs with respect to space geodetic solutions, (iii) discuss the effects of an imprecise alignment of TVs from
a local to a global reference frame. Results of ITRF-like combinations show that terrestrial TVs originate the smallest residuals
in all the three components. In most cases, GPS-based TVs fit space geodetic solutions very well, especially in the horizontal
components (N, E). On the contrary, the estimation of the VLBI RP Up component through GPS technique appears to be awkward,
since the corresponding post fit residuals are considerably larger. Besides, combination tests including multi-temporal TVs
display local effects of residual redistribution, when compared to those solutions where Medicina TVs are added one at a time.
Finally, the combination of TRFs turns out to be sensitive to the orientation of the local tie into the global frame. 相似文献
17.
Using the phase differences between satellite radar images recorded before and after an earthquake, interferometry allows mapping the projection along the line of sight (LOS) of the ground displacement. Acquisitions along multiple LOS theoretically allow deriving the complete deformation vector; however, due to the orbit inclination of current radar satellites, precision is poor in the north-south direction. Moreover, large deformation gradients (e.g., fault ruptures) prevent phase identification and unwrapping and cannot be measured directly by interferometry. Subpixel correlation techniques using the amplitude of the radar images allow measuring such gradients, both in slant-range and in azimuth. In this letter, we use a correlation technique based on the maximization of coherence for a radar pair in interferometric conditions, using the complex nature of the data. In the case of highly coherent areas, this technique allows estimating the relative distortion between images. Applied to ASAR images acquired before and after the December 26, 2003 Bam earthquake (Iran), we show that the near-field information retrieved by this technique is useful to constrain geophysical models. In particular, we confirm that the major gradients of ground displacement do not occur across the known fault scarp but approximately 3 km west of it, and we also estimate directly the amplitude of right lateral slip, while retrieving this value from interferometry requires passing through the use of a model for the earthquake fault and slip. 相似文献
18.
We assess the accuracy of some indirect approaches to invariant point (IVP), or system reference point, determination of satellite
laser ranging (SLR) and very long baseline interferometry (VLBI) systems using both observed and simulated survey data sets.
Indirect IVP determination involves the observation of targets located on these systems during specific rotational sequences
and by application of geometrical models that describe the target motion during these sequences. Of concern is that most SLR
and VLBI systems have limited rotational freedom thereby placing constraint on the reliability of parameter estimation, including
the IVP position. We assess two current approaches to IVP analysis using survey data observed at the Yarragadee (Australia)
SLR and the Medicina (Italy) VLBI sites and also simulated data of a large rotationally constrained (azimuth-elevation) VLBI
system. To improve reliability we introduce and assess some new geometric conditions, including inter-axis, inter-circle and
inter-target conditions, to existing IVP analysis strategies. The error component of a local tie specifically associated with
the indirect determination of SLR and VLBI IVP is less than 0.5 mm. For systems with significant rotational limits we find
that the inter-axis and inter-circle conditions are critical to the computation of unbiased IVP coordinates at the sub-millimetre
level. When the inter-axis and inter-circle geometric conditions are not imposed, we retrieve biased vertical coordinates
of the IVP (in our simulated VLBI system) in the range of 1.2–3.4 mm. Using the new geometric conditions we also find that
the axis-offset estimates can be recovered at the sub- millimetre accuracy (0.5 mm). 相似文献