1,720,993 research outputs found
Very improved KINematic gravimetry: a new approach to aerogravimetry
Get PDFThe regional gravity field modeling by means of classical remove-compute-restore procedures is nowadays widely used in different contexts: from geodetic applications for the regional gravimetric geoid determination to exploration geophysics applications to extrapolate gridded or sparse points values of gravity anomalies (Bouguer, free-air, isostatic, etc.), useful to understand and map geological structures in a specific region. However, the accuracies and resolutions required for such exploration activity, do not consent the exploitation of satellite only gravity field data but need the integration with observation acquired at lower altitude.
Thanks to the development, in the late eighties and early nineties, of Global Navigation Satellite Systems (GNSS) and the consequent availability of accurate navigational data, techniques such as airborne gravimetry, that can provide this complementary information, started to spread worldwide. This technique represents nowadays one of the most efficient techniques ideal to collect gravity observations close to the Earth’s surface, in a fast and cost-effective way. Airborne gravimetry is capable of providing gravity measurements also in challenging environments which can be difficult to access otherwise, such as mountainous areas, rain forests and polar regions.
However due to the relatively high acquisition velocity, the presence of atmospheric turbulence, aircraft vibration, instrumental drift, etc. airborne data are usually contaminated by a very high observation error. For this reason a proper procedure to filter the raw observations both in the low and high frequency should be applied to recover valuable information.
In this work a new methodology to process airborne gravity measurements, named Very Improved KINematic Gravimetry (VIKING) is presented.
The proposed procedure allows to pre-process the raw observations coming from both the GNSS receiver and the gravimeter, with the aim to optimally combine the derived accelerations to compute gravity disturbances. Furthermore it consents to process by means of a filtering and gridding procedure these latter raw gravity disturbances to predict the signal on other points (grids or sparse points).
In details, the pre-processing deals with the manipulation of data acquired from the on board gravimeter and GNSS receiver to correct biases and derive gravity accelerations; while the processing regards the procedure to filter and grid the gravity accelerations data to obtain gravity anomalies/disturbances maps.
The developed algorithms used to pre-process raw GNSS acquired data are basically obtained by manipulating the classical GNSS observation equation to derive a new expression sensitive to the receiver acceleration (which corresponds to the vehicle acceleration) but almost insensitive to its actual position, by means of the implementation of the variometric approach. Regarding the pre-processing of the gravimeter data, the two principal aims of the method are the computation of all the corrections to properly combine gravimeter observations with GNSS observations and the optimal sampling of the gravimeter data, characterized by a very high observation rate, in such a way to not have loss of valuable information in terms of gravity accelerations.
The proposed solution to filter and grid raw airborne observations is a remove-compute-restore like procedure, and consists in a combination of an along track Wiener filter and a classical Least Squares Collocation technique. Basically the proposed procedure is an adaptation to airborne gravimetry of the Space-Wise approach, developed by Politecnico di Milano, to process data coming from the ESA satellite mission GOCE. Among the main differences with respect to the satellite application of this approach there is the fact that, while in processing GOCE data the stochastic characteristics of the observation error can be considered a-priori well known, in airborne gravimetry, due to the complex environment in which the observations are acquired, these characteristics are unknown and should be retrieved from the dataset itself.
The presented VIKING methodology is suited for airborne data analysis in order to be able to quickly filter and grid gravity observations in an easy fast and accurate way. Some innovative theoretical aspects focusing in particular on the theoretical covariance modeling are presented too.
An important part of the whole research project regarded the implementation of a suitable software for airborne gravity data processing. It has been developed in parallel C language and is organized in a set of toolboxes, which can be run independently from all the other ones, or in sequence to perform the whole processing. In order to evaluate the goodness of the whole procedure and its performances, various numerical tests have been performed on a real aerogravimetric dataset. The different tests were mainly focused on the analysis of the optimal choice of some parameters involved in the computation and on the performances in terms of accuracy and computational times of the various modules. The final result of the whole VIKING procedure, once calibrated the different parameters accordingly to the numerical tests performed, shows a predicted signal with ac- curacies of about 1.3 mGal. The obtained result in term of accuracy is in line with the expectations derived from the specific survey characteristics
Improving the computation of the gravitational terrain effect close to ground stations in the GTE software
No full textThe precise computation of the vertical gravitational attraction of the topographic masses (terrain correction) is still being studied both for geodetic and geophysical applications. In fact, it is essential in high precision geoid estimation by means of the well-known remove-compute-restore technique, which is used to isolate the gravitational effects of anomalous masses in exploration geophysics. The terrain correction can be evaluated exploiting a Digital Terrain Model (DTM) in different ways, such as classical numerical integration, prisms, tesseroids, polyhedrons, and/or Fast Fourier Transform techniques. The increasing resolution of recently developed DTMs, the increasing number of observation points, and the increasing accuracy of gravity data represent, nowadays, major challenges for the terrain correction computation. Classical point mass approximation and prism based-algorithms are indeed too slow, while Fourier-based algorithms are usually too much approximate when compared to the required accuracy. In this work, we improve the Gravity Terrain Effects (GTE) algorithm, the innovative tool that exploits a combined prism-Fast Fourier Transform approach especially developed for airborne gravimetry, to compute the terrain correction on the surface of the DTM (i.e. corresponding to the ground stations and/or its vicinity). This required development of a proper adjustment of the algorithms implemented within the GTE software and also to define and implement a procedure to overcome the problems of the computation of the gravitational effects due to the actual slope of the terrain close to the stations. The latter problem is thoroughly discussed and solved by testing different solutions like concentric cylindrical rings, triangulated polyhedrons, or ultra-high resolution squared prisms. Finally, numerical tests to prove the temporal efficiency and the computational performances of the improved GTE software to compute terrain correction for ground stations are also presented
Lithospheric modeling in Iran and the Arabian Peninsula from gravity data including seismic tomographic data: first results.
Get PDFIn this presentation we want show our lithosphere density model of a Middle East area encompassing Iran and the Arabian Peninsula, realized through a Bayesian inversion applied to an optimized density model. The starting model used for the inversion was obtained converting seismic velocities interpolated from local and global tomographies and converted in densities using a simplified version of the Brocher’s relation for velocity-to-density conversion, recalculating new coefficients for the relation. This optimization was realized following a Least Squares method, inverting global gravity field data. The model was divided into five parts: water, sediment, crust, mantle, and a separate crustal layer was defined in the Red Sea zone. Specifically, the Moho depth was obtained using the vertical velocity gradient method as presented in Tadiello and Braitenberg (2021), except for the southeastern zone along the Red Sea suture, which had strong velocity anomalies at the surface, and we relied on those to model a faster intrusive body within the crust, while estimation of the density distribution in the mantle was obtained using Perple_X software. We present the final density model, resulting from the inversion, and discuss it in terms of intra-crustal densification and relation to surface magmatic outcrops, finding that correlations can be identified. These demonstrate the presence of deep-seated crustal density variations which relate to geological provinces identified from surface investigations. A further point to discuss is the rheological properties obtainable from the joint velocity and density model and the relation to the inhomogeneous distribution of seismicity
GTE: a new software for gravitational terrain effect computation: theory and performances
No full textThe computation of the vertical attraction due to the topographic masses, the so-called Terrain Correction, is a fundamental step in geodetic and geophysical applications: it is required in high-precision geoid estimation by means of the remove–restore technique and it is used to isolate the gravitational effect of anomalous masses in geophysical exploration. The increasing resolution of recently developed digital terrain models, the increasing number of observation points due to extensive use of airborne gravimetry in geophysical exploration and the increasing accuracy of gravity data represents nowadays major issues for the terrain correction computation. Classical methods such as prism or point masses approximations are indeed too slow while Fourier based techniques are usually too approximate for the required accuracy. In this work a new software, called Gravity Terrain Effects (GTE), developed to guarantee high accuracy and fast computation of terrain corrections is presented. GTE has been thought expressly for geophysical applications allowing the computation not only of the effect of topographic and bathymetric masses but also those due to sedimentary layers or to the Earth crust-mantle discontinuity (the so-called Moho). In the present contribution, after recalling the main classical algorithms for the computation of the terrain correction we summarize the basic theory of the software and its practical implementation. Some tests to prove its performances are also described showing GTE capability to compute high accurate terrain corrections in a very short time: results obtained for a real airborne survey with GTE ranges between few hours and few minutes, according to the GTE profile used, with differences with respect to both planar and spherical computations (performed by prism and tesseroid respectively) of the order of 0.02 mGal even when using fastest profiles
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
GTE. A new software for gravitational terrain effect computation: theory and performances
Get PDFThe computation of the vertical attraction due to the topographic masses, the so-called Terrain Correction, is a fundamental step in geodetic and geophysical applications: it is required in high-precision geoid estimation by means of the remove–restore technique and it is used to isolate the gravitational effect of anomalous masses in geophysical exploration. The increasing resolution of recently developed digital terrain models, the increasing number of observation points due to extensive use of airborne gravimetry in geophysical exploration and the increasing accuracy of gravity data represents nowadays major issues for the terrain correction computation. Classical methods such as prism or point masses approximations are indeed too slow while Fourier based techniques are usually too approximate for the required accuracy. In this work a new software, called Gravity Terrain Effects (GTE), developed to guarantee high accuracy and fast computation of terrain corrections is presented. GTE has been thought expressly for geophysical applications allowing the computation not only of the effect of topographic and bathymetric masses but also those due to sedimentary layers or to the Earth crust-mantle discontinuity (the so-called Moho). In the present contribution, after recalling the main classical algorithms for the computation of the terrain correction we summarize the basic theory of the software and its practical implementation. Some tests to prove its performances are also described showing GTE capability to compute high accurate terrain corrections in a very short time: results obtained for a real airborne survey with GTE ranges between few hours and few minutes, according to the GTE profile used, with differences with respect to both planar and spherical computations (performed by prism and tesseroid respectively) of the order of 0.02 mGal even when using fastest profiles
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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