1,720,959 research outputs found
Invariant models in the inversion of gravity and magnetic fields and their derivatives.
No full textIn potential field inversion problems we usually solve underdetermined systems and
realistic solutions may be obtained by introducing a depth-weighting function in the
objective function. The choice of the exponent of such power-law is crucial. It was
suggested to determine it from the field-decay due to a single source-block;
alternatively it has been defined as the structural index of the investigated source
distribution. In both cases, when k-order derivatives of the potential field are
considered, the depth-weighting exponent has to be increased by k with respect that of
the potential field itself, in order to obtain consistent source model distributions. We
show instead that invariant and realistic source-distribution models are obtained using
the same depth-weighting exponent for the magnetic field and for its k-order
derivatives. A similar behavior also occurs in the gravity case. In practice we found that
the depth weighting-exponent is invariant for a given source-model and equal to that of
the corresponding magnetic field, in the magnetic case, and of the 1st derivative of the
gravity field, in the gravity case. In the case of the regularized inverse problem, with
depth-weighting and general constraints, the mathematical demonstration of such
invariance is difficult, because of its non-linearity, and of its variable form, due to the
different constraints used. However, tests performed on a variety of synthetic cases
seem to confirm the invariance of the depth-weighting exponent.
A final consideration regards the role of the regularization parameter; we show that the
regularization can severely affect the depth to the source because the estimated depth
tends to increase proportionally with the size of the regularization parameter. Hence,
some care is needed to handle with the combined effect of the regularization parameter
and of the depth weighting
Volume Continuation of potential fields from the minimum-length solution: An optimal tool for continuation through general surfaces
No full textMany methods have been used to upward continue potential field data.Most techniques employ the Fast Fourier
transform, which is an accurate, quick way to compute level-to-level upward continuation or spatially varying
scale filters for level-to-draped surfaces. We here propose a new continuation approach based on the
minimum-length solution of the inverse potential field problem, which we call Volume Continuation (VOCO).
For real data the VOCO is obtained as the regularized solution to the Tikhonov problem. We tested our method
on several synthetic examples involving all types of upward continuation and downward continuation (levelto-
level, level-to-draped, draped-to-level, draped-to-draped). We also employed the technique to upward
continue to a constant height (2500 m a.s.l.), the high-resolution draped aeromagnetic data of the Ischia Island
in Southern Italy. We found that, on the average, they are consistent with the aeromagnetic regional data
measured at the same altitude. The main feature of our method is that it does not only provide continued data
over a specified surface, but it yields a volume of upward continuation. For example, the continued data refers
to a volume and thus, any surface may be easily picked up within the volume to get upward continuation to
different surfaces. This approach, based on inversion of the measured data, tends to be especially advantageous
over the classical techniques when dealing with draped-to-level upward continuation. It is also useful to obtain
a more stable downward continuation and to continue noisy data. The inversion procedure involved in the method
implies moderate computational costs, which are well compensated by getting a 3D set of upward continued
data to achieve high quality results
Self-constrained inversion of potential fields
No full textAbstract
We present a constrained inversion procedure based on a priori information derived exclusively from the analysis on the potential field data themselves (self-constrained inversion). The procedure
effectively applies to underdetermined problems and involves scenarios where the source distribution can be assumed to be of some simple character, but perturbations from this simplified body might occur. To set up effective constraints we first estimate by an analysis of the gravity or magnetic field some or all of the following source parameters: the source depth-to-the-top, the structural index, the horizontal position of the source body edges and their dip. The second step is incorporating the information related to these constraints in the objective function as depth and spatial weighting functions. We show, through some 2D and 3D synthetic and real data examples, that the sole potential field-based constraints are most of times enough to obtain rather good improvements in the density and magnetization models
Inversion of Gravity Gradient Tensor Data: does it provide better resolution?
No full textThe gravity gradient tensor (GGT) has been increasingly used in practical applications, but
the advantages and the disadvantages of the analysis of GGT components versus the analysis
of the vertical component of the gravity field are still debated. We analyse the performance of
joint inversion of GGT components versus separate inversion of the gravity field alone, or of
one tensor component. We perform our analysis by inspection of the Picard Plot, a Singular
Value Decomposition tool, and analyse both synthetic data and gradiometer measurements
carried out at the Vredefort structure, South Africa.We show that the main factors controlling
the reliability of the inversion are algebraic ambiguity (the difference between the number of
unknowns and the number of available data points) and signal-to-noise ratio. Provided that
algebraic ambiguity is kept low and the noise level is small enough so that a sufficient number
of SVD components can be included in the regularized solution, we find that: (i) the choice
of tensor components involved in the inversion is not crucial to the overall reliability of the
reconstructions; (ii) GGT inversion can yield the same resolution as inversion with a denser
distribution of gravity data points, but with the advantage of using fewer measurement stations
Volume upward continuation of potential fields from the minimum-length solution: an application in environmental hazards mapping
No full textWe propose a new approach, based on the
minimum-length solution of the inverse potential field problem.
The method yields a volume upward continuation, and reveals to
be advantageous over the classical techniques when dealing with
truncated anomalies and when draped-to-level upward
continuation is needed. One of the advantages of the novel
approach is that the VUC (Volume Upward Continuation) method
yields a volume upward continuation, and thus it may be used to
extract in a simple way the continuation on any kind of surface
and also to multiple levels
Microgravimetria in ambito urbano: il caso di Cardito (Na)
No full textLe cavità sotterranee in ambito urbano possono produrre dissesti, fenomeni di crollo e movimenti di subsidenza capaci di provocare gravi danneggiamenti alla rete dei sottoservizi e alla popolazione locale. L’esplorazione del sottosuolo con metodologie geofisiche può essere molto utile per ridurre il rischio legato a tali fenomeni. Lo studio del campo microgravimetrico risulta senz’altro tra le tecniche più promettenti in aree urbanizzate, ricche di intense fonti di disturbo per quasi tutti i metodi geofisici. In questo lavoro presentiamo i risultati di un’indagine microgravimetrica svolta presso il comune di Cardito (NA). L’indagine, condotta al di sopra di una cavità nota, ha avuto lo scopo di testare l’efficacia delle procedure utilizzate per la residuazione del segnale gravimetrico misurato, verificandone l’efficacia tramite l’interpretazione dei dati con tecniche multiscal
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Variations on the Author
Full text link“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
Full text linkWe 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
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