1,720,961 research outputs found
Characterisation of shallow sediments by processing of P, SH and SV wavefields in Kaštela (HR)
No full textA seismic study was conducted in a historical town in Croatia to characterize the shallow subsurface and evaluate the response to an earthquake for engineering purposes. The study involves three seismic lines acquired along the existing roads of the town. For each of the lines, we acquired vertical compressional P and two horizontally polarized, S-wavefields; SH and SV, respectively orthogonal and parallel to the direction of the seismic line. We processed the data from these three wavefields with both first-break tomography, to obtain wave-velocity profiles, and reflection seismic imaging, to obtain stacked sections, and integrated the results. From such analyses, we obtained an in-depth characterization of the shallow subsurface. Specifically, we obtained standard engineering parameters like the equivalent shear-wave velocity of the upper 30 m of the subsurface (VS30), which indicates A-class soil, and compressional to shear-wave velocity ratio (VP/VS), which gives an indication regarding the presence of fractures in the rock. In addition to this, we evaluated the presence of anisotropy thanks to the SH- and SV-wave tomography inversions, which allowed to notice that VSV > VSH on most of the area. The presence of anisotropy is consistent with the known geological features of the area, in particular the subvertical bedding of the flysch, of which we were able to estimate the bedding plane orientations (dip and strike angles). Finally, by superimposing the stacked sections obtained from reflection seismic imaging with the velocity profiles computed with the traveltime tomography, we confirmed the reliability of the tomographic velocity models
Application of surface-wave tomography to mineral exploration: a case study from Siilinjärvi, Finland
No full textIn order to assess the feasibility and validity of surface-wave tomography as a toolfor mineral exploration, we present an active seismic three-dimensional case studyfrom the Siilinj ̈arvi mine in Eastern Finland. The aim of the survey is to identify theformation carrying the mineralization in an area south of the main pit, which will bemined in the future. Before acquiring the data, we performed an accurate survey designto maximize data coverage and minimize the time for deployment and recollectionof the equipment. We extract path-averaged Rayleigh-wave phase-velocity dispersioncurves by means of a two-station method. We invert them using a computationallyefficient tomographic code which does not require the computation of phase-velocitymaps and inverts directly for one-dimensional S-wave velocity models. The retrievedvelocities are in good agreement with the data from a borehole in the vicinity, and thepseudo three–dimensional S-wave velocity volume allows us to identify the geologicalcontact between the formation hosting most of the mineralization and the surroundingrock. We conclude that the proposed method is a valid tool, given the small amountof equipment used and the acceptable amount of time required to process the data
Surface Wave Tomography at Exploration Scale
No full textIn this work we present the application of a direct tomographic inversion method, similar to that historically used in global seismology, to exploration scale. We will start with an excursus on the physical background necessary to understand how surface waves propagate and the charachteristics which make these waves suitable for inversion. In the following section the optimal acquisition methods will be described. These are not very different from those for the datasets used for industry-standard reflection seismology imaging; however a few remarks have to be made. For instance, the length of the line, the spacing between receivers and the dominant wavelength are essential parameters to take into account when acquiring a dataset for surface wave analysis. The following section, regarding the processing, will present the most used methods for inversion of surface waves at exploration scale, those at regional (seismological) scale and the method used to perform the inversion. These can be classified into tomographic and non tomographic approaches. In both cases, the aim is to extract phase velocity dispersion curves from the data and invert them to get 1D models. The key difference lies in how the dispersion curves are extracted and the "meaning" we give to them. When performing a non tomographic inversion it is possible to extract the curves in many ways, e.g. by computing a Fourier transform in space and time (f-k domain) and pick the curve as energy maxima, as surface waves are the most energetic event in a seismogram. Most commonly, the f-k spectrum is an average one over a certain number of stations. Each of these curves will be inverted to get a 1D model and each of these models can be connected to the others via constraints (regularisation). On the other hand, when one wants to perform tomographic inversion, the dispersion curves are extracted by computing the average slowness of surface waves travelling along a path from one receiver to another, illuminated by the same source. Therefore, since more paths can cross the same model point, more dispersion curves have to fit the model parameters at each model point. Seismologists, at this point, generally compute phase velocity maps from which they extract further dispersion curves to be inverted. What's new in the method used for this work is the fact that we invert dispersion curves, without computing phase velocity maps. This is why it's called "direct inversion". This has several positive aspects, e.g. the computational efficiency. Another advantage, common to all tomographic approaches is the fact that it uses only two stations and therefore does not need a dataset with a large number of channels. On the other hand, it has some limitations both on the acquisition parameters (especially offset and distance between the two receivers) as well as the fact that it only inverts for the fundamental mode dispersion curve. Two datasets have been analysed: one shot in New Zealand by ETH Zurich, one shot in the area of Torino by the Italian National Centre for Research (CNR). The first constitutes of five 2D seismic lines, one of which at ultra-high resolution (1m receiver spacing and 2 m source spacing). This dataset has already been inverted using the estabilished multichannel analysis of surface waves method. Therefore, it was a good dataset to start with as results could be compared with those from this previous inversion. The CNR dataset is a small 3D dataset, above an artificially made tank filled with loose sand. The fact that the map of the subsurface was already known made it a good dataset to test the validity of the method also in 3D. In the appendix, we show tests performed on a large 3D seismic survey, acquired in Oman for oil exploration purposes. This dataset is known in the geophysical community (e.g. Gouédard et al., 2012) for having a low signal to noise ratio. Furthermore, the subsurface to be mapped shows strong lateral variations. For this reason, preliminary tests were performed in order to find a method to increase the signal to noise ratio and therefore assess the feasibility of a direct tomographic inversion. A good solution could be to stack the traces from two different sources aligned with the same couple of receivers.Applied Geophysics and PetrophysicsGeoscience & EngineeringCivil Engineering and Geoscience
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
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
Surface-wave analysis for static corrections in mineral exploration: A case study from central Sweden
No full textIn mineral exploration, increased interest towards deeper mineralizations makes seismic methods attractive. One of the critical steps in seismic processing workflows is the static correction, which is applied to correct the effect of the shallow, highly heterogeneous subsurface layers, and improve the imaging of deeper targets. We showed an effective approach to estimate the statics, based on the analysis of surface waves (groundroll) contained in the seismic reflection data, and we applied it to a legacy seismic line acquired at the iron-oxide mining site of Ludvika in Sweden. We applied surface-wave methods that were originally developed for hydrocarbon exploration, modified as a step-by-step workflow to suit the different geologic context of hardrock sites. The workflow starts with the detection of sharp lateral variations in the subsurface, the existence of which is common at hard-rock sites. Their location is subsequently used, to ensure that the dispersion curves extracted from the data are not affected by strong lateral variations of the subsurface properties. The dispersion curves are picked automatically, windowing the data and applying a wavefield transform. A pseudo-2D time-average S-wave velocity and time-average P-wave velocity profile are obtained directly from the dispersion curves, after inverting only a reference curve. The time-average P-wave velocity profile is then used for the direct estimation of the one-way traveltime, which provides the static corrections. The resulting P-wave statics from the field data were compared with statics computed through conventional P-wave tomography. Their difference was mostly negligible with more than 91% of the estimations being in agreement with the conventional statics, proving the effectiveness of the proposed workflow. The application of the statics obtained from surface waves provided a stacked section comparable with that obtained by applying tomostatics
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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