1,720,980 research outputs found
IDENTIFICATION AND INVESTIGATION OF SHALLOW PALEOCHANNELS IN THE CHAMELECON VALLEY (HONDURAS): 1D VS 2D ELECTRICAL RESISTIVITY SURVEYS
No full textElectrical Resistivity Tomography (ERT) and resistivity depth sounding surveys were undertaken at two sites located along the Chamelecòn Valley (Honduras). The objective of the surveys was to determine the occurrence, geometry and lithological properties of the Chamelecòn River’s shallow paleochannels. Following the interpretation of aerial photograms which indicated the likely presence of paleochannels, two sites were selected for the surveys. Resistivity depth soundings were used to achieve electro-stratigraphic sections obtained by interpolating 1D inverted data that would assist with assessing the occurrence of paleochannels at a large scale. The presence of paleochannels at the two sites was assessed through interpretation of the electro-stratigraphic sections. At Site 2, the abundant 1D data, the small distance between the soundings and the shallow target, allowed determining the course of the paleoriver as shown by resistivity and electrical transmissivity maps. The results from the 1D data, although affected by strong heterogeneities, are congruent with the 2D inverted models. ERT surveys confirmed the presence of paleochannels and allowed achieving detailed imaging of the alluvial geological bodies. Paleochannel deposits show resistivity values ranging between 70 and 100 ohm m at Site 1, while resistivity values range between 90 and 120 ohm m at Site 2. The latter resistivity values were affected by the presence of coarser deposits. Resistivity values suggest that at both sites paleochannel deposits have a clay content that is lower than 3 %. Other alluvial deposits in the area show resistivity values that range between 19 and 70 ohm m at Site 1, while resistivity values range between 30 and 90 ohm m at Site 2. These lower values suggest that clay content can be as high as 29 %. The presence of paleochannels assessed by means of 1D surveys and the lithological characterisation assessed by means of 2D surveys were confirmed by logs from boreholes drilled at the sites
TOO THIN TO BE DETECTED: WHEN ERT SURVEYS CAN FAIL TO ASSESS AN AQUICLUDE LAYER INTERPOSED BETWEEN TWO AQUIFERS: THE SUNCERI TEST SITE (HONDURAS)
No full textERT surveys were undertaken at the Sunceri test site located in the city of San Pedro Sula (Honduras). Four deep pumping wells are located at the site, together with 26 shallow and 12 deep piezometers drilled through alluvial deposits. These deposits are 100-150 m thick and overlie intrusive and low grade metamorphic rocks. The site is a major public water supply (PWS) for the city with a total groundwater abstraction of 160-200 l/s. The main objective of this paper is to identify if ERT surveys can determine the occurrence and continuity of a clay layer with a variable thickness and an average resistivity of 17 Ω∙m. Based on borehole logs, the clay layer is located at a depth of approximately 25 m. This layer is widespread across the entire area. It separates an upper unconfined aquifer from a deeper confined aquifer. The aquifers have different piezometric levels and hydro-chemical features. It is essential to correctly assess the thickness and paramatise the aquiclude layer so that a correct vulnerability assessment of the groundwater resource can be undertaken. Indeed, pollution of the heavily abstracted deep aquifer from contaminated shallow groundwater should be prevented. The inverted resistivity sections reveal the presence of the clay layer. However, they fail to show that the clay layer is discontinuous across the area. In particular, the clay layer doesn’t appear in the middle portion of the surveyed area where its thickness falls below 4.9 m as revealed by 2D synthetic dataset modelling. 1D modelling indicates a slightly lower value of 3 m. The fact that the thinning of the clay layer (that has a resistivity consistent with a 37 % clay content, i.e. 4 ∙10-3 m/d hydraulic conductivity) occurs in an area affected by a severe drawdown of the piezometric level due to the interference between the depression cones of the pumping wells, increases the vulnerability of the deep aquifer under the current abstraction rates
3D ERT IMAGING OF THE FRACTURED-KARST AQUIFER UNDERLYING THE EXPERIMENTAL SITE OF POITIERS (FRANCE): COMPARING WENNER-SCHLUMBERGER, POLE-DIPOLE AND HYBRID ARRAYS
No full textElectrical Resistivity Tomography (ERT) surveys were undertaken to investigate the Dogger Limestone fractured-karst aquifer at the Hydrogeological Experimental Site (HES) of Poitiers, France. Three-dimensional resistivity imaging was obtained from full inversion of combined 2D ERT data collected along five parallel 470 m long profiles with a 50 m line spacing. A 3D block measuring 515 x 203 m in size with a maximum depth of 100 m was surveyed. Dogger Limestone occurs at a depth ranging between 30 and 120 m and is overlain by argillaceous limestone. This paper compares the imaging obtained from different array sequences. Calibration of the 3D resistivity block with well logs indicates that: the Wenner-Schlumberger (WS) array shows the tendency to enhance layering, to locate bodies at a shallower depth and to laterally extend them; the Pole-Dipole (PD) array shows larger lateral heterogeneities, more compact and vertically extended bodies and poor data fitting; the hybrid array sequence, obtained by the combination of WS and PD array sequences, despite a poor data fitting, similar to PD, shows a better correlation with respect to well log results. In this setting, the hybrid array sequence shows better imaging, due to the combination of the large vertical resolution of WS, large lateral resolution and penetration depth of PD. It allows passing through the thick, low resistivity shallow layer. Indeed, the results are affected by the occurrence of the shallow, 30 m thick, low resistivity argillaceous limestone that reduced the investigation depth as revealed by synthetic datasets modelling and sensitivity analysis. Modelling also revealed that the occurrence of the argillaceous limestone led to a severe underestimate of the Dogger Limestone resistivity values with respect to well resistivity logs; it also allowed verifying the detectability limits when investigating shallow karst limestone intervals located at depths of up to 50 m
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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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