1,720,974 research outputs found
A newly distinguished marine magnetotelluric coast effect sensitive to the lithosphere-Asthenosphere boundary
No full textThe marine magnetotelluric (MT) method is a useful tool for offshore studies aimed at, for example, hydrocarbon exploration and the understanding of Earth's tectonics. Marine MT data are often distorted by coastlines because of the strong resistivity contrast between the conductive ocean and the resistive land. At mid ocean ridges, the resistivity of Earth's structure can be assumed to be two-dimensional, which allows MT data to be decomposed into a transverse electrical (TE) mode, with electric current flowing approximately along the ridge, and a transverse magnetic (TM) mode, with electric current flowing perpendicular to the ridge. We collected marine MT data at the middle Atlantic Ridge which exhibited highly negative TM-mode phases, as large as -180°, at relatively high frequencies (0.1-0.01 Hz). Similar negative phases have been observed in other marineMTdata sets, but have not been the subject of study.We show here that these negative phases are caused by a newly distinguished coast effect. The TM-mode coast effect is not only a galvanic effect, as previously understood, but also includes inductive distortions. TM-mode negative phases are caused by the turning of the Poynting vector, the phase change of electromagnetic fields, and vertically flowing currents in the seafloor. The findings provide a new understanding of the TM-mode coast effect, which can guide our ability to fit the field data with the inclusion of coastlines, and reduce misinterpretation of the data in offshore studies. The study also shows that the TMmode coast effect is sensitive to the depth and conductivity of the asthenosphere, an important feature of the Earth's interior that was the object of our Atlantic Ocean study.</p
The practical application of 2D inversion to marine controlled-source electromagnetic data
No full textAn algorithm is presented for the inversion of marine controlled-source electromagnetic (CSEM) data that uses a 2D finite difference (FD) forward driver. This code is demonstrated by inverting a CSEM data set collected at Hydrate Ridge, Oregon, consisting of 25 seafloor sites recording a 5-Hz transmission frequency. The sites are located across a bathymetric high, with variations in water depth of ?300m along the 16-km profile. To model this complex seafloor bathymetry accurately, the FD grid was designed by careful benchmarking using a different 2D finite element (FE) forward code. A comparison of the FE and FD forward model solutions verifies that no features in the inversion are due to inaccuracies of the FD grid. The inversion includes the local seawater conductivity–depth profile as recorded by the transmitter’s conductivity–temperature-depth gauge, because seawater conductivity is known to have a significant effect on the CSEM responses. An apparent resistivity pseudosection of the CSEM data resembles the 2D inversion in general appearance. However, the inversion provides depth and geometric control of features that cannot be provided by the pseudosection and eliminates artifacts generated from the pseudosection projection
The nature of the Lithosphere‐Asthenosphere Boundary
No full textPlate tectonic theory was developed 50 years ago and underpins most of our understanding of Earth's evolution. The theory explains observations of magnetic lineations on the seafloor, linear volcanic island chains, large transform fault systems, and deep earthquakes near deep sea trenches. These features occur through a system of moving plates at the surface of the Earth, which are the surface expression of mantle convection. The plate consists of the chemically distinct crust and some amount of rigid mantle, which move over a weaker mantle beneath. However, exactly where the transition between stronger and weaker mantle occurs and what determines and defines the plate are still debated. In the classic definition the plate is defined thermally, by the geotherm-adiabat intersection, where the plate is the conductively cooling part of the mantle convection system. Many observations such as heat flow, seafloor bathymetry, seismic imaging, and magnetotelluric (MT) imaging are consistent with general lithospheric thickening with age, which suggests that temperature is an important factor in determining lithospheric thickness. However, while age averages give a good indication of overall properties, the range of lithospheric thicknesses reported is large for any given tectonic age interval, suggesting greater complexity. A number of observations including sharp discontinuities from teleseismic scattered waves and active source reflections and also strong anomalies from surface and body wave tomography and MT imaging cannot be explained by a purely thermal model. Another property or process is required to explain the anomalies and sharpen the boundary. Many subsolidus models have been proposed, although none can universally explain the variety of independent global observations. Alternatively, a small amount of partial melt can easily satisfy a range of observations. The presence of melt could also weaken the mantle over geologic timescales, and it would therefore define the lithosphere-asthenosphere boundary (LAB). The location of melt is important to mantle dynamics and the LAB, although exactly where and exactly how much melt exists in the mantle are debated. Asthenospheric melt interpretations include a variety of forms: in small or large melt triangles beneath spreading ridges, in channels, in layers, along a permeability boundary leading to the ridge, at a depth of neutral buoyancy, punctuated, or pervasively over broad areas and either sharply or gradually falling off with depth. This variability in melt character or geometry may explain the previously described variability in LAB depths. The LAB is likely highly variable laterally as are the locations, forms, and amounts of melt, and the LAB is likely dynamic, dictated by small-scale convection and the dynamics of melt generation and migration. A melt-defined, dynamic LAB and a weak asthenosphere have broad implications for our understanding of Earth systems and planetary habitability. A weak asthenosphere caused by volatiles or melt could enable plate tectonic style convection, allow multiple scales of convection, and dictate the driving forces of the system. A better understanding of plate tectonics has broad implications for life on Earth. These include mitigating natural disasters caused by plate motions including volcanoes, earthquakes, and tsunamis. In addition, uplift and subsidence of the tectonic plates affects the sea level, impacting the level of the paleo-oceans and potentially affecting climate change estimates through geologic time. Finally, plate tectonic processes shape the surface morphology of the planet, making continents that enable our existence on land and the ocean basins that hold our free-surface water. Remarkably, despite large amounts of material transfer into and out of the mantle, and multiple scales of convection, plate tectonics has maintained a hydrosphere over billions of years that is favorable for life.</p
A Lithosphere‐Asthenosphere Boundary and Partial Melt Estimated Using Marine Magnetotelluric Data at the Central Middle Atlantic Ridge
No full textThe differential motion between the lithosphere and the asthenosphere is aseismic, so the magnetotelluric (MT) method plays an important role in studying the depth and nature of the lithosphere-asthenosphere boundary (LAB). In March 2016, we deployed 39 marine MT instruments across the Middle Atlantic Ridge (MAR), 2,000 km away from the African coast, to study the evolution of the LAB with ages out to 45 million years (My). The MT acquisition time was limited to about 60 days by battery life. After analyzing dimensionality and coast effects for the MT data, determinant data were inverted for two-dimensional resistivity models along two profiles north and south of the Chain Fracture Zone (CFZ). The imaged thickness of the lithospheric lid (>100 Ωm) ranges from 20 to 80 km, generally thickening with age. In the north of CFZ, punctuated low-resistivity anomalies (<1 Ωm), likely associated with potential partial melts, occur along its base. In the south of CFZ, the base of the resistive lid is demarcated by a low-resistivity channel (<1 Ωm) most likely fed by deeper melts. Sensitivity analyses and structural recovery tests indicate the robustness of these features. Resistivity models are in good agreement with results of seismic data. These results imply that partial melt is persistent over geologic timescales and that the LAB is dynamic features fed by upward percolation of mantle melt. The melt fraction is about 1–7% based on the resistivity, temperature, pressure, and hydrous basalt models, which is consistent with petrophysical observations.</p
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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