1,721,083 research outputs found
Layer stripping of shear-wave splitting in marine PS waves
No full textThe properties of split S waves can be used to infer: (1) the state of stress and strain in the Earth; (2) the directional dependence of hydraulic conductivity and (3) small changes in pore-fluid pressure in the rock mass that occur in response to dynamic processes, such as the earthquake cycle. Measurements of split S waves are particularly useful in shallow (<1000 m subseabed) marine sediments, where S-wave splitting from an azimuthal elastic anisotropy is typically produced by the presence of near-vertical aligned cracks. Here we present a method of measuring small amounts of S-wave splitting in marine P-to-S mode-converted waves, and illustrate the technique with data from an ocean-bottom seismometer (OBS) deployed on the west Svalbard continental slope. The analysis applies a modified version of the Alford rotation and layer-stripping technique developed for zero-offset S-wave sources and treats PS waves that undergo mode conversion at reflectors that are close to the seabed in comparison with the overlying water depth. When the seismic record contains coherent signal on both the in-plane and out-of-plane components, the layer-stripping technique is capable of decoupling the S-wave splitting from the effects of P-wave velocity anisotropy and reflector dip that influence the downgoing, P wave, part of the ray path. The amount of S-wave splitting in the data is small, however, and we find that this causes a greater practical problem for the analysis than the known theoretical limitations of the layer-stripping theory (such as use of a finite-offset source). For the analysis of the example data we develop a number of procedures that are necessary to mitigate the low signal-to-noise levels. These include using a wide range of shot-receiver azimuths to generate data redundancy, methods of identifying and rejecting poor measurements, and a predictive layer-stripping approach that minimizes the propagation of errors through the analysis that arise from scatter in the layer-by-layer results.With the PS waves of the example data, which have a dominant period about 30 ms, we find the technique is capable of measuring S-wave splitting to a precision of about 0.5 ms for 4 or 5 layers. The number of layers successfully treated would increase if the amount of S-wave splitting were larger than in these data, for which the total cumulative S-wave splitting was about 10 ms over a 450 m depth interval. The orientation of the fast split S wave was measured with a precision of about 15°. Our results give an S-wave velocity anisotropy of 1–2 per cent in the shallowest 25–30 m subseabed that implies the presence of a differential horizontal stress at, or very close to, the seabed. The S-wave splitting accumulated throughout the investigated section at a rate that was consistent with predictions made for a single set of parallel, fluid-filled cracks with crack-density about 0.015. The fast S wave was found to be oriented at 75 ± 15° for the uppermost 150 m, before drifting clockwise to an azimuth of 190–210°. The clockwise drift in the fast S-wave polarization direction implies a transfer of dominance between one set of cracks in the near surface (probably related to the slope of the seabed) and a different set in deeper sediments of tectonic origin. A zone of azimuthal isotropy, where the mechanisms that produce the elastic anisotropy may cancel each other out, occupies the interval between the two almost orthogonal sets of cracks. From interpretation of the azimuthal variation in the P-wave component of PS traveltimes, we found a P-wave velocity anisotropy of 1–2 per cent that was fastest in a direction parallel to the fast S-wave polarization
CSEM Survey of a Methane Vent Site, Offshore West Svalbard
No full textIn July 2012, the scientific cruise JCR269B on RRS James Clarke Ross conducted two Controlled Source ElectroMagnetic (CSEM) surveys over the West Svalbard margin to complement previously collected seismic data. The objective was to evaluate hydrate and free gas saturations in the submarine sediments in this area
Controls on the formation and stability of gas hydrate-related bottom-simulating reflectors (BSRs): A case study from the west Svalbard continental slope
Full text linkThe growth and stability of the free-gas zone (FGZ) beneath gas-hydrate related bottom-simulating seismic reflectors (BSRs) is investigated using analytical and numerical analyses to understand the factors controlling the formation and depletion of free gas. For a model based on the continental slope west of Svalbard (a continental margin of north Atlantic type), we find that the FGZ is inherently unstable under a wide range of conditions because upward flow of under-saturated liquid depletes free gas faster than it is produced by hydrate recycling. In these scenarios, the 150-m-thick FGZ that presently exists there would deplete within 105–106 years. We suggest the FGZ is in a stable state, however, that is formed by a diffusion-dominated mechanism that produces low concentrations of gas in a FGZ of steady state thickness. Gas forms across a thick zone because the upward fluid flux is relatively low and because the gas–water solubility decreases to a minimum several hundred meters below the seabed. This newly understood solubility-curvature effect is complementary to hydrate recycling, but becomes the most important factor controlling the presence and properties of the BSR in environments where the rate of upward fluid flow and the rate of hydrate recycling are both relatively low (i.e., rifted continental margins). If the present-day FGZ is in steady state, we estimate that the upward fluid flux in the west Svalbard site must be less than 0.15 mm a?1
Resistivity image beneath an area of active methane seeps in the west Svalbard continental slope
Full text linkThe Arctic continental margin contains large amounts of methane in the form of methane hydrates. The west Svalbard continental slope is an area where active methane seeps have been reported near the landward limit of the hydrate stability zone. The presence of bottom simulating reflectors (BSRs) on seismic reflection data in water depths greater than 600 m suggests the presence of free gas beneath gas hydrates in the area. Resistivity obtained from marine controlled source electromagnetic (CSEM) data provides a useful complement to seismic methods for detecting shallow hydrate and gas as they are more resistive than surrounding water saturated sediments. We acquired two CSEM lines in the west Svalbard continental slope, extending from the edge of the continental shelf (250 m water depth) to water depths of around 800 m. High resistivities (5-12 Ωm) observed above the BSR support the presence of gas hydrate in water depths greater than 600 m. High resistivities (3-4 Ωm) at 390-600 m water depth also suggest possible hydrate occurrence within the gas hydrate stability zone (GHSZ) of the continental slope. In addition, high resistivities (4-8 Ωm) landward of the GHSZ are coincident with high-amplitude reflectors and low velocities reported in seismic data that indicate the likely presence of free gas. Pore space saturation estimates using a connectivity equation suggest 20-50 per cent hydrate within the lower slope sediments and less than 12 per cent within the upper slope sediments. A free gas zone beneath the GHSZ (10-20 per cent gas saturation) is connected to the high free gas saturated (10-45 per cent) area at the edge of the continental shelf, where most of the seeps are observed. This evidence supports the presence of lateral free gas migration beneath the GHSZ towards the continental shelf.</p
Corrigendum to "Switching of a paleo-ice stream in northwest Svalbard" [Quat. Sci. Rev. 30 (2011), 1710-1725]
No full textIce streams are the fast-flowing zones of ice sheets that can discharge a large flux of ice. The glaciated western Svalbard margin consists of several cross-shelf troughs which are the former ice stream drainage pathways during the Pliocene–Pleistocene glaciations. From an integrated analysis of high-resolution multibeam swath-bathymetric data and several high-resolution two-dimensional reflection seismic profiles across the western and northwestern Svalbard margin we infer the ice stream flow directions and the deposition centres of glacial debris that the ice streams deposited on the outer margin. Our results show that the northwestern margin of Svalbard experienced a switching of a major ice stream. Based on correlation with the regional seismic stratigraphy as well as the results from ODP 911 on Yermak Plateau and ODP 986 farther south on the western margin of Spitsbergen, off Van Mijenfjord, we find that first a northwestward flowing ice stream developed during initial northern hemispheric cooling (starting not, vert, similar2.8–2.6 Ma). A switch in ice stream flow direction to the present-day Kongsfjorden cross-shelf trough took place during a glaciation at not, vert, similar1.5 Ma or probably later during an intensive major glaciation phase known as the ‘Mid-Pleistocene Revolution’ starting at not, vert, similar1.0 Ma. The seismic and bathymetric data suggest that the switch was abrupt rather than gradual and we attribute it to the reaching of a tipping point when growth of the Svalbard ice sheet had reached a critical thickness and the ice sheet could overcome a topographic barrier
Record of methane emissions from the West Svalbard continental margin during the last 23,500years revealed by ?13C of benthic foraminifera
Full text linkThe values of ?13C in benthic foraminifera have been measured in a gas-hydrate-bearing sediment core collected from an area of active methane venting on the Vestnesa Ridge (West Svalbard continental margin) to reconstruct the local history of methane emissions over the past 23,500 years BP. The chronostratigraphic framework of the core has been derived from AMS 14C dates and biostratigraphic analysis. While foraminifera from some intervals have ?13C within the normal marine range (0 to -1‰), five intervals are characterised by much lower ?13C, as low as -17.4‰. These intervals are interpreted to record the incorporation of 13C-depleted carbon in the presence of methane emissions at the seafloor during biomineralization of the carbonate foraminiferal tests and subsequent secondary mineralization. Methane emission events (MEE) occur from the Last Glacial Maximum (LGM) to the Holocene, with the most prominent one, in terms of ?13C depletion, predating the Bølling-Allerød Interstadial (GI-1 in the Greenland ice core record). The lack of correlation between the values of ?13C and ?18O, however, appears to preclude warming of bottom waters as the principal control on methane release. Rather, it seems likely that methane release is a consequence of episodicity in the supply of gas to the hydrate system and in the processes that enable methane gas to migrate through the hydrate stability field to the seabed, or of other geological processes still under debate
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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