22 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
Controls on the formation and stability of gas hydrate-related bottom-simulating reflectors (BSRs): A case study from the west Svalbard continental slope
No full textThe 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
Precision and stability in measurement of split PS-wave time delay and polarization azimuth
No full textCalculating material criticality of transparent conductive electrodes used for thin film and third generation solar cells
No full textThe supply risk and exposure to supply shortage is becoming an important factor in the consideration of a mass low carbon technology roll-out. This study takes current criticality studies, which analyse the criticality of single raw materials, and extends it to calculate the relative criticality of multiple material transparent conductive electrodes (TCE). It compares the calculated criticality with the TCE's Haacke figure of merit. It is found that more recent TCEs developed to replace the commonly used indium tin oxide, such as flurine doped tin oxide and silver nanowires, can have a higher criticality, even though the materials themselves are currently less expensive
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Korean art and the avant-garde dilemma
No full textThe thesis covers Korean avant-garde art history and the dilemma that faced Korean artists at the end of the Japanese Colonial Period (1910-1945). Current literature adequately details avant-garde as progressive fine arts; however, there is limited literature on Korean art in this period. This thesis suggests the term avant-garde dilemma to indicate Korean artists‘ difficulty in style selection to follow a traditional aesthetical trend or progressive socio-political attitude for the foundation of Korean post-modernism. A salient démarche is found when Korean political avant-garde artists meet this dilemma in the midst of the Korean Demonstration Era (1976-1989) that initiates the decline of aesthetical activism and Demonstrative Art. Several styles of avant-garde dilemma after the Korea War are critiqued in the avant-garde evolution; subsequently, there arise hybrid styles between socio-political avant-garde and aesthetical avant-garde styles in Contemporary Korean Art. The examples included are Nam-Jun Baik‘s Video Art (a combination of art and technology), Do-Ho Suh‘s combination of meticulous sculpture with installation to satire Korean neo-capitalist society, Doo-Shik Lee‘s combination of oriental color with western gesture, and Suk-Chang Hong‘s free calligraphy to combine still-life, landscape, calligraphy, and scribbling
Formation of the bottom-simulating reflector and its link to vertical fluid flow
No full textMany places where natural gas hydrate occurs have a regionally extensive, bottom-simulating seismic
reflector (BSR) at the base of the gas hydrate stability zone (GHSZ). This reflection marks the top of an
underlying free-gas zone (FGZ). Usually, hydrate recycling (that produces gas as the stability field moves
upward relative to sediments) is invoked to explain the presence and properties of the sub-BSR FGZ.
However, this explanation is not always adequate: FGZs are often thicker in passive-margin environments
where hydrate recycling is relatively slow, than in convergent-margin environments where hydrate
recycling is relatively fast (e.g. Blake Ridge compared with Cascadia). Furthermore, some areas with thick
FGZs and extensive BSRs (e.g. west Svalbard) have similar rates of hydrate recycling to northern Gulf or
Mexico, yet the latter has no regional BSR.
Here we discuss a gas-forming mechanism that operates in addition to hydrate recycling, and which
produces a widespread, regional, BSR when gas is transported upward through the liquid phase; this
mechanism is dominant in tectonically passive margins. If the gas-water solubility decreases downward
beneath the GHSZ (this occurs where the geothermal gradient and the pressure are relatively high), low
rates of upward fluid flow enable pore water to become saturated in a thick layer beneath the GHSZ. The
FGZ that this produces achieves a steady-state thickness that is primarily sensitive to the rate of upward
fluid flow. Consequently, geophysical observations that constrain the thickness of sub-BSR FGZs can be
used to estimate the regional, diffuse, upward fluid flux through natural gas-hydrate systems.Non UBCUnreviewe
HIGH-FLUX GAS VENTING IN THE EAST SEA, KOREA, FROM ANALYSIS OF 2D SEISMIC REFLECTION DATA.
No full textSeismic reflection data from a multi-channel streamer deployed offshore Korea reveal evidence of hydrateforming
gases being vented into the ocean. Numerous, localised vent structures are apparent from reduced
seismic reflection amplitude, high seismic velocities, and reflector pull-up. These structures penetrate
upward from the base of the gas hydrate stability zone (GHSZ) and are typically several hundred metres
wide, and only a few hundred metres high. Underlying zones of reduced reflection amplitude and low
velocities indicate the presence of gas many kilometers below the seabed, which migrates upward through
near-vertical conduits to feed the vent structures. Where the local geology and underlying plumbing
indicates a high flux of gases migrating through the system, the associated vent structures show the greatest
change of reflector pull-up (the greatest concentration of hydrate) to be near the seabed; where the local
geology and underlying plumbing indicates a moderate flux of gases, the greatest change of reflector pullup
(the greatest concentration of hydrate) is near the base of the GHSZ. The distribution of gas hydrate in
the high-flux gas vent is consistent with the recent salinity-driven model developed for a rapid and
continuous flow of migrating gas, while the hydrate distribution in the lower-flux vent is consistent with a liquid-dominated system. The high-flux vent shows evidence of recent activity at the seabed, and it is likely
that a substantial amount of gas is passing, or has passed, through this vent structure directly into the
overlying ocean.Non UBCUnreviewe
SEISMIC REFLECTION BLANK ZONES IN THE ULLEUNG BASIN, OFFSHORE KOREA, ASSOCIATED WITH HIGH CONCENTRATIONS OF GAS HYDRATE
No full textIt has recently been recognized that abundant gas hydrates occur in localized zones of upwelling
fluids, with concentrations much higher than in regional distributions associated with bottomsimulating
reflectors (BSRs). We report a study of multi-channel seismic reflection data across
such structures in the Ulleung Basin, East Sea backarc offshore Korea, an area with few BSRs.
The structures are commonly up to several km across and a few hundred meters in depth extent,
and are characterized by reduced reflectivity and bowed-up sediment reflectors on time-migrated
sections. The seismic pull-up mainly results from higher velocities, although physical
deformation due to folding and faulting is not ruled out. Some of the features extend upward
close to the seafloor and others only partway through the gas hydrate stability zone. The base of
gas hydrate stability zone (BGHSZ), calculated assuming a regional average constant heat flow
of 110 mW/m2, is confirmed by the presence of gas inferred from reduced instantaneous
frequencies and high instantaneous amplitudes, and from a decrease in seismic velocities. The vents are fed by upward migrating free gas or gas-rich fluids through near-vertical conduits
probably due to regional, upward fluid flow caused by tectonic compression of the basin.Non UBCUnreviewe
A review of health utilities using the EQ-5D in studies of cardiovascular disease
No full textBackground
The EQ-5D has been extensively used to assess patient utility in trials of new treatments within the cardiovascular field. The aims of this study were to review evidence of the validity and reliability of the EQ-5D, and to summarise utility scores based on the use of the EQ-5D in clinical trials and in studies of patients with cardiovascular disease.
Methods
A structured literature search was conducted using keywords related to cardiovascular disease and EQ-5D. Original research studies of patients with cardiovascular disease that reported EQ-5D results and its measurement properties were included.
Results
Of 147 identified papers, 66 met the selection criteria, with 10 studies reporting evidence on validity or reliability and 60 reporting EQ-5D responses (VAS or self-classification). Mean EQ-5D index-based scores ranged from 0.24 (SD 0.39) to 0.90 (SD 0.16), while VAS scores ranged from 37 (SD 21) to 89 (no SD reported). Stratification of EQ-5D index scores by disease severity revealed that scores decreased from a mean of 0.78 (SD 0.18) to 0.51 (SD 0.21) for mild to severe disease in heart failure patients and from 0.80 (SD 0.05) to 0.45 (SD 0.22) for mild to severe disease in angina patients.
Conclusions
The published evidence generally supports the validity and reliability of the EQ-5D as an outcome measure within the cardiovascular area. This review provides utility estimates across a range of cardiovascular subgroups and treatments that may be useful for future modelling of utilities and QALYs in economic evaluations within the cardiovascular area.The authors are grateful for the funding support of the EuroQol Group (PI: Buxton)
