794 research outputs found
Assessment of decadal-scale ecological change at a deep Mid-Atlantic hydrothermal vent and reproductive time-series in the shrimp Rimicaris exoculata
No full textThis study presents a comparison of distribution and abundance of dominant megafaunal species at the TAG hydrothermal mound on the Mid-Atlantic Ridge from 1994 to 2004. A Geographical Information System (GIS) database was compiled from georeferenced observations of faunal abundances at 534 locations on the TAG hydrothermal mound, determined by image analysis of ROV dive footage from November 2004. These data are compared with observations from submersible dives in 1994 to assess changes in the extent and population density of aggregations of the shrimp Rimicaris exoculata at the central black smokers of TAG. The GIS database was also used to assess changes in abundance and distribution of the anemone Maractis rimicarivora by simulating the path of a biotransect conducted in 1994 and 1995. There was no evidence of a decline in the extent of shrimp aggregations at the central black smokers of TAG between 1994 and 2004. This result indicates that occasional exposure to high-intensity submersible lighting, which took place during several scientific expeditions in the intervening period, does not pose an immediate conservation threat to populations of R. exoculata. Similarly, there were no significant differences in the distribution and abundance of anemones between 1994 and 2004. These results indicate a constancy in the identity, distribution and abundance of dominant species at TAG that contrasts with other vent sites where quantitative time-series have been established. The reproductive pattern of R. exoculata was also examined by dissection and direct measurement of oocytes from females collected in September 1994 and November 2004, providing the first comparison of reproductive development in samples from different months for this species. There was no significant difference in oocyte size-frequency distributions of females collected in these samples, indicating a lack of seasonal reproduction in R. exoculata
The response of borehole water levels in an ophiolitic, peridotite aquifer to atmospheric, solid Earth, and ocean tides
No full textPeridotite aquifers are ubiquitous on Earth, but most are in the deep-sea, and thus difficult to access. Ophiolites provide a unique opportunity to study peridotite aquifers, and the Oman Drilling Project established a Multi-Borehole Observatory in a peridotite terrain of the Samail ophiolite. We use the water level response of two 400-m deep boreholes (BA1B, BA1D) to solid Earth, ocean, and atmospheric tides to investigate the hydromechanical structure of the aquifer. The two boreholes are offset by ∼ 100 m but exhibit markedly different tidal responses, indicating a high degree of short-length-scale heterogeneity. Hole BA1B does not respond to tidal strain or barometric loading, consistent with the behavior of an unconfined aquifer. Hole BA1D responds to both tidal strain and barometric loading, indicating some degree of confinement. The response to applied strain, which includes a non-negligible ocean tidal loading component, is consistent with a partially confined, low conductivity aquifer. The response to barometric loading appears to be affected by the complex hydrological structure of the surficial zone and we were not able to fit the observations to within error. Aquifer conductivity estimates for Hole BA1D based on the response to tidal strain are within a factor of ∼ 3 of pumping test estimates
Bayesian probabilistic damage detection of a reinforced-concrete bridge column
No full textA Bayesian probabilistic approach for damage detection has been proposed for the continuous monitoring of civil structures (Sohn H, Law KH. Bayesian probabilistic approach for structure damage detection. Earthquake Engineering and Structural Dynamics 1997; 26: 1259-1281). This paper describes the application of the Bayesian approach to predict the location of plastic hinge deformation using the experimental data obtained from the vibration tests of a reinforced-concrete bridge column. The column was statically pushed incrementally with lateral displacements until a plastic hinge is fully formed at the bottom portion of the column. Vibration tests were performed at different damage stages. The proposed damage detection method was able to locate the damaged region using a simplified analytical model and the modal parameters estimated from the vibration tests, although (1) only the first bending and first torsional modes were estimated from the experimental test data, (2) the locations where the accelerations were measured did not coincide with the degrees of freedom of the analytical model, and (3) there existed discrepancies between the undamaged test structure and the analytical model. The Bayesian framework was able to systematically update the damage probabilities when new test data became available. Better diagnosis was obtained by employing multiple data sets than just by using each test data set separately. Copyright (C) 2000 John Wiley & Sons, Ltd.The "rst author wishes to express his sincere thanks to Professor Gerard C. Pardoen and his students at the
University of California at Irvine for providing an opportunity to observe the bridge column test. Also, the
authors are indebted to Dr Charles R. Farrar and Dr Scott W. Doebling of the Los Alamos National
Laboratory for providing the vibration test data. This research was partially sponsored by the National
Science Foundation under Grant No. CMS-95261-2
Gas Migration Episodes Observed During Peridotite Alteration in the Samail Ophiolite, Oman
No full textSerpentinization and carbonation of mantle rocks (peridotite alteration) are fundamentally important processes for a spectrum of geoscience topics, including arc volcanism, earthquake processes, chemosynthetic biological communities, and carbon sequestration. Data from a hydrophone array deployed in the Multi‐Borehole Observatory (MBO) of the Oman Drilling Project demonstrates that free gas generated by peridotite alteration and/or microbial activity migrates through the formation in discrete bursts of activity. We detected several, minutes‐long, swarms of gas discharge into Hole BA1B of the MBO over the course of a 9 month observation interval. The episodic nature of the migration events indicates that free gas accumulates in the permeable flow network, is pressurized, and discharges rapidly into the borehole when a critical pressure, likely associated with a capillary barrier at a flow constriction, is reached. Our observations reveal a dynamic mode of fluid migration during serpentinization, and highlight the important role that free gas can play in modulating pore pressure, fluid flow, and alteration kinetics during peridotite weathering
Troca ou produção? Uma introdução crítica à proposta teórica de Alfred Sohn-Rethel
No full textThis paper explores and analyzes the critique of the modern-traditional epistemology of the philosopher Alfred Sohn-Rethel, focusing on the categories of "social synthesis" and "real abstraction" that the author postulated in his magnum opus Manual Work and Intellectual Work (2001) . Sohn-Rethel\u27s perspective seeks to critically adapt Marx\u27s materialist conception of history to the analysis of the emergence of the fundamental categories of scientific knowledge, as formulated in European modernity. However, the author\u27s emphasis on exchange over production ends up being inconsistent with basic postulates derived from the critique of political economy formulated by Marx in Capital. Thus, from the review of Marx\u27s categories and taking into account the position of other authors, the research emphasizes the limitations derived from the Sohn-Rethel proposal when analyzing the social forms of knowledge.El presente artículo explora y analiza la crítica a la epistemología moderna-tradicional del filósofo Alfred Sohn-Rethel centrándose en las categorías de “síntesis social” y “abstracción real” que el autor postuló en su obra magna Trabajo manual y trabajo intelectual (2001). La perspectiva de Sohn-Rethel busca adecuar críticamente la concepción materialista de la historia de Marx al análisis de la emergencia de las categorías fundamentales del conocimiento científico, tal y como se formularon en la modernidad europea. Sin embargo, el énfasis que el autor sostiene del intercambio por encima de la producción termina por ser incongruente con postulados básicos derivados de la crítica de la economía política formulada por Marx en El Capital. Así, a partir de la revisión de las categorías de Marx y tomando en cuenta la posición de otros autores, la investigación enfatiza en los limitantes derivados de la propuesta de Sohn-Rethel al analizar las formas sociales del conocimiento.Este artigo explora e analisa a crítica à epistemologia moderno-tradicional do filósofo Alfred Sohn-Rethel, com foco nas categorias de "síntese social" e "abstração real" que o autor postulou em sua magnum opus Trabalho Manual e Trabalho Intelectual (2001). ). A perspectiva de Sohn-Rethel procura adaptar criticamente a concepção materialista da história de Marx à análise da emergência das categorias fundamentais do conhecimento científico, tal como formuladas na modernidade europeia. No entanto, a ênfase do autor na troca sobre a produção acaba sendo inconsistente com os postulados básicos derivados da crítica da economia política formulada por Marx em O Capital. Assim, a partir da revisão das categorias de Marx e levando em conta a posição de outros autores, a pesquisa enfatiza as limitações derivadas da proposta de Sohn-Rethel ao analisar as formas sociais de conhecimento
Application of time reversal guided waves to field bridge testing for baseline free damage diagnosis
No full textThis research is partially supported by Pennsylvania Infrastructure Technology Alliance (PITA) Program and Pennsylvania Department of Transportation (PennDOT). The first author would like to acknowledge the graduate fellowship program from Samsung Lee Kun Hee Scholarship Foundation in Seoul, Korea. The authors would like to thank Paul Majoris and James Johnson for granting access to the Buffalo Creek Bridge site. The authors also would like to thank Profs. David W. Greve and Irving J. Oppenheim for providing invaluable comments and making the scaled girder specimen available for our study
Development of Dual PZT Transducers for Reference-Free Crack Detection in Thin Plate Structures
No full textA new Lamb-wave-based nondestructive testing (NDT) technique, which does not rely on previously stored baseline data, is developed for crack monitoring in plate structures. Commonly, the presence of damage is identified by comparing "current data" measured from a potentially damaged stage of a structure with "baseline data" previously obtained at the intact condition of the structure. In practice, structural defects typically take place long after collection of the baseline data, and the baseline data can be also affected by external loading, temperature variations, and changing boundary conditions. To eliminate the dependence on the baseline data comparison, the authors previously developed a reference-free NDT technique using 2 pairs of collocated lead zirconate titanate (PZT) transducers placed on both sides of a plate. This reference-free technique is further advanced in the present study by the necessity of attaching transducers only on a single surface of a structure for certain applications such as aircraft. To achieve this goal, a new design of PZT transducers called dual PZT transducers is proposed. Crack formation creates Lamb wave mode conversion due to a sudden thickness change of the structure. This crack appearance is instantly detected from the measured Lamb wave signals using the dual PZT transducers. This study also suggests a reference-free statistical approach that enables damage classification using only the currently measured data set. Numerical simulations and experiments were conducted using an aluminum plate with uniform thickness and fundamental Lamb waves modes to demonstrate the applicability of the proposed technique to reference-free crack detection.This research is supported by the Radiation Technology Program under
Korea Science and Engineering Foundation (KOSEF) and the Ministry
of Science and Technology (M20703000015-07N0300-01510) and
Korea Research Foundation Grant funded by the Korean Government
(MOEHRD, Basic Research Promotion Fund) (KRF-2007-331-D00462).
Any opinions, findings, and conclusions or recommendations expressed
in this material are those of the author(s) and do not necessarily reflect
the views of the funding agencies
Continuous fatigue crack monitoring without baseline data
No full textIn order to overcome the susceptibility of conventional non-destructive testing (NDT) techniques to operational and environmental variations, a new damage detection technique that does not require direct comparison with baseline data was previously developed by the authors for detecting a crack in a plate structure. This reference-free technique employs two pairs of collocated lead zirconate titanate transducers (PZTs) placed on both sides of the plate to generate and measure Lamb waves. Then, the existence of mode conversion due to the crack is identified from the Lamb wave signals instantly measured by PZTs. In this study, the effectiveness of the proposed technique is tested using a steel girder specimen. A cyclic loading is applied to the girder resulting in fatigue cracks, and the proposed technique detects the appearance of fatigue damage solely based on the measured Lamb waves at the present stage. Experimental results are presented to demonstrate the applicability of the proposed technique to fatigue crack monitoring, and issues related to PZT installation are discussed.This research is supported by the Radiation Technology
Program under Korea Science and Engineering Foundation
(KOSEF) and the Ministry of Science and Technology
(M20703000015-07N0300-01510) andKorea Research
Foundation Grant funded by the Korean Government
(MOEHRD, Basic Research Promotion Fund)
(KRF-2007-331-D00462). Any opinions, findings and
conclusions or recommendations expressed in this material
are those of the author(s) and do not necessarily reflect
the views of the funding agencies. The authors also
would like to thank Professors Kent A. Harris and Piervincenzo
Rizzo at University of Pittsburgh for designing
the steel girder specimen and conducting fatigue loading
tests
Energy-based Reference-Free Damage Diagnosis using A Single Pair of Collocated PZTs
No full textThis research is supported by the Radiation Technology Program (M20703000015-07N0300-01510) under Korea Science and Engineering Foundation (KOSEF) and U-Eco city project (C007L7510001-08L015800110) under Korea Institute of Construction & Transportation technology Evaluation and Planning (KICTEP). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the funding agencies
Parameter estimation of the generalized extreme value distribution for structural health monitoring
No full textStructural health monitoring (SHM) can be defined as a statistical pattern recognition problem which necessitates establishing a decision boundary for damage identification. In general, data points associated with damage manifest themselves near the tail of a baseline data distribution, which is obtained from a healthy state of a structure. Because damage diagnosis is concerned with outliers potentially associated with damage, improper modeling of the tail distribution may impair the performance of SHM by misclassifying a condition state of the structure. This paper attempts to address the issue of establishing a decision boundary based on extreme value statistics (EVS) so that the extreme values associated with the tail distribution can be properly modeled. The generalized extreme value distribution (GEV) is adopted to model the extreme values. A theoretical framework and a parameter estimation technique are developed to automatically estimate model parameters of the GEV. The validity of the proposed method is demonstrated through numerically simulated data, previously published real sample data sets, and experimental data obtained from the damage detection study in a composite plate. (c) 2005 Elsevier Ltd. All rights reserved.The authors would like to thank Prof. Keith Worden’s
contribution to the initial implementation of the differential
evolution code used in this study. The authors also wish to
recognize the Weapon Response Group (ESA-WR) of Los
Alamos National Laboratory for providing the experimental
data for the composite plate test for this study. The first
author would like to acknowledge the Post-doctoral Fellowship
Program of the Korea Science & Engineering Foundation
(KOSEF) in 2003
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