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21.
Josh Kohut Hugh Roarty Evan Randall-Goodwin Scott Glenn C. Sage Lichtenwalner 《Ocean Dynamics》2012,62(6):953-968
The National High Frequency (HF) Surface Current Mapping Radar Network is being developed as a backbone system within the U.S. Integrated Ocean Observing System. This paper focuses on the application of HF radar-derived surface current maps to U.S. Coast Guard Search and Rescue operations along the Mid-Atlantic coast of the USA. In that context, we evaluated two algorithms used to combine maps of radial currents into a single map of total vector currents. In situ data provided by seven drifter deployments and four bottom-mounted current meters were used to (1) evaluate the well-established unweighted least squares (UWLS) and the more recently adapted optimal interpolation (OI) algorithms and (2) quantify the sensitivity of the OI algorithm to varying decorrelation scales and error thresholds. Results with both algorithms were shown to depend on the location within the HF radar data footprint. The comparisons near the center of the HF radar coverage showed no significant difference between the two algorithms. The most significant distinction between the two was seen in the drifter trajectories. With these simulations, the weighting of radial velocities by distance in the OI implementation was very effective at reducing both the distance between the actual drifter and the cluster of simulated particles as well as the scale of the search area that encompasses them. In this study, the OI further reduced the already improved UWLS-based search areas by an additional factor of 2. The results also indicated that the OI output was relatively insensitive to the varying decorrelation scales and error thresholds tested. 相似文献
22.
C.Geoffrey Wheat Hans W. Jannasch Josh N. Plant 《Earth and Planetary Science Letters》2003,216(4):549-564
Osmotically pumped fluid samplers were deployed in four deep-sea boreholes that were drilled during Ocean Drilling Program (ODP) Leg 168 on the eastern flank of the Juan de Fuca Ridge. Samplers were recovered from ODP Sites 1024 and 1027 and aliquots were analyzed for a variety of dissolved ions. Results from both of the samplers show a drastic change in the major ion composition within the first 20-40 days after the borehole was sealed at the seafloor followed by a more gradual change in composition. This gradual change ceased after 820 days at Site 1024 but continued throughout the 3-year deployment at Site 1027. We modeled this change in composition to estimate the flux of formation fluid through the open borehole. The rapid early change requires a flow of ∼1500 kg of formation fluid per day. The more gradual later change requires flow rates of 38 kg/day at Site 1024 and 17.5 kg/day at Site 1027. The latter fluxes require a minimum average specific discharge of meters to hundreds of meters per year through the surrounding basaltic matrix. Trace element data show surprisingly little contamination given the presence of steel casing, Li-organic-rich grease at each joint, cement, and drilling muds. Observed changes in trace element concentrations relative to those of bottom seawater provide a measure for the global significance of cool (23°C; ODP Site 1024) ridge flank hydrothermal systems relative to warm (64°C; Baby Bare and ODP Site 1027) hydrothermal systems and illustrate the importance of these cooler systems to global geochemical budgets. 相似文献
23.
Lora R. Stevens Jeffery R. Stone Josh Campbell Sherilyn C. Fritz 《Quaternary Research》2006,65(2):264-274
A 2200-yr long, high-resolution (∼5 yr) record of drought variability in northwest Montana is inferred from diatoms and δ18O values of bio-induced carbonate preserved in a varved lacustrine core from Foy Lake. A previously developed model of the diatom response to lake-level fluctuations is used to constrain estimates of paleolake levels derived from the diatom data. High-frequency (decadal) fluctuations in the de-trended δ18O record mirror variations in wet/dry cycles inferred from Banff tree-rings, demonstrating the sensitivity of the oxygen-isotope values to changes in regional moisture balance. Low frequency (multi-centennial) isotopic changes may be associated with shifts in the seasonal distribution of precipitation. From 200 B.C. to A.D. 800, both diatom and isotope records indicate that climate was dry and lake level low, with poor diatom preservation and high organic carbon: nitrogen ratios. Subsequently, lake level rose slightly, although the climate was drier and more stable than modern conditions. At A.D. 1200, lake level increased to approximately 6 m below present elevation, after which the lake fluctuated between this elevation and full stage, with particularly cool and/or wetter conditions after 1700. The hydrologic balance of the lake shifted abruptly at 1894 because of the establishment of a lumber mill at the lake's outlet. Spectral analysis of the δ18O data indicates that severe droughts occurred with multi-decadal (50 to 70 yr) frequency. 相似文献
24.
Kerry L. Sublette J. Bryan Tapp J. Berton Fisher Eleanor Jennings Kathleen Duncan Greg Thoma Josh Brokaw Tim Todd 《Applied Geochemistry》2007
For almost a decade the Tallgrass Prairie Preserve in Oklahoma has been used as a field laboratory for the investigation of aspects of the remediation and restoration of oil and brine spills. Objectives of this work have included: (1) simplification of the remediation process and lowering the cost of remediation; (2) the development of methods to accelerate or jump-start the restoration process; and (3) determining appropriate metrics for assessing the status of soil ecosystem recovery. This research has resulted in a number of lessons learned that will be presented here which can be exported to other exploration and production sites, especially sites located in sensitive ecosystems. Key observations have included the role of a fertilizer amendment in linking the remediation and restoration process at an oil-impacted site, the use of nematodes as ecological indicators in the restoration of oil- and brine-impacted sites, and the development of a two-stage process for remediation of brine impacted sites that does not include significant use of gypsum. 相似文献
25.
近年来,在全球能源转型的大背景下,高温地热发电得到世界各国极大的重视,前景广阔。要实现对高温地热能源的开采,采用的钻井技术是关键。北美从20世纪50年代开始,对地热能源商业化开采、并用以发电的技术进行了持续性的研究和试验。在高温硬岩地热钻井技术上形成了完整的体系,比如高温地热井设计与井筒完整性技术,高温、超高温坚硬岩石钻头设计与生产技术,高温井底动力钻具,高温井下测量工具,高温泥浆与堵漏技术,高温水泥浆体系以及高温井控技术等。本文结合作者在国内地热井钻完井及北美地热井的工作经验,概要介绍北美的地热钻井技术现状、地热钻井技术难点以及与之相关的关键技术。 相似文献
26.
The behaviour of Li and Mg isotopes during primary phase dissolution and secondary mineral formation in basalt 总被引：1，自引：0，他引：1
Josh Wimpenny Sigurður R. Gíslason Abdelmouhcine Gannoun Philip A.E. Pogge Von Strandmann 《Geochimica et cosmochimica acta》2010,74(18):5259-6026
This study presents lithium (Li) and magnesium (Mg) isotope data from experiments designed to assess the effects of dissolution of primary phases and the formation of secondary minerals during the weathering of basalt. Basalt glass and olivine dissolution experiments were performed in mixed through-flow reactors under controlled equilibrium conditions, at low pH (2-4) in order to keep solutions undersaturated (i.e. far-from equilibrium) and inhibit the formation of secondary minerals. Combined dissolution-precipitation experiments were performed at high pH (10 and 11) increasing the saturation state of the solutions (moving the system closer to equilibrium) and thereby promoting the formation of secondary minerals.At conditions far from equilibrium saturation state modelling and solution stoichiometry suggest that little secondary mineral formation has occurred. This is supported by the similarity of the dissolution rates of basalt glass and olivine obtained here compared to those of previous experiments. The δ7Li isotope composition of the experimental solution is indistinguishable from that of the initial basalt glass or olivine indicating that little fractionation has occurred. In contrast, the same experimental solutions have light Mg isotope compositions relative to the primary phases, and the solution becomes progressively lighter with time. In the absence of any evidence for secondary mineral formation the most likely explanation for these light Mg isotope compositions is that there has been preferential loss of light Mg during primary phase dissolution.For the experiments undertaken at close to equilibrium conditions the results of saturation state modelling and changes in solution chemistry suggest that secondary mineral formation has occurred. X-ray diffraction (XRD) measurements of the reacted mineral products from these experiments confirm that the principal secondary phase that has formed is chrysotile. Lithium isotope ratios of the experimental fluid become increasingly heavy with time, consistent with previous experimental work and natural data indicating that 6Li is preferentially incorporated into secondary minerals, leaving the solution enriched in 7Li. The behaviour of Mg isotopes is different from that anticipated or observed in natural systems. Similar to the far from equilibrium experiments initially light Mg is lost during olivine dissolution, but with time the δ26Mg value of the solution becomes increasingly heavy. This suggests either preferential loss of light, and then heavy Mg from olivine, or that the secondary phase preferentially incorporates light Mg from solution. Assuming that the secondary phase is chrysotile, a Mg-silicate, the sense of Mg fractionation is opposite to that previously associated with silicate soils and implies that the fractionation of Mg isotopes during silicate precipitation may be mineral specific. If secondary silicates do preferentially remove light Mg from solution then this could be a possible mechanism for the relatively heavy δ26Mg value of seawater. This study highlights the utility of experimental studies to quantify the effects of natural weathering reactions on the Li and Mg geochemical cycles. 相似文献
27.
Victor?GiurgiutiuEmail author Kent?Harries Michael?Petrou Joel?Bost Josh?B.?Quattlebaum 《地震工程与工程振动(英文版)》2003,2(2):213-223
The capability of embedded piezoelectric wafer active sensors (PWAS) to perform in-situ nondestructive evaluation (NDE) for structural health monitoring (SHM) of reinforced concrete (RC) structures strengthened with fiber reinforced polymer (FRP) composite overlays is explored. First, the disbond detection method were developed on coupon specimens consisting of concrete blocks covered with an FRP composite layer. It was found that the prescnce of a disbond crack drastically changes the electromecbanical (E/M) impedance spectrum measured at the PWAS terminals. The spectral changes depend on the distance between the PWAS and the crack tip. Second, large scale experiments were conducted on a RC beam strengthened with carbon fiber reinforced polymer (CFRP) composite overlay. The beam was subject to an acccleratcd fatigue load regime in a three-point bending configuration up to a total of807,415 cycles. During these fatigue tests, the CFRP overlay experienced disbonding beginning at about 500,000 cycles. The PWAS were able to detect the disbonding before it could be reliably seen by visual inspection. Good correlation between the PWAS readings and the position and extent of disbond damage was observed. These preliminary results demonstrate the potential of PWAS technology for SHM of RC structures strengthened with FRP composite ovcrlays. 相似文献
28.
Josh E. Moncrieff Laurence R. Bentley Heyddy Calderón Palma 《Hydrogeology Journal》2008,16(1):183-197
Nicaragua’s León-Chinandega aquifer has seen extensive contamination by persistent organochlorine pesticides applied over
decades of intensive agricultural activity. Models of flow and transport of a 330 km2 sub-region of the aquifer were developed to test conceptual models of contaminant transport, to constrain the value of certain
key transport parameters, and to investigate contamination-related concerns raised in past studies. To support these models,
a variety of hydrogeologic and geochemical data were collected. It was concluded that the organochlorine pesticides seen in
groundwater originate in soils, and are transported to the water table through widespread preferential flow, through shortcutting
around wells, or through wind-blown particles delivered to poorly protected hand-dug wells. The distribution coefficient (K
d) of these pesticides is estimated to be between 0.1 and 100 ml/g and the concentration of pesticides being delivered to the
water table is estimated to be between 102 and 105 ng/L. It was found that the distribution and concentration of pesticides in the aquifer would be affected by an increase
in groundwater abstraction within the region.
Résumé L’aquifère du León-Chinandega au Nicaragua a été soumis à des contaminations étendues par des pesticides organochlorés persistants qui ont été répandus pendant des dizaines d’années d’agriculture intensive. Des modèles d’écoulement et de transport d’une sous-région de 330 km2 de l’aquifère ont été construits pour tester des modèles conceptuels du transport de contaminants, pour contraindre la valeur de certains paramètres de transport clefs, et pour étudier les problèmes liés au transport de contaminations qui avaient été rencontrés lors d’études précédentes. Différents types de données hydrogéologiques et géochimiques ont été collectés pour réaliser ces modèles. La conclusion montre que les pesticides organochlorés rencontrés dans les eaux souterraines proviennent des sols et qu’ils sont transportés jusqu’à l’aquifère via de grandes zones d’écoulement préférentiel, via des raccourcis hydrauliques autour des puits, ou encore via des particules emportées par le vent et tombées dans des puits creusés à la main et mal protégés. Le coefficient de distribution (K d) de ces pesticides est estimé entre 0.1 et 100 ml/g et la concentration de pesticides arrivant dans la nappe est estimée entre 102 et 105 ng/L. L’étude a montré qu’une augmentation des prélèvements d’eau souterraine dans la région aurait un impact sur la distribution et la concentration des pesticides dans l’aquifère.
Resumen El acuifero León-Chinandega (Nicaragua) presenta una contaminación extensiva por la aplicación de plaguicidas organoclorados en la agricultura intensiva durante décadas. Se han desarrollado modelos de flujo y transporte de una subregión del acuífero de 330 km2 para comprobar los modelos conceptuales del transporte de contaminantes, estimar los valores de ciertos parámetros clave de transporte e investigar los temas relacionados con la contaminación en estudios anteriores. Para elaborar estos modelos, se recogió una variedad de datos hidrogeológicos y geoquímicos. Se ha concluido que los plaguicidas organoclorados presentes en el agua subterránea se originan en el suelo y son transportados hacia el nivel freático a través de flujos preferentes generalizados, a través de cortes alrededor de los pozos o a través de partículas transportadas por el viento depositadas en pozos excavados a mano con poca protección. El coeficiente de distribución (K d) de estos plaguicidas se ha estimado entre 0.1 y 100 ml/g y la concentración de plaguicidas que sale hacia el nivel freático se estima que puede estar entre 102 y 105 ng/L. Se ha concluido que el incremento de la extracción de aguas subterráneas dentro de la región afectaría la distribución y concentración de plaguicidas en el acuífero.相似文献
29.
Observations of surface velocity data from August 2002 to February 2004 were collected by a series of four long-range high-frequency (HF) radars along the coast of New Jersey. The shelf observations of the central Mid-Atlantic Bight (MAB) were compared to historical observations of surface flow characteristics in the area. The time-averaged spatial mean velocity of 4 cm/s in the down-shelf along-shelf direction and 3 cm/s in the offshore across-shelf direction compared very well to historical surface measurements in the study region. However, as the spatial resolution of the data set revealed, this simple measure masked significant spatial variations in the overall and seasonal mean flow structures. Three regions – the south bank of the Hudson Shelf Valley, the southern New Jersey inner shelf (LEO-15) region, and the region offshore of the Delaware Bay mouth (southwest corner) – had mean flows that favor offshore transport of surface water. In terms of temporal variability, maps of the principle axes showed that the across-shelf (minor) axis contribution was not insignificant in the surface layer ranging from 0.3 to 0.9 of along-shelf (major) axis and that there were seasonal differences in orientation and ellipticity. Analysis of the spatial changes in the temporal and spatial correlation scales over the shelf showed that shelf position, in addition to site separation, contributed to the differences in these properties. Furthermore, observations over the Hudson Shelf Valley region suggested that this was a region of transition in which the orientation of along- and across-shelf components begin to change. 相似文献
30.
Seismic interferometry is a relatively new technique to estimate the Green's function between receivers. Spurious energy, not part of the true Green's function, is produced because assumptions are commonly violated when applying seismic interferometry to field data. Instead of attempting to suppress all spurious energy, we show how spurious energy associated with refractions contains information about the subsurface in field data collected at the Boise Hydrogeophysical Research Site. By forming a virtual shot record we suppress uncorrelated noise and produce a virtual refraction that intercepts zero offset at zero time. These two features make the virtual refraction easy to pick, providing an estimate of refractor velocity. To obtain the physical parameters of the layer above the refractor we analyse the cross‐correlation of wavefields recorded at two receivers for all sources. A stationary‐phase point associated with the correlation between the reflected wave and refracted wave from the interface identifies the critical offset. By combining information from the virtual shot record, the correlation gather and the real shot record we determine the seismic velocities of the unsaturated and saturated sands, as well as the variable relative depth to the water‐table. Finally, we discuss how this method can be extended to more complex geologic models. 相似文献