A mobile coherent low-frequency acoustic range     

Spindel  R. Porter  R. Webb  D. 《Oceanic Engineering, IEEE Journal of》1977,2(4):331-337

A portable system for the measurement of certain underwater acoustic propagation phenomena is described. Acoustic sources and receivers of special design are used. A precision tracking system enables coherent signal reception in the presence of source-receiver motion. The major elements of the acoustic range are described and examples of data are presented.  相似文献   

Using tree stems in multi-temporal terrestrial lidar scanning data to monitor landslides on vegetated slopes     

van Veen  Megan  Porter  Michael  Lato  Matt  Mitchell  Andrew  Fish  Colleen  Van Gassen  Wim 《Landslides》2022,19(4):829-840

Landslides - Terrestrial lidar scanning (TLS) has become a widely accepted expert tool for monitoring geohazards on bare or sparsely vegetated slopes through change detection. While trees can be an...  相似文献   

Dissimilatory sulfate reduction in hypersaline coastal pans: Activity across a salinity gradient   总被引：1，自引：0，他引：1  

Donovan Porter  Donald Cowan 《Geochimica et cosmochimica acta》2007,71(21):5102-5116

The impact of salinity on the metabolic activity of sulfate-reducing bacteria in five highly saline to hypersaline coastal pans was studied using a radioactive tracer technique. We recorded sulfate reduction at in situ porewater salinities of up to 422. Furthermore, enumeration of sulfate reduction rates in whole core incubations conducted under in situ conditions suggested a high variability in the activity of sulfate-reducers. Average reduction rates (27-3685 nmol cm⁻³ d⁻¹) varied according to depth, season and site sampled. The highest reduction rates measured in the hypersaline pan were comparable to the highest reported rates from highly productive salt marsh and microbial mat ecosystems. Correspondingly, the depth-integrated rates (integrated to 12 cm) varied from 6 to 241 mmol m⁻² d⁻¹ and were also among the highest ever reported rates. The reduction rates decreased down-core and, surprisingly, were highest in the winter season when the lowest sediment temperatures were encountered.High salt concentrations did not inhibit sulfate reduction rates. Rather, higher rates were measured at pans with higher in situ salinities. In laboratory slurry incubation experiments, sediments from the saltpans were treated with increasing salt concentrations. Regression analysis suggested that the short term response of microbial consortia to up-shock was an increase in sulfate reduction activity up to salinities of 272-311 and 134-244, in hypersaline and highly saline pans, respectively. Beyond these salinities, the cells showed evidence of reduced activities.  相似文献   

The impact of resuspension on sediment mercury dynamics, and methylmercury production and fate: A mesocosm study   总被引：5，自引：0，他引：5  

Eun-Hee Kim  Robert P. Mason  Elka T. Porter  Heather L. Soulen 《Marine Chemistry》2006,102(3-4):300-315

The objective of this study was to investigate the effects of resuspension on the fate and bioaccumulation of mercury (THg) and methylmercury (MeHg) in shallow estuarine environments, using mesocosms. Two 4-week experiments were conducted in July (Experiment 1) and October (Experiment 2) of 2001 with Baltimore Harbor sediments. Hard clams, Mercenaria mercenaria, were introduced into sediments for Experiment 2. Tidal resuspension (4 h on and 2 h off cycles) was simulated, with 3 replicate tanks for each treatment—resuspension (R) and non-resuspension (NR). Sediment cores were collected during the experiments for THg, MeHg, organic content and AVS analyses, and for the determination of methylation/demethylation using Hg stable isotopes (¹⁹⁹Hg(II) and CH₃¹⁹⁹Hg(II)). Zooplankton samples were collected once a week while clams were taken before and after Experiment 2 for THg and MeHg analyses. Our results suggest that the interplay between Hg methylation and MeHg degradation determines the overall MeHg pool in sediments. Sediment resuspension does not appear to directly impact the Hg transformations but can lead to changes in the association to Hg binding phases, influencing Hg methylation. The bioaccumulation results indicate that sediment resuspension can play an important role in transferring sediment MeHg into organisms.  相似文献   

Hydrodynamic coefficients of a harmonically oscillated tower     

Subrata K. Chakrabarti  Dennis C. Cotter  Alan R. Libby 《Applied Ocean Research》1983,5(4):226-233

A tower hinged at the bottom was oscillated mechanically in a sinusoidal fashion in a plane in still water. An instrumented section in the tower measured the inline and transverse forces locally on the tower due to the hydrodynamic effects. These forces are analyzed for the added mass, drag and lift coefficients which are presented as functions of Keulegan-Carpenter and Reynolds number. The lift force frequencies are also investigated. The measured overall reactions on the tower are used to verify the values of the local coefficients. The results presented here are not only applicable to articulated towers but to other moving elements of an offshore structure, e.g. risers, tension-legs, etc.  相似文献   

Stable isotope evidence for regional-scale fluid migration in a Barrovian metamorphic terrane,Vermont, USA     

Libby A. Stern  C. Page Chamberlain  Daniel E. Barnett  John M. Ferry 《Contributions to Mineralogy and Petrology》1992,112(4):475-489

We measured the C- and O-isotopic composition of carbonate minerals in the Waits River Formation, eastern Vermont, to determine the extent of fluid infiltration during regional metamorphism over an approx. 2000 km² area in a deep-seated (>25 km) Barrovian terrane. From a petrologic study of this terrane, Ferry proposed the existence of a large regional metamorphic hydrothermal system with two first-order features: (1) on the scale of the entire terrane fluid flow was focused into the axes of two major antiforms of regional extent; (2) on a smaller scale (about 100 km²) flow was further focused around synmetamorphic granitic plutons that intruded along the axes of the antiforms. We find isotopic evidence for both the regional hydrothermal activity along the antiforms and the more intense fluid flow around synmetamorphic plutons. The evidence for hydrothermal activity around the plutons is large heavy isotope depletions, up to 6–9 in ¹⁸O_carb and ¹³C_carb, in diopside zone rocks adjacent to the plutons. These isotopic shifts are greater than can be explained solely by prograde metamorphic reactions. We find two lines of evidence for the more diffuse regional flow that was focused into axes of the antiforms. First, ¹⁸O_carb and ¹³C_carb, within individual outcrops become increasingly homogeneous with increasing grade towards antiform axes, indicating that the rocks equilibrated with a permeating fluid. Second, there are depletions in ¹⁸O near the margins of the Waits River Formation which can be interpreted as a dispersed, advective infiltration front displaced toward the antiform axes. These fronts were modelled using Eq. 13 of Bickle and Baker and imply time-integrated fluid of 10⁵–10⁶ cm³/cm², which are consistent with values derived by Ferry from measured progress of prograde devolatilization reactions.  相似文献   

Playing the 'disabled role' in local travel     

Alison Porter 《Area》2000,32(1):41-48

Summary This paper describes how people with a range of physical impairments use the 'disabled role' to enable them to make local journeys in Swansea. It examines the processes by which interaction between people and social institutions serves to overcome transport disability and to reinforce prevailing meanings and structures of disability.  相似文献   

Spectroscopy on Small Telescopes: The Echelle Spectrograph     

Martin J. Porter 《Astrophysics and Space Science》2000,273(1-4):217-224

In recent years there has been a trend toward the automation ofmodest sized observatories, especially those involved withphotoelectric photometry. Although undoubtedly useful, there isclearly a need for more advanced instrumentation such asspectrographs. The relatively small telescope sizes places severelimits on the choice of spectrograph configuration. A fiber-fedechelle spectrograph is arguably the most suitable design forthese applications.Presented here is a brief discussion of the design philosophy andoperating principles. Both theoretical and measured performancedata for the prototype University of Queensland EchelleSpectrograph are also presented.  相似文献   

Spring-summer temperatures since AD 1780 reconstructed from stable oxygen isotope ratios in white spruce tree-rings from the Mackenzie Delta,northwestern Canada     

Trevor J. Porter  Michael F. J. Pisaric  Robert D. Field  Steven V. Kokelj  Thomas W. D. Edwards  Peter deMontigny  Richard Healy  Allegra N. LeGrande 《Climate Dynamics》2014,42(3-4):771-785

High-latitude δ¹⁸O archives deriving from meteoric water (e.g., tree-rings and ice-cores) can provide valuable information on past temperature variability, but stationarity of temperature signals in these archives depends on the stability of moisture source/trajectory and precipitation seasonality, both of which can be affected by atmospheric circulation changes. A tree-ring δ¹⁸O record (AD 1780–2003) from the Mackenzie Delta is evaluated as a temperature proxy based on linear regression diagnostics. The primary source of moisture for this region is the North Pacific and, thus, North Pacific atmospheric circulation variability could potentially affect the tree-ring δ¹⁸O-temperature signal. Over the instrumental period (AD 1892–2003), tree-ring δ¹⁸O explained 29 % of interannual variability in April–July minimum temperatures, and the explained variability increases substantially at lower-frequencies. A split-period calibration/verification analysis found the δ¹⁸O-temperature relation was time-stable, which supported a temperature reconstruction back to AD 1780. The stability of the δ¹⁸O-temperature signal indirectly implies the study region is insensitive to North Pacific circulation effects, since North Pacific circulation was not constant over the calibration period. Simulations from the NASA-GISS ModelE isotope-enabled general circulation model confirm that meteoric δ¹⁸O and precipitation seasonality in the study region are likely insensitive to North Pacific circulation effects, highlighting the paleoclimatic value of tree-ring and possibly other δ¹⁸O records from this region. Our δ¹⁸O-based temperature reconstruction is the first of its kind in northwestern North America, and one of few worldwide, and provides a long-term context for evaluating recent climate warming in the Mackenzie Delta region.  相似文献   

Dynamics of a mesoscale eddy off Cape Ann, Massachusetts in May 2005     

Mingshun Jiang  Meng Zhou  Scott P. Libby  Donald M. Anderson 《Deep Sea Research Part I: Oceanographic Research Papers》2011,58(11):1130-1146

Observations and numerical modeling indicate that a mesoscale anti-cyclonic eddy formed south of Cape Ann at the northern entrance of Massachusetts Bay (MB) during May 2005, when large river discharges in the western Gulf of Maine and two strong Nor'easters passing through the regions led to an unprecedented toxic Alexandrium fundyense bloom (red tide). Both model results and field measurements suggest that the western Maine Coastal Current separated from Cape Ann around May 7–8, and the eddy formed on around May 10. The eddy was trapped at the formation location for about a week before detaching from the coastline and moving slowly southward on May 17. Both model results and theoretical analysis suggest that the separation of the coastal current from the coast and subsequent eddy formation were initiated at the subsurface by an adverse pressure gradient between Cape Ann and MB due to the higher sea level set up by onshore Ekman transport and higher density in downstream MB. After the formation, the eddy was maintained by the input of vorticity transported by the coastal current from the north, and local vorticity generation around the cape by the horizontal gradients of wind-driven currents, bottom stress, and water density induced by the Merrimack River plume. Observations and model results indicate that the anti-cyclonic eddy significantly changed the pathway of nutrient and biota transport into the coastal areas and enhanced phytoplankton including Alexandrium abundances around the perimeter of the eddy and in the western coast of MB.  相似文献