Radiometric investigation of different snow covers in Svalbard     

Ruggero Casacchia  Francesca Lauta  Rosamaria Salvatori  Anselmo Cagnati  Mauro Valt  Jon B. Ørbæk 《Polar research》2001,20(1):13-22

This paper examines the relationship between reflectance and physical characteristics of the snow cover in the Arctic. Field data were acquired for different snow and ice surfaces during a survey carried out at Ny-Ålesund, Svalbard, in spring 1998. In each measurement reflectance in the spectral range 350 - 2500 nm, snow data (including temperature, grain size and shape, density and water content), surface layer morphology, and vertical profile of the snow pack were recorded detailed analysis of reflectance based on the physical was performed. Field reflectance data were also re-sampled at the spectral intervals of Landsat TM to compare the ability of identifying different snow targets at discrete wavelength intervals. This analysis shows that reliable data on snow structure and thickness are necessary to understand albedo changes of the snow surfaces.  相似文献   

Seismic hazard evaluation for dams in northern Colorado, U.S.A.     

Ute R. Vetter  Jon P. Ake  Roland C. Laforge 《Natural Hazards》1996,14(2-3):227-240

A seismic hazard evaluation for three dams in the Rocky Mountains of northern Colorado is based on a study of the historical seismicity. To model earthquake occurrence as a random process utilizing a maximum likelihood method, the catalog must exhibit random space-time characteristics. This was achieved using a declustering procedure and correction for completeness of recording. On the basis of the resulting a- and b-values, probabilistic epicentral distances for a 2 × 10^–5 annual probability were calculated. For a random earthquake of magnitude M _L 6.0–6.5, this distance is 15 km. Suggested ground motion parameters were estimated using a probabilistic seismic hazard analysis. Critical peak horizontal accelerations at the dams are 0.22g if median values are assumed and 0.39g if variable attenuation and seismicity rates are taken into account. For structural analysis of the dams, synthetic acceleration time series were calculated to match the empirical response spectra. In addition, existing horizontal strong motion records from two Mammoth Lakes, California earthquakes were selected and scaled to fit the target horizontal acceleration response spectra.  相似文献   

Seasonal maturity and size at age of Loligo pealeii in waters of southern New England     

Macy  William K.  III; Brodziak  Jon K. T. 《ICES Journal of Marine Science》2001,58(4):852-864

  相似文献   

A SeaMARC II survey of recent submarine volcanism near Easter Island     

Rick A. Hagen  Nancy A. Baker  David F. Naar  R. N. Hey 《Marine Geophysical Researches》1990,12(4):297-315

SeaMARC II side-scan sonar data reveal that a large area of seafloor north and west of Easter Island has been disrupted by recent submarine volcanism. A large volcanic area begins approximately 60 km WNW of the island and extends for over 130 km to the west. The volcanic field is characterized by high backscatter intensity in the side-scan sonar records and is elevated 400–1000 m above the N-S seafloor fabric that surrounds it. This field, the Abu Volcanic Field, covers at least 2500 km² and appears to consist of recent lava flows and small volcanoes. Backscatter intensity of the Abu Volcanic Field is similar to that of the adjacent ridge flank which is less than 0.4 Ma, suggesting a similar age for its formation. Two additional areas of high backscatter immediately north of Easter Island cover a combined area of over 300 km². The sidescan sonar records show that these features are clearly of volcanic origin and are not debris flows from the nearby island. The flows are nearly 300 m thick and are morphologically similar to subaerial pahoehoe lava shields. Their high backscatter indicates that they are also the products of relatively recent submarine volcanic activity. The presence of these large areas of recent volcanism in the vicinity of Easter Island has important implications for the various models that have been proposed to explain the origin of the Easter Seamount Chain. In addition, the similar ages of Easter Island and the Easter Microplate suggest that the presence of a hotspot near or beneath this fast-spreading portion of the East Pacific Rise about 4.5 m.y. ago may have initiated the large-scale rift propagation that created the microplate.  相似文献   

Chirp sub-bottom profiler source signature design and field testing   总被引：1，自引：0，他引：1  

Gutowski  Martin  Bull  Jon  Henstock  Tim  Dix  Justin  Hogarth  Peter  Leighton  Tim  White  Paul 《Marine Geophysical Researches》2002,23(5-6):481-492

Chirp sub-bottom profilers are marine sonar systems which use a highly repeatable source signature to facilitate the acquisition of correlated data with decimetre vertical resolution in the top 20–30 m of sediments. Source signatures can be readily developed and implemented, but an applicable methodology for assessing resolution and attenuation characteristics of these wide-band systems did not exist. Methodologies are developed and applied to seven contrasting source signatures which occupy the same frequency band, but differ in their Envelope and Instantaneous Frequency functions. For the Chirp source signatures tested, a Sine-Squared envelope function is shown to produce seismic data with the optimum resolution and penetration characteristics.  相似文献   

Reference points and management strategies: lessons from quantum mechanics     

Schnute  Jon T.; Haigh  Rowan 《ICES Journal of Marine Science》2006,63(1):4-11

  相似文献   

Drag and Sediment Dispersion Over Sand Waves     

Jon J. Williams 《Estuarine, Coastal and Shelf Science》1995,41(6)

Estimates of the drag coefficient over sand waves during calm weather in the southern North Sea have been obtained from measurements of the water slope and currents at different heights (z) above the sea-bed using the log profile and momentum balance methods. An observed phase difference between principal terms in the momentum balance equation is examined theoretically. Drag coefficient estimates are found to agree broadly with previous studies. Owing to bedform asymmetry, average drag coefficient values obtained atz=1 m (C₁₀₀) are found to be 0·0021 and 0·0029 for flood and ebb tides, respectively. Systematic changes in bed roughness are not detected. Using a momentum balance approach, the average drag coefficient value (C_d) atz=10 m is found to be 0·0056. Changes in 10-min averageC_dvalues over sand waves during the tidal cycle are found to be small with bedform asymmetry having no detectable effect. Correlation betweenC_dandC₁₀₀is found to be poor and separation of skin friction and form drag terms is not possible with existing measurements. The inclusion of form drag inC₁₀₀values at the present site leads to over-estimation of the bed shear stress ({q) available to mobilize and transport sediment. Mobile sediment, detected through the use of tracers and a transmissometer, was not found to have any measurable effect on eitherC_dorC₁₀₀in calm weather conditions.  相似文献   

Consequence of swimbladder model choice and fish orientation to target strength of three New Zealand fish species     

McClatchie  Sam; Alsop  Jon; Ye  Zhen; Coombs  Roger F. 《ICES Journal of Marine Science》1996,53(5):847-862

  相似文献   

Optimum operation of restoration techniques for eutrophic water bodies     

N. M. Hagen  H. -B. Kleeberg 《Surveys in Geophysics》1994,15(3):299-310

Operating rules have been applied in water resources management for a long time in order to control and supply a requiredquantity (volume) of water. The operating rules have to guarantee the optimum management of the reservoir(s). Thequality of the stored water has been satisfactory for the desired utilization up to the sixties. Due to the deterioration of reservoir water quality through human impacts, however, increased attention had to be paid since. Eutrophication of stagnant waters is still an unsolved problem. Through means of various restoration techniques, i.e., dilution/flushing or hypolimnetic withdrawal, the quality of the stored water can be improved. Continuous operation or appropriate time or depth variant operating rules are required to achieve this goal. The paper presents such rules for long-term operation. They have been established for the first time and can he represented in two or three-dimensional graphs depending on the number of included components (e.g., actual water storage and quality). The quality operating rules take into account the dynamics of the processes in aquatic ecosystems. Simplifications with regard to application and acceptance (e.g., clarity) are developed and tested. The general validity and efficiency of the operating rules have been proved in a case study (a multi-purpose reservoir) and a fictitious lake.  相似文献   

Nomadic pastoralism and irrigated agriculture in Somalia     

Jon D. Unruh 《GeoJournal》1991,25(1):91-108

The persistent interplay of food production problems, land degradation, and social and climatic difficulties on the Horn of Africa result in recurring famines in spite of vast sums of money spent on agricultural development. As land resources — which undergird both social and production systems in Africa — become increasingly degraded, development efforts, especially in problematic areas, need to become part of comprehensive resource use programs that take into account the existing regional land use ecology. Designs which disrupt the ecology of established land uses can lead to extensive degradation because such uses are linked to wider areas; and the effects of such disruption can ultimately threaten the viability of the proposed schemes themselves.While African agriculture has traditionally met greater food needs by expanding the area under cultivation and irrigation, the increasing scarcity of new high quality arable land means that multiple use of high potential areas will become a priority. This paper describes a multiple land use in a high potential river basin of Somalia, in the context of the existing use patterns involved in irrigated agriculture and nomadic pastoralism. The spatial and temporal access and use of resources are analyzed, and recommendations made for improving the integration of these production systems.  相似文献