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981.
On low-pass digital filters in oceanography 总被引:1,自引:0,他引:1
-Two types of filters are widely used to remove semidirunal and diurnal tidal signals and other high frequency noises in oceanography. The first type of filters uses moving average with weights in time domain, and can be easily operated. Some data will be lost at each end of the time series, especially for the low low-pass filters. The second type of filters uses the discrete Fourier transform filter (DFTF) which operates in the frequency domain, and there are no data loss at the ends for the forward transform. However, owing to the Gibbs phenomenon and the discrete sampling (Nyquist effect) , ringing appears in the inverse transformed data, which is especially serious at each end. Thus some data at the ends are also discarded. The present study tries to find out what causes the ringing and then to seek for methods to overcome the ringing. We have found that there are two kinds of ringings, one is the Gibbs phenomenon, as defined before. The other is the "Nyquist"ringing due to sampling Nyquist critical 相似文献
982.
Q. J. Huggett A. K. Cooper M. L. Somers A. R. Stubbs 《Marine Geophysical Researches》1992,14(1):47-63
GLORIA side-scan sonographs from the Bering Sea Basin show a complex pattern of interference fringes sub-parallel to the ship's track. Surveys along the same trackline made in 1986 and 1987 show nearly identical patterns. It is concluded from this that the interference patterns are caused by features in the shallow subsurface rather than in the water column. The fringes are interpreted as a thin-layer interference effect that occurs when some of the sound reaching the seafloor passes through it and is reflected off a subsurface layer. The backscattered sound interferes (constructively or desctructively) with the reflected sound. Constructive/destructive interference occurs when the difference in the length of the two soundpaths is a whole/half multiple of GLORIA's 25 cm wavelength. Thus as range from the ship increases, sound moves in and out of phase causing bands of greater and lesser intensity on the GLORIA sonograph. Fluctuations (or wiggles) of the fringes on the GLORIA sonographs relate to changes in layer thickness. In principle, a simple three dimensional image of the subsurface layer may be obtained using GLORIA and bathymetric data from adjacent (parallel) ship's tracks. These patterns have also been identified in images from two other systems; SeaMARC II (12 kHz) long-range sonar, and TOBI (30 kHz) deep-towed sonar. In these, and other cases world-wide, the fringes do not appear with the same persistence as those seen in the Bering Sea. 相似文献
983.
The radioelement and heavy mineral distribution in river, beach and innershelf areas of the southern Kerala coast is related
to placer mineral concentration on the beaches at Chavara. Southern Kerala rivers—Neyyar, Karamana and Vamanapuram—transport
higher amounts of radioactive elements than the larger Kallada River due to higher radioactive minerals in the hinterland
rocks. Coastal configurations and the seasonal longshore current pattern seems to control along-shore distribution of minerals.
The proposed model for placer concentration suggests that the energy difference and seasonal current direction along this
coast is important. 相似文献
984.
Anaerobic salt marsh sediments were amended with a variety of organic pollutants and the effects on methanogenesis, sulfate reduction and carbon dioxide evolution were examined. Addition of 1000 μg g?1 (dry weight sediment) Arochlor 1221, lindane, endrin, benzene and phenanthrene resulted in no significant effects on the activities studied. Methanogenesis was inhibited by 1000 μg g?1toxaphene, PCP, chlordane, naphthalene, DDT, Kepone and heptachlor and by 100 μg g?1 PCP and toxaphene. At 1000 μg g?1 naphthalene and toxaphene and 100 μg g?1 PCP, a period of initial inhibition of methanogenesis was followed by stimulation relative to controls. Arochlor 1254 (1000 μg g?1) and Temik (500 and 10 μg g?1) stimulated methanogenesis from the outset. Temik at 500 μg g?1 gave the greatest stimulation of methanogenesis (900% of controls) of any of the compounds studied. Sulfate reduction was inhibited by 1000 μg g?1 PCP, toxaphene, naphthalene and chlordane and by 500 μg g?1 atrazine and 100 μg g?1 heptachlor. Sustained inhibition of sulfate reduction by naphthalene, toxaphene and PCP may have contributed to the stimulation of methanogenesis. Carbon dioxide evolution was not significantly affected by most of the compounds studied except for 100 μg g?1 PCP and 1000 μg g?1 aphthalene, each of which gave significant inhibition in only one of three experiments.Concentrations of individual organic pollutants required to cause observable effects were high. It is concluded that, except for highly polluted sediments, methanogenesis, sulfate reduction and CO2 evolution would not be affected by the compounds studied here at concentrations typically found in the environment. 相似文献
985.
This paper presents a method to statistically predict the magnitude of impact pressure (including extreme values) produced by deep water waves breaking on a circular cylinder representing a column of an ocean structure. Breaking waves defined here are not those whose tops are blown off by the wind but those whose breaking is associated with steepness. The probability density function of wave period associated with breaking waves is derived for a specified wave spectrum, and then converted to the probability density function of impact pressure. Impacts caused by two different breaking conditions are considered; one is the impact associated with waves breaking in close proximity to the column, the other is an impact caused by waves approaching the column after they have broken. As an example of the application of the present method, numerical computations are carried out for a wave spectrum obtained from measured data in the North Atlantic. 相似文献
986.
K. G. Robertson 《Marine Geophysical Researches》1990,12(1-2):3-8
Accurate navigation forms an essential part of all research at sea and the deep ocean imposes it's own unique problems. This chapter discusses several of the techniques in current use on the research vessels of the Natural Environment Research Council (NERC), concentrating on those systems which provide global navigation facilities, as opposed to the more localised, coastal aids. Whilst most of the systems rely on surface propagation of radio waves, the use of acoustics and sea-bed mapping instruments constitute accurate alternatives for some sub-sea applications. 相似文献
987.
The first map of the sea bed morphology and sedimentary features within the RMSTitanic search area is proposed from the interpretation of SAR side-scan sonar images. Downslope sedimentary features such as erosional
furrows and crown scarps constitute a 7 km wide instability corridor. A large field (15 km2) of asymmetrical sediment waves indicating a downslope transport is identified. Current-induced features corresponding to
associated sand ribbons and barchan dunes resulting from the Western Boundary Undercurrent action are mapped. The morphology
of theTitanic Canyon is also precised from the SAR images. Finally, the origin of the sea bed features is discussed in an attempt to link
each bed form to a sedimentary process. 相似文献
988.
Descriptive Analysis and Classification of Benthic Communities in Some Mediterranean Coastal Lagoons (Central Italy) 总被引:3,自引:0,他引:3
Abstract. Benthic samples were taken seasonally at 32 stations in five coastal lagoons of Central Italy (Fogliano, Monaci, Caprolace, Fondi, and Lungo) during a two-year period (1982–1984). Composition and distribution of benthic populations in each lagoon as well as seasonal trends of species richness, density, diversity, and redundancy are presented and discussed.
The faunal differentiation appeared to be mainly due to the different degree of water exchange, both with marine and continental systems. 相似文献
The faunal differentiation appeared to be mainly due to the different degree of water exchange, both with marine and continental systems. 相似文献
989.
AN ECOSYSTEM MODEL ANALYSIS OF THE EFFECT OF MINE TAILINGS ON THE EUPHOTIC ZONE OF A PELAGIC ECOSYSTEM 总被引:1,自引:0,他引:1
A computer analysis was performed on experimental results obtained when mine tailings were added to seawater. The Mixed Upper-Layer Ecotrophie Simulation (MULES) model was tested by changing the extinction coefficient and the abundance of heterotrophic zooflagellates. Increasing the extinction coefficient resulted in a delay of phytoplankton growth, an increase in zooplankton standing stock and better growth of autotrophic flagellates compared with diatoms. Zooflagellates in the ecosystem influence the growth of zooplankton; secondary production by zooplankton was markedly depressed at low levels of zooflagellates. These results are believed to be of general significance for the diagnosis of suspended sediment effects on planktonic ecosystems. 相似文献
990.
The paper describes an analytical method to determine an operability index for a marine vehicle when its geometric characteristics, the geographical area in which the vessel is supposed to operate, and the limiting criteria for operations are known. The computer program based on strip theory provides reliable results not only for conventional hullforms, but also for catamarans, offshore vehicles, etc. The results are presented in the form of figures for different vessels, namely, crane barge, naval vessels and SWATH type catamarans. The authors are of the opinion that the proposed method would provide the designer with a valid tool to improve the seakeeping qualities of a vessel, while taking into account the limiting conditions imposed due to seaway operations. 相似文献