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排序方式: 共有301条查询结果,搜索用时 15 毫秒
1.
Abstract

Shallow water bathymetry has proved to be a challenging task for remote sensing applications. In this work, Green-Wavelength Terrestrial Laser Scanning (GWTLS) is employed to survey nearshore bathymetry under clear atmospheric and water conditions. First, the obtained seabed points were corrected for refraction and then geo-registration, and filtering processes were exerted to obtain an accurate bathymetric surface. Terrain analysis was performed with respect to a reference surface derived from classical surveying techniques. The overall analysis has shown that the best results stem from 35° to 50° incident angles, whereas for angles higher than 65° measurements are not acceptable, although for the same angle in front and close to the instrument accuracy is considered acceptable due to the high laser power. Also, high resolution micro-topography, shallower than 1?m water depth, was managed to be captured. Systematic experimental approaches are expected to improve the GWTLS technique to detect bathymetry, which is anticipated to assist in mapping very shallow foreshore, tidal, and deltaic environments, to contribute conceptual into developing hybrid observation systems for coastal monitoring, and also to be applied in various maritime applications.  相似文献   
2.
NLOS GPS signal detection using a dual-polarisation antenna   总被引:2,自引:2,他引:0  
The reception of indirect signals, either in the form of non-line-of-sight (NLOS) reception or multipath interference, is a major cause of GNSS position errors in urban environments. We explore the potential of using dual-polarisation antenna technology for detecting and mitigating the reception of NLOS signals and severe multipath interference. The new technique computes the value of the carrier-power-to-noise-density (C/N 0) measurements from left-hand circular polarised outputs subtracted from the right-hand circular polarised C/N 0 counterpart. If this quality is negative, NLOS signal reception is assumed. If the C/N 0 difference is positive, but falls below a threshold based on its lower bound in an open-sky environment, severe multipath interference is assumed. Results from two experiments are presented. Open-field testing was first performed to characterise the antenna behaviour and determine a suitable multipath detection threshold. The techniques were then tested in a dense urban area. Using the new method, two signals in the urban data were identified as NLOS-only reception during the occupation period at one station, while the majority of the remaining signals present were subject to a mixture of NLOS reception and severe multipath interference. The point positioning results were dramatically improved by excluding the detected NLOS measurements. The new technique is suited to a wide range of static ground applications based on our results.  相似文献   
3.
Geophysical data sets are growing at an ever-increasing rate, requiring computationally efficient data selection(thinning)methods to preserve essential information. Satellites, such as Wind Sat, provide large data sets for assessing the accuracy and computational efficiency of data selection techniques. A new data thinning technique, based on support vector regression(SVR), is developed and tested. To manage large on-line satellite data streams, observations from Wind Sat are formed into subsets by Voronoi tessellation and then each is thinned by SVR(TSVR). Three experiments are performed. The first confirms the viability of TSVR for a relatively small sample, comparing it to several commonly used data thinning methods(random selection, averaging and Barnes filtering), producing a 10% thinning rate(90% data reduction), low mean absolute errors(MAE) and large correlations with the original data. A second experiment, using a larger dataset, shows TSVR retrievals with MAE < 1 m s-1and correlations 0.98. TSVR was an order of magnitude faster than the commonly used thinning methods. A third experiment applies a two-stage pipeline to TSVR, to accommodate online data. The pipeline subsets reconstruct the wind field with the same accuracy as the second experiment, is an order of magnitude faster than the nonpipeline TSVR. Therefore, pipeline TSVR is two orders of magnitude faster than commonly used thinning methods that ingest the entire data set. This study demonstrates that TSVR pipeline thinning is an accurate and computationally efficient alternative to commonly used data selection techniques.  相似文献   
4.
Fitting the spectral energy distributions (SEDs) of galaxies is an almost universally used technique that has matured significantly in the last decade. Model predictions and fitting procedures have improved significantly over this time, attempting to keep up with the vastly increased volume and quality of available data. We review here the field of SED fitting, describing the modelling of ultraviolet to infrared galaxy SEDs, the creation of multiwavelength data sets, and the methods used to fit model SEDs to observed galaxy data sets. We touch upon the achievements and challenges in the major ingredients of SED fitting, with a special emphasis on describing the interplay between the quality of the available data, the quality of the available models, and the best fitting technique to use in order to obtain a realistic measurement as well as realistic uncertainties. We conclude that SED fitting can be used effectively to derive a range of physical properties of galaxies, such as redshift, stellar masses, star formation rates, dust masses, and metallicities, with care taken not to over-interpret the available data. Yet there still exist many issues such as estimating the age of the oldest stars in a galaxy, finer details of dust properties and dust-star geometry, and the influences of poorly understood, luminous stellar types and phases. The challenge for the coming years will be to improve both the models and the observational data sets to resolve these uncertainties. The present review will be made available on an interactive, moderated web page (), where the community can access and change the text. The intention is to expand the text and keep it up to date over the coming years.  相似文献   
5.
The storm period of 8–12 November 2004 offers an opportunity for insight into the phenomena of low-latitude ionospheric structure during geomagnetically disturbed times because of the strength of the disturbances, the timing of the storms, and the instrumentation that was operating during the interval. We will take advantage of these factors to model the ambient ionosphere and the plasma turbulence responsible for radio scintillation within it, using the AFRL low-latitude ambient/turbulent ionospheric model and the storm-time model features described in the companion paper [Retterer, J.M., Kelley, M.C., 2009. Solar-wind drivers for low-latitude ionospheric models during geomagnetic storms. J. Atmos. Solar-Terr. Phys., this issue]. The model plasma densities show very good agreement with the densities measured by the Jicamarca ISR as well as with the total electron content (TEC) measured by the Boston College South American chain of GPS receivers. The detection by the radar of coherent returns from plasma turbulence match well the times of predicted ionospheric instability. The predicted geographic extent of the occurrence of equatorial plasma bubbles was matched by DMSP satellite observations and our forecasts of scintillation strength were validated with measurements of S4 at Ancon and Antofagasta by stations of the AFRL SCINDA network.  相似文献   
6.
Stress mapping is a numerical modelling technique used to determine the distribution and relative magnitude of stress during deformation in a mineralised terrane. It is based on the general principle that fluid flow in the Earth's crust is primarily related to pressure gradients. It is best applied to epigenetic hydrothermal mineral deposits, where fluid flow and fluid flux are enhanced in dilational sections of structures and in sites of enhanced rock permeability due to high fracture density. These are defined by sites of low minimum principal stress (σ3). Most stress mapping is carried out in two dimensions in plan view using geological maps. This is suitable for terranes with steeply dipping lithostratigraphy and structures in which the distribution of mineral deposits is largely controlled by fault structures portrayed on the maps. However, for terranes with gently dipping sequences and structures, and for situations where deposits are sited in and near the hinges of complex fold structures, stress mapping in cross‐section is preferable. The effectiveness of stress mapping is maximised if mineralisation was late in the evolutionary history of the host terrane, and hence the structural geometry of the terrane and contained deposits were essentially that expressed today. The orientation of syn‐mineralisation far‐field stresses must also be inferred. Two examples of orogenic gold deposits, which meet the above criteria, are used to illustrate the potential of stress mapping in cross‐section. Sunrise Dam, located in the Archaean Yilgarn Craton, is a lode‐gold deposit sited in a thrust‐fold belt. Stress mapping illustrates the heterogeneity of stress distribution in the complex structural geometry of the deposit, and predicts the preferential siting of ore zones around the intersections of more steeply dipping, linking thrusts and banded iron‐formation units, and below the controlling more gently dipping basal thrust, the Sunrise Shear. The Howley Anticline in the Pine Creek block hosts several Palaeoproterozoic gold deposits, sited in complex anticlinal structures in greywacke sequences. Stress mapping indicates that gold ores should develop in the hinge zones of symmetrical anticlines, in the hinge zones and more steeply dipping to overturned limbs of asymmetric anticlines, and in and around thrusts in both anticlines and parasitic synclines. The strong correlation between the predictions of the stress mapping, based on the distribution of low σ3, and the location of gold ores emphasises the potential of stress mapping in cross‐section, not only as an exploration tool for the discovery of additional resources or deposits, but also as a test of geological models. Knowledge of the potential siting of gold ores and their probable orientations also provides a guide to drilling strategies in both mine‐ and regional‐scale exploration.  相似文献   
7.
The Pagassitikos Gulf in Greece is a semi-enclosed bay with a maximum depth of 102 m. According to the present-day bathymetric configuration and the sea level during the latest Pleistocene, the gulf would have been isolated from the open sea, forming a palaeolake since ~32 cal. ka b.p. Sediment core B-4 was recovered from the deepest sector of the gulf and revealed evidence of a totally different depositional environment in the lowest part of the core: this contained light grey-coloured sediments, contrasting strongly with overlying olive grey muds. Multi-proxy analyses showed the predominance of carbonate minerals (aragonite, dolomite and calcite) and gypsum in the lowest part of the core. Carbonate mineral deposition can be attributed to autochthonous precipitation that took place in a saline palaeolake with high evaporation rates during the last glacial–early deglacial period; the lowest core sample to be AMS 14C dated provided an age of 19.53 cal. ka b.p. The palaeolake was presumably reconnected to the open sea at ~13.2 cal. ka b.p. during the last sea-level rise, marking the commencement of marine sedimentation characterised by the predominance of terrigenous aluminosilicates and fairly constant depositional conditions lasting up to the present day.  相似文献   
8.
The Corinthia lode‐gold deposit in amphibolite‐facies greenstone belt rocks in the Southern Cross Province of the Archaean Yilgarn Block contains a largely undeformed pegmatite dyke emplaced during the last phases of movement along the Fraser's‐Corinthia shear zone. Gold mineralization and shear zone development were synchronous, and a Pb‐Pb isochron age of 2620 ±6 Ma for pegmatite emplacement either indirectly dates mineralization, or places a minimum age constraint on the timing of mineralization. This age is in accord with a broadly synchronous dominant episode of Archaean lode‐gold mineralization throughout the Yilgarn Block.  相似文献   
9.
This study aims to explore the condensation and fractionation trends of persistent organic pollutants (POPs) in the karst soils. The tiankeng is a karst surface expression that can act as a focal point for introduction of contaminants to a karst aquifer, which may serve both as condenser for vapor phase POPs and as barrier/sink for particulate associated less volatile POPs. The fractionation of POPs in soils from the upper rim and floor of tiankeng is of interest in understanding the role of tiankeng in the long-distance transport of POPs. In the present study, polycyclic aromatic hydrocarbons (PAHs) in the surface soils from the upper rim and floor of Dashiwei tiankeng in Southern China were analyzed. The total PAH concentrations in soils were 23.40–190 ng g−1, with phenanthrene being the most abundant. The distribution patterns of PAH compounds in the soil samples matched well with their properties. It indicated the heavy PAHs were susceptible to retention by the floor soils of tiankeng than light PAHs. A plot of Cfloor/Crim against PAH molecular weight gave a good positive relationship in the molecular weight range of 152–276. It is suggested that the floor soils can be focal points of more concentrated PAH and deserve attention. The concentrations of total PAHs in the floor soils (43.40–190 ng g−1, mean 87.76 ng g−1) were higher than those in the upper rim (23.40–88.94 ng g−1, mean 57.74 ng g−1). In addition, there was a shift in compound pattern with an increase in the proportion of light PAHs (2–3 rings), a decrease in heavy PAHs (5–6 rings) and a relatively stable content of 4-ring PAHs. A combination of particulate scavenging and cold condensation is proposed as the major mechanism for the compositional fractionation of PAHs in the soils from the upper rim and floor of tiankeng.  相似文献   
10.
Abstract

A theoretical analysis of pseudo two-dimensional, finite-amplitude, thermal convection is made for an infinite Prandtl number fluid which is subjected to a constant heat flux out of the top boundary and insulated at the bottom. For large Rayleigh numbers the convective flow becomes intermittent and the system is characterized by the following cyclic process: the formation of a thermal boundary layer by diffusion, the instability of this layer when it becomes sufficiently thick, the destruction of the layer by the convective flow, the dying down of the convection, and the reforming of the thermal boundary layer by diffusion. The periodicity and the horizontal wave number of the intermittent convective flow are found to be independent of the depth of the fluid layer but depend on the rate of cooling and the properties of the fluid.  相似文献   
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