Late Quaternary primary tephras in Sacred Lake sediments, northeast Mount Kenya, Kenya     

D.O. Olago  F.A. Street-Perrott  R.A. Perrott  M. Ivanovich  D.D. Harkness 《Journal of African Earth Sciences》2000,30(4):1113

In this paper, a sequence of five Late Quaternary tephras occurring as discrete, well-preserved horizons in lake sediments on the northeastern flank of Mount Kenya are characterised and their ages determined by a combination of high-resolution indirect radiocarbon dating and direct dating. The grain size characteristics suggest that the tephras are of fairly local origin. These Na---K-rich alkali pyroclasts with a trachytic chemical composition have a highly correlated chemistry and mineralogy, suggesting that they were probably derived from the same genetic series and possibly erupted from a single source vent. Morphological differences are attributed to the peculiar characteristics of each eruption episode. The magma source was probably a small, highly differentiated magma chamber following the olivine basalt-trachyandesite-trachyte-phonolite series, which broadly reflects the Quaternary rock suite of Mount Kenya.  相似文献   

Nonstationary Spatial Texture Estimation Applied to Adaptive Speckle Reduction of SAR Data     

D'Hondt  O. Ferro-Famil  L. Pottier  E. 《Geoscience and Remote Sensing Letters, IEEE》2006,3(4):476-480

This letter proposes a new model for the second-order statistics of spatial texture in synthetic aperture radar images. The autocovariance function is locally approximated by a two-dimensional anisotropic Gaussian kernel (AGK) to characterize texture by its local orientation and anisotropy. The estimation of texture parameters at a given scale is based on the gradient structure tensor operator and does not require the explicit computation of the autocovariance. Finally, a new filter called AGK minimum mean square error (MMSE) that takes into account this spatial information is introduced and compared with the refined MMSE filter. The proposed filter has better performance in terms of texture preservation and structure enhancement  相似文献   

Generalized polarization relationships for acoustic gravity waves in the nonisothermal atmosphere with the wind     

O. N. Savina  P. A. Bespalov  V. O. Rapoport  N. A. Ryzhov 《Geomagnetism and Aeronomy》2006,46(2):247-253

The nonlinear equation of the first order of the Riccati type has been obtained for the wave impedance of acoustic gravity waves in the nonisothermal atmosphere. The vertically nonuniform horizontal wind and the effect of viscosity on the horizontal components of the velocity field have been taken into account in the calculations. The boundary-value problem for the Riccati equation is defined by the boundary emission condition at high altitudes. Upon finding the wave impedance along with the generalized polarization relationship, all remaining disturbances of the atmospheric parameters related to acoustic gravity waves are found with the help of a simple integration. The results of using a developed formalism are illustrated by the numerical computation of acoustic gravity wave fields in the atmosphere with real vertical profiles of temperature and horizontal field velocity.  相似文献   

Unusual spatial-temporal dynamics of geomagnetic disturbances during the main phase of the extremely strong magnetic storm of November 7–8, 2004     

O. V. Kozyreva  N. G. Kleimenova  T. A. Kornilova  K. Kauriste  J. Manninen  A. Ranta 《Geomagnetism and Aeronomy》2006,46(5):580-592

The spatial dynamics of geomagnetic variations and pulsations, auroras, and riometer absorption during the development of the main phase of the extremely strong magnetic storm of November 7–8, 2004, has been studied. It has been indicated that intense disturbances were observed in the early morning sector of auroral latitudes rather than in the nighttime sector, as usually takes place during magnetic storms. The unusual spatial dynamics was revealed at the beginning of the storm main phase. A rapid poleward expansion of disturbances from geomagnetic latitudes of 65°–66° to 74°–75° and the development of the so-called polar cap substorm with a negative bay amplitude of up to 2500 nT, accompanied by precipitation of energetic electrons (riometer absorption) and generation of Pi2–Pi3 pulsations, were observed when IMF B _z was about ?45 nT. The geomagnetic activity maximum subsequently sharply shifted equatorward to 60°–61°. The spatial dynamics of the westward electrojet, Pi2–Pi3 geomagnetic pulsations, and riometer absorption was similar, which can indicate that the source of these phenomena is common.  相似文献   

Modeling studies of the upper ocean response to a tropical cyclone     

Steven L. Morey  Mark A. Bourassa  Dmitry S. Dukhovskoy  James J. O’Brien 《Ocean Dynamics》2006,56(5-6):594-606

A coupled ocean and boundary layer flux numerical modeling system is used to study the upper ocean response to surface heat and momentum fluxes associated with a major hurricane, namely, Hurricane Dennis (July 2005) in the Gulf of Mexico. A suite of experiments is run using this modeling system, constructed by coupling a Navy Coastal Ocean Model simulation of the Gulf of Mexico to an atmospheric flux model. The modeling system is forced by wind fields produced from satellite scatterometer and atmospheric model wind data, and by numerical weather prediction air temperature data. The experiments are initialized from a data assimilative hindcast model run and then forced by surface fluxes with no assimilation for the time during which Hurricane Dennis impacted the region. Four experiments are run to aid in the analysis: one is forced by heat and momentum fluxes, one by only momentum fluxes, one by only heat fluxes, and one with no surface forcing. An equation describing the change in the upper ocean hurricane heat potential due to the storm is developed. Analysis of the model results show that surface heat fluxes are primarily responsible for widespread reduction (0.5°–1.5°C) of sea surface temperature over the inner West Florida Shelf 100–300 km away from the storm center. Momentum fluxes are responsible for stronger surface cooling (2°C) near the center of the storm. The upper ocean heat loss near the storm center of more than 200 MJ/m² is primarily due to the vertical flux of thermal energy between the surface layer and deep ocean. Heat loss to the atmosphere during the storm’s passage is approximately 100–150 MJ/m². The upper ocean cooling is enhanced where the preexisting mixed layer is shallow, e.g., within a cyclonic circulation feature, although the heat flux to the atmosphere in these locations is markedly reduced.  相似文献   

Primary production and phytoplankton characteristics as eutrophication criteria of Kursiu Marios Lagoon, the Baltic Sea     

S. V. Aleksandrov  O. A. Dmitrieva 《Water Resources》2006,33(1):97-103

The results of studies of phytoplankton production, biomass, and species composition conducted in Kursiu Marius Lagoon in 2001, 2002 are presented. A significant effect of blue-green alga blooming on the trophic status and processes in the coastal part of Kursiu Marius Lagoon is revealed. It is shown that the mass development of algae in combination with setup phenomena can cause alga accumulation in the coastal zone, oxygen deficiency, and fish kills.  相似文献   

Spectral time method of identification of seismic events at distances of 15°–40°     

E. O. Kedrov  O. K. Kedrov 《Izvestiya Physics of the Solid Earth》2006,42(5):398-415

The spectral time method is applied to the identification of underground nuclear explosions (UNEs) and earthquakes from P wave records obtained at stations at epicentral distances of 15°–40°. It is necessary to utilize data at such great distances because the density of stations of the International Monitoring System is small in several areas of the Earth. At such distances, it is difficult to detect S and surface LR waves and, therefore, the efficiency of the identification using the diagnostic parameters (discriminants) S/P and LR/P is drastically reduced. As a result, the role of spectral and spectral time methods in the identification of events from P waves becomes much more significant. An advantage of the spectral time method (STM) is that it is applicable in the region of interest, provided that data from one or two reference underground chemical explosions and a representative sample of earthquake records obtained at network stations of the region are available. Moreover, in the majority of cases, the STM identifies earthquakes that could not be recognized with the use of spectral diagnostic parameters. We used records of UNEs and earthquakes in Central Asia received from the archives of the Center of the Incorporated Research Institution for Seismology (IRIS) in Washington, the National Data Center in Dubna (Russian Federation), and the International Data Center (IDC) in Vienna. The effectiveness of earthquake record selection using the given method amounted to about 80%. Supposedly, the application of this method of identification can increase the percentage of filtration (screening) of regional events having a natural origin.  相似文献   

Assessment of the natural resources of groundwater by using balance-hydrodynamic models     

V. A. Vsevolozhskii  S. O. Grinevskii 《Water Resources》2006,33(4):374-380

The specific features of formulation and implementation of balance-hydrodynamic simulation during different-scale studies of natural resources of groundwater. Are an example of detailed estimate of the resources and balance of groundwater in the Vologda River basin is given. It is noted that the use of balance-hydrodynamic models, in addition to an increase in the general reliability of estimates, allows the calculation of normal annual characteristics of groundwater resources with different occurrence (exceedance probability), which is impracticable when hydrodynamic methods in other formulations are used.  相似文献   

Cosmic ray flux variations, modulated by the solar and earth’s magnetic fields, and climate changes. 1. Time interval from the present to 10–12 ka ago (the Holocene Epoch)     

V. A. Dergachev  P. B. Dmitriev  O. M. Raspopov  H. Jungner 《Geomagnetism and Aeronomy》2006,46(1):118-128

Direct and indirect data on variations in cosmic rays, solar activity, geomagnetic dipole moment, and climate from the present to 10–12ka ago (the Holocene Epoch), registered in different natural archives (tree rings, ice layers, etc.), have been analyzed. The concentration of cosmogenic isotopes, generated in the Earth’s atmosphere under the action of cosmic ray fluxes and coming into the Earth archives, makes it possible to obtain valuable information about variations in a number of natural processes. The cosmogenic isotopes ¹⁴C in tree rings and ¹⁰Be in ice layers, as well as cosmic rays, are modulated by solar activity and geomagnetic field variations, and time variations in these concentrations gives information about past solar and geomagnetic activities. Since the characteristics of natural reservoirs with cosmogenic ¹⁴C and ¹⁰Be vary with climate changes, the concentrations of these isotopes also inform about climate changes in the past. A performed analysis indicates that cosmic ray flux variations are apparently the most effective natural factor of climate changes on a large time scale.  相似文献   

A nonstationary model of level variations in a drainless water body: The Caspian Sea     

V. A. Rumyantsev  A. O. Rumyantsev  Yu. A. Trapeznikov 《Water Resources》2006,33(4):396-401

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