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761.
David M. Pyle Tjeerd H. van Andel Panayiotis Paschos Paul van den Bogaard 《Quaternary Research》1998,49(3):280-286
A newly recognized 2-m-thick trachytic volcanic ash deposit from northwestern Greece is dated at 374,000 ± 7000 yr and correlated with the Middle Pleistocene volcanic activity of central Italy. The deposit represents ash fallout from one of the largest volcanic eruptions in Europe of the past 400,000 yr and should provide an important stratigraphic marker within the poorly dated Middle Pleistocene deposits of Italy and Greece. 相似文献
762.
763.
David R. Geist Mark C. Joy David R. Lee Tom Gonser 《Ground Water Monitoring & Remediation》1998,18(1):78-82
An impact drive point method is described for emplacing piezometers in a cobble river bottom where this has previously been difficult without the use of drilling rigs. To force the drive point piezometers through coble, the vibrational impact of an air-powered hammer was carried directly to the drive point by the use of an internal drive rod. After insertion to depth, the drive rod was removed from the lower portion of the piezometer and a standpipe was added to extend the piezometer above the river level. Piezometers installed in this way have permitted water quality analysis and dynamic measurement of vertical potentials in cobble sediments ranging in size from 2.5 to >30 cm and the method has been successfully used in the Columbia River, USA, and Töss River, Switzerland. This innovative method provides information on the hydrodynamics of pore water in highly permeable, cobble deposits that are common in high-energy river and lake bottoms. Piezometers installed using the internal drive rod method facilitate the assessment of the temporal and spatial dynamics of recharge and discharge at the ground water/surface water interface and analyses of the ecological connectivity between the hyporheic zone and surface water of rivers and streams. This information will lead to improved management decisions related to our nation's ground water and surface water supplies. 相似文献
764.
The Apuseni Mountains, part of the Western Carpathians, are distinctive in cultural terms because of the highly dispersed
settlement patterns that have developed on the high erosion surfaces where the Moti people cleared the forests to allow for
an expansion of family farming in the eighteenth and nineteenth centuries. These small ‘crang’ settlements remain, but the
population is declining in the face of limited employment opportunities and poor services. The question arises as to the most
appropriate rural development strategy for the region, following the communist period which encouraged centralisation through
the provision of housing and industry in key villages and new towns. The emphasis on private farming and the expansion of
tourism could protect the inherited cultural landscape but money must be found to improve rural services. There is also a
strong conservation movement which supports the establishment of a national park, but this could constrain development through
controls on grazing and woodcutting. Hence the dialogue continues to find the best compromise and the concept of a ‘natural
park’ is being discussed with such a reconciliation in mind.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
765.
A geophysical survey was conducted to determine the depth of the base of the water-table aquifer in the southern part of
Jackson Hole, Wyoming, USA. Audio-magnetotellurics (AMT) measurements at 77 sites in the study area yielded electrical-resistivity
logs of the subsurface, and these were used to infer lithologic changes with depth. A 100–600 ohm-m geoelectric layer, designated
the Jackson aquifer, was used to represent surficial saturated, unconsolidated deposits of Quaternary age. The median depth
of the base of the Jackson aquifer is estimated to be 200 ft (61 m), based on 62 sites that had sufficient resistivity data.
AMT-measured values were kriged to predict the depth to the base of the aquifer throughout the southern part of Jackson Hole.
Contour maps of the kriging predictions indicate that the depth of the base of the Jackson aquifer is shallow in the central
part of the study area near the East and West Gros Ventre Buttes, deeper in the west near the Teton fault system, and shallow
at the southern edge of Jackson Hole. Predicted, contoured depths range from 100 ft (30 m) in the south, near the confluences
of Spring Creek and Flat Creek with the Snake River, to 700 ft (210 m) in the west, near the town of Wilson, Wyoming.
Received, May 1997 · Revised, February 1998 · Accepted, April 1998 相似文献
766.
Marek J. Kukula James S. Dunlop David H. Hughes & Steve Rawlings 《Monthly notices of the Royal Astronomical Society》1998,297(2):366-382
Although radio-quiet quasars (RQQs) constitute ≳ 90 per cent of optically identified quasar samples, their radio properties are only poorly understood. In this paper we present the results of a multi-frequency VLA study of 27 low-redshift RQQs. We detect radio emission from 20 objects, half of which are unresolved (≤ 0.24 arcsec). In cases where significant structure can be resolved, double, triple and linear radio sources on scales of a few kpc are found. The radio emission (typically) has a steep spectrum (α ∼ 0.7, where S ∝ ν−α ), and high brightness temperatures ( T B ≥ 105 K) are measured in some of the radio components. The RQQs form a natural extension to the radio luminosity–absolute magnitude distribution of nearby Seyfert 1s. We conclude that a significant fraction of the radio emission in RQQs originates in a compact nuclear source directly associated with the quasar. There are no significant differences between the radio properties of RQQs with elliptical hosts and those in disc galaxies within the current sample. 相似文献
767.
Simon J. Woodings rew J. Williams Ralph Martin Ronald R. Burman & David G. Blair 《Monthly notices of the Royal Astronomical Society》1998,301(1):L5-L9
Following the proposal by Damineli that the central object of Eta Carinae may be an early-type binary, we perform numerical simulations of the X-ray emission from colliding stellar winds. A synthetic light curve has been generated which qualitatively agrees with the recent X-ray variability, and provides further support for the binary model. In particular, the model predicts a rise in the observed X-ray emission towards periastron, followed by a sharp drop and subsequent recovery. This is indeed what is seen in the RXTE light curve, although some problems concerning the X-ray spectrum at periastron still need to be explained. The simulations suggest that the width of the periastron dip will provide strong constraints on the binary and stellar wind properties of the components of Eta Car. 相似文献
768.
769.
David E Fisher 《Geochimica et cosmochimica acta》1981,45(2):245-249
He and Ar were extracted from the Weekeroo Station octahedrite in six heating steps. The Ar isotopic ratios were searched for any indication of an indigenous component with atmospheric-like values. Negative results were obtained. 相似文献
770.