排序方式： 共有21条查询结果，搜索用时 15 毫秒
11.
GPS-levelling points are widely used to control gravimetric geoid or quasigeoid models. Direct comparison is often interpreted
to reveal the accuracy of the gravimetric model, using GPS-levelling as a reference. However, both GPS and levelled heights
contain errors, and in order to achieve a centimeter-accuracy geoid, these should be investigated. The Norwegian Height System
NN1954 is known to contain large systematic errors due to postglacial land uplift in the area. In this study, the current
height system and two revised versions, corrected for uplift, are applied to compute three sets of control quasigeoid heights
in the southern part of Norway. These heights are then compared to various Nordic gravimetric quasigeoid models generated
during the last two decades. In contradiction to some earlier studies, the accuracy of gravimetric quasigeoid models for this
area are found to improve near-linearly with time. This is in accordance with expectations, since both data coverage and computation
methods have progressed during this time. However, this study shows the importance of establishing accurate and error-free
control data for geoid comparisons. 相似文献
12.
Lasafam Iturrizaga 《GeoJournal》1999,47(1-2):277-339
An abridged version of a geomorphological inventory and typology of Postglacial debris accumulations in High Asia is presented,
with selected examples from the Hindu Kush, the Karakoram and the Himalayas. The debris accumulations were surveyed in the
course of four research expeditions lasting a total of ten months in selected valley systems of High Asia (the eastern Hindu
Kush, the northwestern Karakoram, the Nanga Parbat massif (Pakistan), the Ladakh and Zanskar ranges, the Nun Kun massif, the
Kumaon and Garhwal Himalayas with the Kamet, Trisul and Nanda Devi massifs (India) and in the central Himalayas with the Kanjiroba,
Annapurna, Manaslu and Makalu massifs (Nepal)). The study areas being widely scattered, a supraregional comparison of the
debris accumulations proved possible. The debris accumulations are considered in centre-to-periphery sequences from the mountain
interior to the mountain fringes, and in vertical sequences, i.e. altitudinal zones, taking into account their topographical
relationship to adjoining elements of the landscape. Supraregional and climate-specific types of debris accumulation are distinguished
and it is recognized that the debris accumulations of the Karakoram and the Himalayas resemble each other more closely with
increasing elevation.
The core of the study is the dominant role played by past glaciation in the formation of Postglacial debris accumulations
in the high mountains of Asia. This glacial-history-oriented concept of debris accumulation stands in sharp contrast to previous
opinions about the genesis of the debris accumulation landscape in the extreme high mountains of Asia. The study shows that
at many places morainic deposits mask extensive portions of the valley sides up to several hundred metres above the valley
floor. These moraines are the main debris sources and exert a strong influence on, or even suppress, the purely slope-related
formation of debris accumulations. Resedimentation of morainic material in combination with additional talus delivery leads
to numerous characteristic composite types of debris accumulations, which are here termed transitional glacial debris accumulations.
Various stages in the transition from moraine to slope-related debris accumulations were observed, making it necessary to
consider the evolutional element in the development of debris accumulations by taking into account both genetic series of
debris accumulations and formations of debris accumulations. A significant proportion of debris accumulations are also due
to collapse processes which result from pressure release at the valley sides after deglaciation and occur in the course of
glacial trough valleys being transformed into more stable fluvial V-shaped valleys.
The residual morainic landscape has left debris accumulations that are basically similar in study areas of different climate
– i.e. in the Hindu Kush and the Karakoram on the one hand, and the Himalayas on the other. The age classification of the
debris accumulations was based on the location of the slope-derived debris accumulations in relation to the corresponding
stages of glaciation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
Detailed litho‐ and biostratigraphical analyses from three coastal sites in contrasting coastal settings on the Isle of Skye, Scotland, UK, reveal evidence for several changes in relative sea level during the Late Devensian and Holocene. At the start of the record, relative sea level in the area was high at ca. 12 500 14C (ca. 14 800 cal.) yr BP but then fell, reaching a low point during the Younger Dryas, at ca. 11 000–10 000 14C (ca. 13 000–11 600 cal.) yr BP, when a rock platform, correlated with the Main Rock Platform, was formed. In the early–middle Holocene, relative sea level was rising by ca. 8000 14C (ca. 8800 cal.) yr BP and in northeast Skye a lagoonal surface, correlated with the Main Postglacial Shoreline, was formed at ca. 6600 14C (ca. 7500 cal.) yr BP. By the late Holocene, relative sea level was again falling, but a rise, registered at at least two sites, began probably before ca. 4000 14C (ca. 4500 cal.) yr BP, and a second lagoonal surface in northeast Skye, correlated with the Blairdrummond Shoreline, was formed, although by ca. 3000 14C (ca. 3200 cal.) yr BP relative sea level in the area had resumed its downward trend. The pattern of relative sea‐level changes disclosed is compared with evidence elsewhere in Scotland. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
14.
Paleoclimate research is essential to determine the natural variability of climate and to place the current climate change into its natural context. The current need is to generate the highest temporal resolution paleoclimatic reconstructions possible in order to assess the natural variability of the climate system, but also to test the ability of numerical models to simulate conditions different from the ones observed with the relatively short instrumental records. In this paper, we show that CAT-scan analysis of sedimentary sequences, with its 1 mm downcore resolution, can be used to identify millennial to seasonal cycles in sedimentary sequences. In examples from the St. Lawrence Estuary, Eastern Canada, spectral analysis of the CAT-scan data from Holocene postglacial sediments revealed millennial- to centennial-scale oscillations possibly associated with either solar variability, changes in relative sea-level or tidal amplitude. Similarly, spectral analysis of Holocene and Sangamonian glaciomarine sequences revealed decadal- to annual-scale oscillations with periods close to the one previously associated with the North Atlantic Oscillation (NAO), whereas spectral analysis of the CAT-scan data from the Sangamonian rhythmites possibly revealed seasonal cycles. 相似文献
15.
本文利用甚长基线干涉测量（VLBI）技术提供的实测站速度和地球物理模型ICE_4G提供的数据,采用实测站速度法和基线长度变化率法分别检测了冰期后地壳回弹的垂直运动和水平运动.对用空间技术求解的实测值和用ICE_4G模型求解的估计值进行线性拟合和相关分析,两者的相关系数达到08～09,表明空间技术已经能够检测1～10mm/a的冰后回弹运动;在方向上两者基本一致,在量级上,实测结果的绝对值比模型估计值系统性的偏大约20%,表明当前冰期后的地壳回弹运动比地质时期要猛烈. 相似文献
16.
An improved and extended GPS-derived 3D velocity field of the glacial isostatic adjustment (GIA) in Fennoscandia 总被引：1，自引：1，他引：1
Martin Lidberg Jan M. Johansson Hans-Georg Scherneck James L. Davis 《Journal of Geodesy》2007,81(3):213-230
We present a new GPS-derived 3D velocity field for the Fennoscandia glacial isostatic adjustment (GIA) area. This new solution
is based upon ∼3,000 days of continuous GPS observations obtained from the permanent networks in Fennoscandia. The period
encompasses a prolongated phase of stable observation conditions after the northern autumn of 1996. Several significant improvements
have led to smaller uncertainties and lower systematic errors in the new solutions compared to our previous results. The GPS
satellite elevation cut-off angle was lowered to 10°, we fixed ambiguities to integers where possible, and only a few hardware
changes occurred over the entire network. The GAMIT/GLOBK software package was used for the GPS analysis and reference frame realization. Our new results confirmed earlier findings
of maximum discrepancies between GIA models and observations in northern Finland. The reason may be related to overestimated
ice-sheet thickness and glaciation period in the north. In general, the new solutions are more coherent in the velocity field,
as some of the perturbations are now avoided. We compared GPS-derived GIA rates with sea-level rates from tide-gauge observations,
repeated precise leveling, and with GIA model computations, which showed consistency. 相似文献
17.
冰期后地壳回弹运动的空间大地测量检测 总被引：3，自引：0，他引：3
本文讨论了用空间大利测量地壳运动预测量（基线变化率、站速度）检测冰期后地壳加弹运动的方法和结果。通过最近获得的空间大地测量地壳运动观测结果与最新冰期后地壳回弹模型ＩＣＥ－４Ｇ的估计值的比较和相关分析，发现两者有较好的一致性，相关系数高达０．８０￣０．９０，说明目前的空间大地测量技术已能够检测出１￣１０ｍｍ／ａ量级的冰期后地壳回弹运动。但实测结果比ＩＣＥ－４Ｇ估计值在绝对值上总体偏大约１．５倍，有可 相似文献
18.
Postglacial rebound is a major geological process which plays an important role in many areas in the earth sciences. Up to
now, most of the images derived from studies of the glacial isostatic adjustment phenomenon have been concerned with surface
signatures, such as the uplift and gravity anomalies and not much attention has been paid on the dynamical responses in the
mantle. We will make use of the 3D visualization package Amira to depict both the external and internal deformation histories
of the transient viscoelastic flow inside the mantle induced by postglacial uplift. Of particularly great interest are the
transient displacement fields and shear heating inside the mantle. This same visualization technology can be brought to bear
in the future for visualizing tsunami waves in ocean basins excited by earthquakes, volcanic eruptions and InSAR images. We
have also integrated the visualization results into the Google Earth virtual globe by combining this scheme with the Amira
package to provide a better geographical and dynamical context.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
19.
Michael Marden Colin Mazengarb Alan Palmer Kelvin Berryman Donna Rowan 《Geomorphology》2008,99(1-4):404-419
The sediment flux generated by postglacial channel incision has been calculated for the 2150 km2, non-glacial, Waipaoa catchment located on the tectonically active Hikurangi Margin, eastern North Island, New Zealand. Sediment production both at a sub-catchment scale and for the Waipaoa catchment as a whole was calculated by first using the tensioned spline method within ARC MAP to create an approximation of the aggradational Waipaoa-1 surface (contemporaneous with the Last Glacial Maximum), and second using grid calculator functions in the GIS to subtract the modern day surface from the Waipaoa-1 surface. The Waipaoa-1 surface was mapped using stereo aerial photography, and global positioning technology fixed the position of individual terrace remnants in the landscape. The recent discovery of Kawakawa Tephra within Waipaoa-1 aggradation gravels in this catchment demonstrates that aggradation was coincidental with or began before the deposition of this 22 600 14C-year-old tephra and, using the stratigraphic relationship of Rerewhakaaitu Tephra, the end of aggradation is dated at ca 15 000 14C years (ca 18 000 cal. years BP). The construction of the Waipaoa-1 terrace is considered to be synchronous and broadly correlated with aggradation elsewhere in the North Island and northern South Island, indicating that aggradation ended at the same time over a wide area. Subsequent downcutting, a manifestation of base-level lowering following a switch to postglacial incision at the end of glacial-age aggradation, points to a significant Southern Hemisphere climatic warming occurring soon after ca 15 000 14C years (ca 18 000 cal. years BP) during the Older Dryas interval. Elevation differences between the Waipaoa-1 (c.15 ka) terrace and the level of maximum channel incision (i.e. before aggradation since the turn of the 20th century) suggest about 50% of the topographic relief within headwater reaches of the Waipaoa catchment has been formed in postglacial times. The postglacial sediment flux generated by channel incision from Waipaoa catchment is of the order of 9.5 km3, of which ~ 6.6 km3 is stored within the confines of the Poverty Bay floodplain. Thus, although the postglacial period represented a time of high terrigenous sediment generation and delivery, only ~ 30% of the sediment generated by channel incision from Waipaoa catchment probably reached the marine shelf and slope of the Hikurangi Margin during this time. The smaller adjacent Waimata catchment probably contributed an additional 2.6 km3 to the same depocentre to give a total postglacial sediment contribution to the shelf and beyond of ~ 5.5 km3. Sediment generated by postglacial channel incision represents only ~ 25% of the total sediment yield from this landscape with ~ 75% of the estimated volume of the postglacial storage offshore probably derived from hillslope erosion processes following base-level fall at times when sediment yield from these catchments exceeded storage. 相似文献
20.
Marie Gagnoud Patrick Lajeunesse Gaston Desrosiers Bernard Long Suzanne Dufour Jacques Labrie Florian Mermillod-Blondin Georges Stora 《Engineering Geology》2009,103(3-4):106-111
Marine silty clay deposited during the Late-Wisconsinian postglacial marine transgression of eastern Québec (Goldthwait Sea) is ubiquitous in the sedimentary column of intertidal zones of the St-Lawrence Estuary. This mud is very compact and limits the penetration of organisms composing the modern Macoma balthica community. In order to describe the characteristics of intertidal sediments containing Goldthwait Sea mud, axial tomography (CT-Scan) is used. CT-Scan is a non-destructive method that can be used to describe sediment characteristics (grain size, mineralogy, primary and secondary sedimentary structures, fabric, shape and roundness, bedding contact), and to obtain high resolution, 3D representations of structures within sediment cores. Based on differences in the densities of analysed materials, the different lithologies, lithofacies, and organisms within the core can be discriminated, and a quantification of the volume occupied by the different components of the material can be made. Here, CT-Scan images provide information on the distribution, orientation and interweaving of thanatocœnosis shell beds that alternate with massive or faintly laminated postglacial marine mud beds, as well as on ichnofacies characteristics. In addition, we show 3D images of bioturbation structures within the recent sediment layer, which is distinguished from the underlying Goldthwait Sea mud. When coupled with conventional sedimentary (grain size statistics) and radiochronological (14C) analyses, these data provide information which is valuable for identifying depositional processes within sedimentary environments. 相似文献