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991.
Accurate geocentric three dimensional positioning is of great importance for various geodetic and oceanographic applications.
While relative positioning accuracy of a few centimeters has become a reality using Very Long Baseline Interferometry (VLBI), the uncertainty in the offset of the adopted coordinate system origin from the geocenter is still believed to be of the
order of one meter. Satellite Laser Ranging (SLR) is capable of determining this offset to better than10 cm, though, because of the limited number of satellites, this requires a long arc of data. The Global Positioning System (GPS) measurements provide a powerful alternative for an accurate determination of this origin offset in relatively short period
of time. Two strategies are discussed, the first utilizes the precise relative positions predetermined byVLBI, where as the second establishes a reference frame by holding only one of the tracking sites longitude fixed. Covariance
analysis studies indicate that geocentric positioning to an accuracy of a few centimeters can be achieved with just one day
of preciseGPS pseudorange and carrier phase data. 相似文献
992.
993.
994.
V. M. Zobin E. I. Gordeev V. F. Bakhtiarov E. I. Ivanova Yu. M. Khatkevich V. N. Khodenko V. E. Levin V. P. Mityakin 《Natural Hazards》1992,6(1):51-70
The earthquake of 6 October 1987 (M = 6.6), which occurred near the Shipunsky Cape, Kamchatka, was the largest crustal event in the vicinity of the main city of Kamchatka — Petropavlovsk-Kamchatsky — during the last three decades. It was followed by numerous aftershocks. This earthquake allowed us to test the effectiveness of the seismic hazard monitoring in Kamchatka, including the seismological, geodetic and hydrogeochemical surveys. The seismic survey provided the location and source nature of the main shock and aftershocks and the seismic environment of the main shock. The geodetic and hydrogeochemical surveys have yielded data on the response to earthquakes of the Earth's surface deformations, water level, and chemical elements concentration in the underground water. As a result, the following data were obtained: The earthquake of 6 October had a seismic moment 4–10 E18 Nm, thrust type of faulting and the source volume of 20 × 20 × 10 km3. The maximum intensity was VI–VII (MSK-64 scale) and maximum acceleration 88 cm/s2. Before this event, a relative increase in the number of the upper mantle (depth more than 100 km) moderate magnitude earthquakes during 5 years and a one-year period of seismic quiescence for small shallow earthquakes, were recognized. Significant anomalies in HCO3 and H3BO3 concentrations in the underground waters were observed in the wells a week before the main shock. 相似文献
u
995.
Shude Mao Hans J. Witt Leon V. E. Koopmans 《Monthly notices of the Royal Astronomical Society》2001,323(2):301-307
Recent observations indicate that many if not all galaxies host massive central black holes. In this paper we explore the influence of black holes on the lensing properties. We model the lens as an isothermal ellipsoid with a finite core radius plus a central black hole. We show that the presence of the black hole substantially changes the critical curves and caustics. If the black hole mass is above a critical value, then it will completely suppress the central images for all source positions. Realistic central black holes are likely to have masses below this critical value. Even in such subcritical cases, the black hole can suppress the central image when the source is inside a zone of influence, which depends on the core radius and black hole mass. In the subcritical cases, an additional image may be created by the black hole in some regions, which for some radio lenses may be detectable with high-resolution and large dynamic range VLBI maps. The presence of central black holes should also be taken into account when one constrains the core radius from the lack of central images in gravitational lenses. 相似文献
996.
997.
Palaeomagnetism of Holocene lake sediments from north Poland 总被引:1,自引:0,他引:1
K. M. Creer E. Hogg Z. Malkowski J. E. Mojski E. Niedziolka-Krol P. W. Readman P. Tucholka 《Geophysical Journal International》1979,59(2):287-313
Summary. Detailed palaeomagnetic investigations of 17 cores of Holocene lake sediments from four lakes in Poland show a pattern of declination variations similar to those found in Lake Windermere, England and in addition a well defined pattern of inclination variations. A total of nine inclination swings are observed compared to five declination swings and their phase relationships are such that both sets of swings cannot be periodic. Exact dating of the palaeomagnetic record is not possible as radiocarbon ages are over 1000 yr older than those deduced from palynological studies. Thermomagnetic and X-ray studies on magnetic concentrates confirm the results of studies of remanence cooling, IRM acquisition and coercivity experiments which are consistent with fine grained magnetite as the main magnetic mineral in the sediments. 相似文献
998.
Bipolar active regions (ARs) are thought to be formed by twisted flux tubes, as the presence of such twist is theoretically
required for a cohesive rise through the whole convective zone. We use longitudinal magnetograms to demonstrate that a clear
signature of a global magnetic twist is present, particularly, during the emergence phase when the AR is forming in a much
weaker pre-existing magnetic field environment. The twist is characterised by the presence of elongated polarities, called
“magnetic tongues”, which originate from the azimuthal magnetic field component. The tongues first extend in size before retracting
when the maximum magnetic flux is reached. This implies an apparent rotation of the magnetic bipole. Using a simple half-torus
model of an emerging twisted flux tube having a uniform twist profile, we derive how the direction of the polarity inversion
line and the elongation of the tongues depend on the global twist in the flux rope. Using a sample of 40 ARs, we verify that
the helicity sign, determined from the magnetic polarity distribution pattern, is consistent with the sign derived from the
photospheric helicity flux computed from magnetogram time series, as well as from other proxies such as sheared coronal loops,
sigmoids, flare ribbons and/or the associated magnetic cloud observed in situ at 1 AU. The evolution of the tongues observed in emerging ARs is also closely similar to the evolution found in recent MHD
numerical simulations. We also found that the elongation of the tongue formed by the leading magnetic polarity is significantly
larger than that of the following polarity. This newly discovered asymmetry is consistent with an asymmetric Ω-loop emergence,
trailing the solar rotation, which was proposed earlier to explain other asymmetries in bipolar ARs. 相似文献
999.
D. P. Rao 《Journal of the Indian Society of Remote Sensing》1977,5(2):67-70
Visual Interpretation of LANDSAT imagery of Eastern Ghats-Godavari delta area on scale 1:1,000,000 on spectral bands 5 and 7 was attempted. A neotectonic and geomorphic map was prepared from these images showing among other features major and minor lineaments. The importance of some of these lineaments for sedimentation, oil migration and localisation of ore bodies is discussed. 相似文献
1000.
Ancient human activities have resulted in important elemental enrichments in soils at archaeological sites. Nevertheless, the spatial extent of such elemental enrichment signals is rarely studied. Our research addresses this theme by studying the characteristics and extent of the geochemical enrichment halo around the monumental Colonnaded Street of the Hellenistic to Byzantine city of Sagalassos in southwest Turkey. Given the complex geology of the area, a strategy applying multivariate statistical data analysis techniques is proposed to identify whether the enrichments have a geological or an anthropogenic source. In addition, we evaluate how a wide variety of natural and anthropogenic processes may influence the distribution of elements around the site. In this paper, it is shown that enrichments of P and Pb are present up to a distance of 450 m and 150 m away from the city, respectively. Hence, we conclude that the extent of chemical enrichment haloes around archaeological sites may be element specific. 相似文献