CT定位用于眼面部异物诊断治疗     

霍艳芬 《CT理论与应用研究》1995,4(4):32-33

本文报告２６例由于外力作用致使眼面部广泛异物存留。临床应用Ｘ线的归咎常见漏诊及定位困难，因此采取ＣＴ定位达到异物诊断治疗目的。  相似文献   

论γ射线的点放射源和非点放射源的辐射研究     

刘恩承 《CT理论与应用研究》1995,4(4):36-38,49

本文讨论点放射源和非点放射源的辐射，为简化理论模型，暂不考虑二次γ射线，只讨论点源和线源和柱状源的辐射，本文的讨论有助于工业ＣＴ的设计及系统的优化。  相似文献   

富钆镝易解石的发现   总被引：1，自引：0，他引：1  

蔡根庆 《矿物学报》1987,(1)

该矿物产于铀钍混合矿化的中粗粒黑云母花岗岩中。本文研究了产状、共生矿物、结晶特性、物理及化学性质并与易解石、钍易解石、铀易解石、钇易解石等作了对比。该矿物铀、钍、铅等含量均较高,在稀土含量中,钆、镝含量高达稀土总量的27％,故定名富钆镝易解石。  相似文献   

A study of tropospheric wave disturbances over India in winter     

I. Subbaramayya  A. S. N. Raju 《Pure and Applied Geophysics》1982,120(3):437-452

The wave disturbances in the troposphere over north and central India during winter when the subtropical westerlies prevail in the upper air over the area have been investigated by power-and cross-spectrum analysis. The power-spectrum study revealed the existence of basically two systems of periods 10.0–12.5 days and 4.5–5.5 days in the middle and upper troposphere. Both the long-and short-period systems have been found to be propagating eastwards at rates 5° and 10° long./day respectively. In both the cases lower temperatures are found to the east of the troughs. It is also noted that these systems build up the meridional temperature gradient and hence strengthen the upper tropospheric westerlies. Synoptic study of the transient systems is presented.  相似文献   

A tectonic model for the metamorphic evolution of the Basal Gneiss Complex, Western South Norway   总被引：1，自引：0，他引：1  

S. J. CUTHBERT  M. A. HARVEY  D. A. CARSWELL 《Journal of Metamorphic Geology》1983,1(1):63-90

A review of currently available information relevant to the Basal Gneiss Complex (BGC) of Western South Norway, combined with the authors'own observations, leads to the following conclusions.
1. Most of the BGC consists of Proterozoic crystalline rocks and probably subordinate Lower Palaeozoic cover.
2. The last major deformation of these rocks was during the Caledonian orogeny and involved large-scale thrusting, recumbent folding and doming. The structural development of the BGC is closely tied in with that of the Caledonian allochthon.
3. The whole eclogite-bearing part of the BGC has suffered a high pressure metamorphism with conditions of between 550°C, 12.5 kbar (Sunnfjord) and about 750°C, 20 kbar (Møre og Romsdal) at the metamorphic climax.
4. This metamorphism was of Caledonian age, probably rather early in the Caledonian tectonic history of the BGC and is considered to have been a rather transient event.
By setting these conclusions in a framework provided by geophysical evidence for the deep structure of the crust in southern Norway we have constructed a geotectonic model to explain the recorded metamorphic history of the BGC. It is suggested that considerable crustal thickening was caused by imbrication of the Baltic plate margin during continental collision with the Greenland plate. This resulted in high pressure metamorphism in the resulting nappe stack. Progradation of the suture caused underthrusting of the Baltic foreland below the eclogite-bearing terrain causing it to emerge at the Earth's surface, aided by tectonic stripping and erosion.
Application of isostacy equations to the model shows that eclogites can be formed by in-situ metamorphism in crustal rocks and reappear at the land surface above a normal thickness of crust in a single orogenic episode of approximately 65-70 Ma duration.  相似文献   

Deformation and metamorphism in the Bard area of the Sesia Lanzo Zone, Western Alps, during subduction and uplift   总被引：2，自引：0，他引：2  

P. F. WILLIAMS  R. COMPAGNONI 《Journal of Metamorphic Geology》1983,1(2):117-140

The structure, microstructure and petrology of a small area close to the village of Bard in Val d'Aosta (Italy) has been studied in detail. The area lies across the contact between the Gneiss Minuti (GM) and the Eclogitic Micaschist (EMS) Complexes of the Lower element of the Sesia portion of the Sesia-Lanzo Zone (Western Alps). Both complexes have undergone high-pressure metamorphism, but the metamorphic assemblages indicate a sudden increase in pressure in going across the contact from the GM to the EMS. Therefore, we interpret the contact as a thrust dividing the lower element of the Sesia into two sub-elements. This interpretation is supported by structural evidence.
The early Alpine (90-70 Ma) metamorphic history is best preserved in the EMS and is one of increasing pressure associated with thrusting. The maximum P/T recorded in the EMS is >1500 MPa (>15kbar) and 550°C and in the GM is < 1500-1300 MPa (< 15-13 kbar) and 500-550°C. We suggest that the rocks were probably in an active Benioff zone during this time.
From then on the histories of the GM and EMS are the same. Deformation continued and the thrust and thrust slices were folded during decreasing pressure. We interpret the first postthrusting deformation in terms of uplift associated with continued shortening of the crust and underplating after the Benioff zone had become inactive and a new Benioff zone had developed further to the north-west.
A still later deformation and the Lepontine metamorphism (38 Ma) are related to continued uplift. Much of this deformation is characterized by structures indicative of vertical shortening and lateral spreading as the mountains rose above the general level of the surface.  相似文献   

A Note on the Mixing Length Theory and Massive Star Evolution     

Li－CaiDeng  Da－RunXiong 《中国天文和天体物理学报》2001,1(1):50-56

1 INTRODUCTIONThe mixing length theory (MLT) for stellar convection originally developed by Vitense(1953, 1958) has been the most popularly used local convection theory in the studies of stellarstructure and evolution. The theory was later modified and revised by many investigators,who suggested some different expressions. In fact, MLT is not a real hydrodynamic theory,rather, it is a simple "ballistic" theory which traces the motion of imaginary convective elements. In reality j stell…  相似文献   

Hydrogen recombination lines near 327 MHz−II: A Galactic plane survey with a 2°×6′ beam     

D. Anish Roshi  K. R. Anantharamaiah 《Journal of Astrophysics and Astronomy》2001,22(1):81-119

In a previous paper we presented a low-resolution (2°×2°) survey of radio recombination lines (RRLs) at 327 MHz in the longitude rangel=330° to 0° to 89°. In this paper, we present the results of a higher resolution (2°×6′) survey of RRLs from seven 2°-wide fields and two 6°-wide fields in the same longitude range. Observations were made using the Ooty Radio Telescope (ORT). A total of 252 spectra that were obtained are presented. RRLs were detected in almost all the individual positions within the fields withl<35° and at several individual positions within the fields in the longitude rangel=35° to 85°. Detailed analysis of the data towards the field centered at G45.5+0.0, shows that the line emission consists of discrete zones of ionized gas. The angular extent of these zones are likely to be one degree or more corresponding to a linear size of >110 pc at the kinematic distance.  相似文献   

High-Resolution Seismic Reflection Evidence for Middle Holocene Environmental Change, Owasco Lake, New York   总被引：2，自引：0，他引：2  

Henry T. Mullins  John D. Halfman 《Quaternary Research》2001,55(3):322

Approximately 70 km of new decimeter-resolution seismic reflection profile data from Owasco Lake, New York define a middle Holocene (4600 ¹⁴C yr B.P.) erosion surface in the north end of the lake at water depths as great as 26 m. Beneath the lake, post-glacial sediments are up to 9 m thick and represent about 10% of the total sediment fill. Early to middle Holocene sediments, 6 m thick, contain biogenic gas at the south end of the basin and a large (4 km×300 m×15 m) subaqueous slide deposit along the east-central portion of the lake. Late Holocene sediments are thinner or absent, particularly at the north end of the lake. The middle Holocene erosion surface may have been produced by a drop in lake level. Alternatively, it may represent a change in climate during the transition between the relatively warm Holocene hypsithermal and cool neoglacial. At this time (4600 ¹⁴C yr B.P.) circulation in Owasco Lake appears to have evolved from sluggish to active. The increased circulation, which persists today, probably resulted from atmospheric cold fronts with strong southwesterly winds that piled up water at the north end of the lake. The increased water circulation may have been ultimately driven by decreasing insolation, which produced an increased pole-to-equator thermal gradient and, thus, stronger global winds that began at the transition between the hypsithermal and neoglacial.  相似文献   

Gravity constraints on the dynamics of the crust–mantle system during Calabrian subduction     

A. M. Marotta  R. Barzaghi  A. Borghi  E. Spelta 《Geophysical Journal International》2007,171(3):977-985

The thermomechanic evolution of the lithosphere–upper mantle system during Calabrian subduction is analysed using a 2-D finite element approach, in which the lithosphere is compositionally stratified into crust and mantle. Gravity and topography predictions are cross-checked with observed gravity and topography patterns of the Calabrian region. Modelling results indicate that the gravity pattern in the arc-trench region is shaped by the sinking of light material, belonging to both the overriding and subduction plates. The sinking of light crustal material, up to depths of the order of 100–150 km is the ultimate responsible for the peculiar gravity signature of subduction, characterized by a minimum of gravity anomaly located at the trench, bounded by two highs located on the overriding and subducting plates, with a variation in magnitude of the order of 200 mGal along a wavelength of 200 km, in agreement with the isostatically compensated component of gravity anomaly observed along a transect crossing the Calabrian Arc, from the Tyrrhenian to the Ionian Seas. The striking agreement between the geodetic retrieved profiles and the modelled ones in the trench region confirms the crucial role of compositional stratification of the lithosphere in the subduction process and the correctness of the kinematic hypotheses considered in our modelling, that the present-day configuration of crust–mantle system below the Calabrian arc results from trench's retreat at a rate of about 3 cm yr⁻¹, followed by gravitational sinking of the subducted slab in the last 5 Myr.  相似文献