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141.
Familiar since antiquity, and subject in contemporary times to various characterization schemes, the exact nature of solid bitumen is not yet fully known. Bitumens have ‘random polymer-like’ molecular structures, are mobile as highly viscous fluids or were once fluids but have since turned into solids. Solid bitumens consist mainly of large moieties, of polyclyclic aromatic hydrocarbons, occasionally with finely admixed, fine-grained cryptocrystalline graphite. Solid bitumens are distinguished from kerogen, which is the syngenetic and generally finely dispersed particulate organic matter in sedimentary rock that virtually does not migrate following its deposition. Occurrences of solid bitumens are relevant to petroleum exploration as well as the search for, and evaluation of, a variety of metallic mineral deposits. Genesis of bitumen is in many cases linked to the thermal and hydrothermal history of organic matter in sedimentary rock. Apparently bitumen, or more specifically organic acids generated along with bitumen during diagenesis, may alter porosity of reservoir rocks or otherwise prepare the ground for ore deposition. Bitumen is also relatively sensitive to alteration processes, some of which, such as oxidative weathering, water leaching, biodegradation (contact) metamorphism and ionizing radiation may likewise affect its nature. Elemental composition of bitumen commonly reflects the nature of mineral deposits. Is is possible that in petroleum exploration, trace metal abundances of bitumen may eventually allow prediction of crude oil types and volumes anticipated from a given source rock? Beside transition elements, notably Ni and V, highly anomalous concentrations of U, Pt and Au occur in some solid bitumens. During the generation of petroleum from kerogen, the trend in δ13C is toward lighter values. The opposite seems to occur when liquid petroleum is subjected to thermal cracking (and /or related processes) yielding solid bitumen enriched in 13C, and isotopically light methane. In fact, except for deasphalting and possibly some irradiation processes, the result of thermal cracking, oxidation, water leaching, inspissation (drying) and bacterial degradation of crude oil is that lower molecular weight hydrocarbons are removed leaving bitumen residues enriched in aromatic hydrocarbons, heteroatomic compounds (NSO) and 13C. Such phenomena are relevant to bitumen paragenesis in petroleum reservoir rocks, to certain Phanerozoic occurrences of multiple generations of bitumens, and to bitumens in mineral deposits. 相似文献
142.
Historical Changes in the Flood Hazard at Khartoum, Sudan: Lessons and Warnings for the Future 总被引:1,自引:0,他引:1
The 1988 floods at Khartoum were frequently described as "unprecedented". However, an examination of evidence for floods caused by the Nile and rain storms during the nineteenth and twentieth centuries leads to the conclusion that neither the Nile flood nor the rain storms nor their coincidence in one season were unprecedented and that similar situations will recur.We argue that most problems were created by complacency, lack of planning and mismanagement. We conclude with some simple suggestions for flood amelioration in the future and warn that complacency is the single most important phenomenon likely to recur to the detriment of Greater Khartoum in the future. 相似文献
143.
144.
The restricted problem of 2 + 2 bodies when one of the infinitesimal masses is further acted upon by the light pressure of the two primaries, is considered. The stationary solutions of this problem are found out. A short discussion is devoted to the stability of these solutions. 相似文献
145.
Formation of saline groundwaters in the Baltic region through freezing of seawater during glacial periods 总被引:1,自引:0,他引:1
Typical Ca---Cl brines occur in crystalline and metamorphic rocks below freshwater horizons at various localities in Sweden and Finland. Total dissolved solids (TDS) range in concentration between 2 and 120 gl−1 and have long been thought to derive from water-rock interactions. The relationships between Na, Cl and Br in these brines suggest, however, that they were derived from freezing of seawater during glacial periods. The brines were subsequently diluted by meteoric waters and their Ca/Mg ratio was increased through water-rock interactions in the subsurface. The hydrogeological model for both the formation of freeze-derived marine brines and their lateral intrusion involves restricted inland marine basins in recent and subrecent polar climatic belts. Seawater in such basins gradually freezes in response to glaciation. The solutes which concentrate in the remaining water body are residual after precipitation of a sequence of minerals which include carbonates, mirabilite and hydrohalite. Hydraulic pressure of the growing ice sheet over the frozen seas is gradually added to the ambient hydrostatic pressure exerted on the brines. This, together with their increased density, increases the intrusional potential of the brines. As the land ice cannot exert hydraulic pressure on continental groundwater in the aquifers, the balance of pressure favours deep landward intrusion of brines. Post-glacial processes cause the subsurface dilution and replacement of the brines both by seawater and fresh waters. The presence of such brines also far from present-day coastal settings reflects the shifting of coastlines as a result of isostatic movements and eustatic sea-level changes associated with glaciation and deglaciation. 相似文献
146.
Perspectives for the Radio-Optical Telescope 54/32/2.6 are given for scientific research in radioastronomy. Its characteristics and potential scientific fields are summarized in the frame of the present French-Armenian collaboration. A 3-phase upgrade plan including a detailed technical evaluation of the antenna is presented.Published in Astrofizika, Vol. 38, No. 4, pp. 645–648, October–December, 1995. 相似文献
147.
We have studied the influence of the magnetic helicity on solar particle propagation using the IMF data observed by the HELIOS spacecraft in the range 0.31–0.95 AU, during eight solar proton events. For this, we have derived power and helicity spectra of the turbulence of the magnetic field during the time of the events. These are used to compute the particle pitch-angle scattering coefficients according to the quasi-linear theory (QLT) treatment of particle propagation in turbulent magnetic fields. The results show that in all the cases the helicity effects are negligible and the particle's mean free paths deduced from the pitch-angle diffusion coefficients are the same regardless of whether or not helicity effects are included in the calculations. The computed mean free paths are quite different in each case.Deceased 10 April, 1995. 相似文献
148.
The Waterman Metamorphic Complex of the central Mojave Desert was exposed as a consequence of early Miocene detachment-dominated extension. However, it has evidence consistent with a more extensive geological history that involves collision of a crustal fragment(s), tectonic thickening by overthrusting and two periods of extension. The metamorphic complex contains granitoid intrusives and felsic mylonitic gneisses as well as polymetamorphic rocks that include marble, calc-silicate, quartzite. mafic granulite, pyribolite, amphibolite, migmatite and biotite schist. The latter group of rocks was affected by an initial series of high-grade metamorphic events (M1 and M2) and a localized lower grade overprint (M3). The initial metamorphism (M1) can be separated into two stages along its high-grade P–T path: M1a, a granulite facies metamorphism at 800–850° C and 7.5–9 kbar and Mlb, an upper amphibolite facies overprint at 750–800° C and 10–12 kbar. M1a developed mineral assemblages and textures consistent with granulite facies conditions at a reduced activity of H2O and is associated with intense ductile deformation (D1) and minor local partial melting. M1b overprinted the granulite assemblages with a series of hydrous phases under conditions of increasing pressure and H2O activity and is accompanied by little or no deformation. M2 developed at lower pressures and temperatures (650–750° C, 4.5–5.5 kbar) and is distinguished by a second local overprint of hydrous phases that reflects an input of aqueous fluids probably associated with the intrusion of a series of granitic dykes and veins. Effects of M3 are confined to the Mitchel detachment zone, an anastomosing early Miocene detachment fault, and are characterized by local ductile/brittle deformation (D2) of the pre-existing high-grade rocks and granitoid intrusives and by the production of mylonites and mylonitic gneisses under greenschist facies conditions (300–350° C, 3–5 kbar). The initial overprint (M1a) represents metamorphism, devolatilization and minor partial melting of supracrustal rocks under granulite facies conditions as a consequence of tectonic and, possibly, magmatic thickening. The increasing pressure transition of M1a to M1b reflects a period of continued compressional tectonism, thrusting and influx of H2O, in part, locally related to crystallization of partial melts. The near isothermal decompression between M1b and M2 probably represents a pre-112-Ma extensional episode that may have been the result of a decompressional readjustment of a thickened crust. Following the initial extensional event, the metamorphic complex remained at depths of 10–17 km for at least 90 Ma until it was uplifted following Miocene extension. M3 develops locally in response to this second extensional period resulting from the early Miocene detachment faulting. 相似文献
149.
An efficient and robust method has been developed to locate multiple impulsive sources in an ocean environment. Global position system (GPS) receivers were installed on sonobuoys to obtain their locations within a few meters of accuracy. A sonobuoy field was deployed in a ring-type pattern. Charges were then set off at arbitrary locations within the ring, High-resolution plots were used to obtain direct path and/or first bottom bounce arrivals on each buoy. A model grid of arrival times was constructed, corresponding to the dimensions of the buoy field. A ray model previously developed here at the Applied Research Laboratories at the University of Texas at Austin (ARL:UT) was used to obtain model travel times. The minimum value of the least-square-type error between the real arrival times and the modeled travel times resulted in an unambiguous location of the source, within the limits of the grid spacing chosen. This value was calculated by picking one receiver as the reference and then summing the timing errors of the remaining receivers relative to the reference. Successive iterations with finer grid spacings result in source localization within the accuracy of the buoy locations. The localization routine was extended by allowing permutations of the pulse arrivals on each buoy to account for multiple sources closely separated in time and/or space. An automated correlation technique is presented as an alternative to the leading edge-detection method used here for obtaining relative arrival times. Two proof-of-concept experiments were performed and some results of data obtained at Lake Travis and the Gulf of Mexico are presented 相似文献
150.
Studies of soil productivity must compensate for the effects of temporal trends in order to examine the pattern of crop yields along spatial gradients. An analysis of the published yield estimates for 30 soils in 233 counties, however, did not find consistent yield increases over the past three decades. On the contrary, the yield estimates for many soils were markedly uniform since 1972. The uniformity appears to have two causes: the acknowledged difficulty of making yield estimates in a time of increasing variability in soil and crop management, both within and between regions, and surveyors' awareness of data stored in a national soils data base. The effect is to cast doubt on soil productivity data reported in county soil surveys published between 1973 and 1988. 相似文献