MAPPING NON-REFLECTING VELOCITY INTERFACES BY NORMAL MOVEOUT VELOCITIES OF UNDERLYING HORIZONS*     

TH. KREY 《Geophysical Prospecting》1980,28(3):359-371

The normal moveout velocity of a reflecting bed is a function of the dips and curvatures of all overlying velocity interfaces. Now let the (N– 1)th velocity interface be a non- (or badly) reflecting bed, whereas the other interfaces, including the base of the Nth layer, reflect satisfactorily, and let the velocities U_{N– 1} and U_N of the (N– 1)th and Nth layer, respectively, be known. Then the normal moveout velocity for the base of the Nth layer, if known in one direction at a certain part of the surface of the earth, provides a second order differential equation in the horizontal coordinates x and y for the depth Z_{N – 1}(x, y) of the unknown interface. The mathematics becomes rather simple in the case of two-dimensional geological structures. For this case and N= 2 the differential equation mentioned can be solved by stepwise integration or by iteration. One of the many possible applications of the new concept is the determination of the structure of the base of an overthrusting sheet.  相似文献   

Garnet peridotite xenoliths in South African kimberlite pipes and their petrogenesis   总被引：1，自引：0，他引：1  

D. A. Carswell  J. B. Dawson 《Contributions to Mineralogy and Petrology》1970,25(3):163-184

Whole rock analyses are presented for nine garnet peridotite, two garnet pyroxenite, one garnet free peridotite and two highly altered garnet peridotite xenoliths from South African kimberlite pipes. Garnets from eight of these samples have also been chemically analysed, while olivine compositions in the twelve fresh samples have been estimated by X-ray diffraction. The chemistry and mineralogy of these specimens are compared with the results of previous studies, and the controversies concerning the petrogenesis of these xenoliths are discussed in detail.  相似文献   

Microbiological characterisation of a diesel contaminated beach site     

Paton GI  Iroegbu CO  Dawson JJ 《Marine pollution bulletin》2003,46(7):903-906

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A parallel, implicit, cell‐centered method for two‐phase flow with a preconditioned Newton–Krylov solver     

Clint N. Dawson  Héctor Klíe  Mary F. Wheeler  Carol S. Woodward 《Computational Geosciences》1997,1(3-4):215-249

A new parallel solution technique is developed for the fully implicit three‐dimensional two‐phase flow model. An expandedcell‐centered finite difference scheme which allows for a full permeability tensor is employed for the spatial discretization, and backwardEuler is used for the time discretization. The discrete systems are solved using a novel inexact Newton method that reuses the Krylov information generated by the GMRES linear iterative solver. Fast nonlinear convergence can be achieved by composing inexact Newton steps with quasi‐Newton steps restricted to the underlying Krylov subspace. Furthermore, robustness and efficiency are achieved with a line‐search backtracking globalization strategy for the nonlinear systems and a preconditioner for each coupled linear system to be solved. This inexact Newton method also makes use of forcing terms suggested by Eisenstat and Walker which prevent oversolving of the Jacobian systems. The preconditioner is a new two‐stage method which involves a decoupling strategy plus the separate solutions of both nonwetting‐phase pressure and saturation equations. Numerical results show that these nonlinear and linear solvers are very effective.  相似文献   

Indirect crustal contamination: evidence from isotopic and chemical disequilibria in minerals from alkali basalts and nephelinites from northern Tanzania     

C. R. Paslick  Alex N. Halliday  R. A. Lange  D. James  J. B. Dawson 《Contributions to Mineralogy and Petrology》1996,125(4):277-292

 Alkali basalts and nephelinites from the volcanic province of northern Tanzania contain pyroxene and nepheline that show evidence for chemical and/or isotopic disequilibria with their host magmas. Olivine, pyroxene, nepheline and plagioclase all appear to be partially xenocrystic in origin. Five whole rock/mineral separate pairs have been analyzed for Sr, Nd, and Pb isotopic compositions. The ²⁰⁶Pb/²⁰⁴Pb ratios are distinct by as much as 20.94 (whole rock) vs. 19.10 (clinopyroxene separate). The Sr and Nd isotopic disequilibria vary from insignificant in the case of nepheline, to Δ ⁸⁷Sr/⁸⁶Sr of 0.0002 and Δɛ_Nd of 0.7 in the case of clinopyroxene. The mineral chemistry of 25 samples indicates the ubiquitous presence of minerals that did not crystallize from a liquid represented by the host rock. The northern Tanzanian magmas are peralkaline and exhibit none of the xenocrystic phases expected from crustal assimilation. The disequilibria cannot be the result of mantle source variations. Rather the xenocrystic phases present appear to have been derived from earlier alkali basaltic rocks or magmas that were contaminated by the crust. Material from this earlier magma was then mixed with batches of magma that subsequently erupted on the surface. Disequilibrium in volcanic rocks has potentially serious consequences for the use of whole rock data to identify source reservoirs. However, mass balance calculations reveal that the ²⁰⁶Pb/²⁰⁴Pb isotopic compositions of the erupted lavas were changed by less than 0.25% as a result of this indirect crustal contamination. Received: 15 February 1995 / Accepted: 4 May 1996  相似文献   

Reduced sapphirine granulite xenoliths from the Lace Kimberlite,South Africa; implications for the deep structure of the Kaapvaal Craton   总被引：1，自引：0，他引：1  

J. B. Dawson  J. V. Smith 《Contributions to Mineralogy and Petrology》1987,95(3):376-383

Sapphirine-bearing granulites, together with sapphirine-free granulites and eclogites, occur as xenoliths in the kimberlite of the Lace diatreme that penetrates the Kaapvaal craton, S. Africa. Absence of (calculated) Fe³⁺ in sapphirine, garnet and sillimanite, together with presence of graphite and sulphides, suggests highly reducing conditions of metamorphism. Chemical considerations and comparisons with experimental investigations suggest metamorphism of a sedimentary (?chlorite-montmorillonite) protolith at 900–1000° C and > 10 Kb; high Cr in the assemblage may point to a basic/ultrabasic precursor. The xenoliths indicate the presence of a very-high-grade granulite terrain, possibly similar to Enderby Land (Antarctica), beneath the Kaapval craton.  相似文献   

Wet removal of highly soluble gases     

Peter Brimblecombe  G. A. Dawson 《Journal of Atmospheric Chemistry》1984,2(1):95-107

Partition, not kinetics, ultimately determines the concentration of highly soluble gases in cloud droplets. Partition equations are formulated and applied to idealized air-mass thunderclouds and precipitating stratus. Contribution to aqueous concentrations from sub-cloud scavenging of highly soluble gases is estimated at between 10 and 20% under relatively unpolluted conditions. Data indicate that evaporation can produce enhancements in concentration of as much as a factor of 3. The calculations give large-scale mean coefficients of wet removal of highly soluble gases of about 2.8×10^-6 s^-1 (4-day residence time) for air-mass thunderclouds and precipitating stratus. Removal is so effective that the mean scale heights of these gases should be decreased to 2 km or less. The criterion of high solubility in this paper is that K ^H (Henry's Law coefficient) > 10⁵ mol l^-1 atm^-1. Gases that are effectively highly soluble include HCl, HNO₃, H₂SO₄, H₂O₂, NH₃ in acid droplets, SO₂ in oxidizing droplets (and probably some light amines and sulfonic acids), but not SO₂ in the absence of oxidants, nor HCHO. A variation of removal coefficient and scale height with solubility is presented. A comparison of atmospheric NH₃ concentrations deduced from rain NH₄ ⁺ and measured directly gives reasonable agreement.  相似文献   

Oxidation states of the upper mantle recorded by megacryst ilmenite in kimberlite and type A and B spinel lherzolites   总被引：2，自引：0，他引：2  

R. J. Arculus  J. B. Dawson  R. H. Mitchell  D. A. Gust  R. D. Holmes 《Contributions to Mineralogy and Petrology》1984,85(1):85-94

The intrinsic oxygen fugacities of homogeneous, inclusion-free, megacryst ilmenites from the Frank Smith, Excelsior, Sekameng and Mukorob kimberlite pipes in southern Africa, and the alnöitic breccia in the Solomon Islands have been determined. Similar measurements have been made of the type A and B spinel peridotites from San Carlos in Arizona. The type A peridotites are characterised by oxygen fugacities close to the iron-wüstite buffer, similar to those of equivalent peridotite specimens from other continental and island arc environments. In strong contrast, the type B peridotites and all of the ilmenite megacrysts range between the oxygen fugacities defined by the nickelnickel oxide and fayalite-magnetite-quartz buffers. A close relationship between type B peridotites, oxidized metasomatizing fluids in the upper mantle and oxidized, silicaundersaturated magma types is suggested. It is unlikely that a solid elemental carbon phase can be an equilibrium crystallization product of kimberlite magmas if the ilmenite megacrysts represent the redox state of kimberlite melts. The ultimate source of the oxidizing fluids and the development of such a wide dispersion (>4 orders of magnitude) in oxygen fugacities of the upper mantle is not clear, but may involve recycled lithosphere, fluids from the lower mantle or result from the relatively rapid diffusion of H₂, compared with other potential volatile species, in the mantle.  相似文献   

地磁场的空间相关性及其在地震预报中的应用   总被引：13，自引：0，他引：13  

中国科学院地球物理研究所第十研究室一组 《地球物理学报》1977,20(3):169-184

本文扼要叙述了震磁关系研究的现状,阐述了“震磁效应”分析的基本原理,提出了“空间相关”和“现象相关”两种“震磁效应”分析的基本方法。作者应用我国华北地区若干台站资料,以局部地区地磁场空间相关性的分析为基础,重点讨论了“空间相关法”,给出了具体震例分析的初步结果。最后,就有关“震磁效应”分析的几个问题进一步做了简要讨论。  相似文献   

GPS on the Web: Internet-Based Geodetic GPS Processing at AUSLIG     

John Dawson  Ramesh Govind  John Manning 《GPS Solutions》2002,5(3):91-93

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