Propagation of guided waves in a fluid layer bounded by two viscoelastic transversely isotropic solids     

Ahmed S. Ashour   《Journal of Applied Geophysics》2000,44(4)

The analysis of Stoneley wave propagation in a fracture is essential for the identification and evaluation of fracture parameters from the borehole Stoneley wave. Also, it is important for many geophysics considerations, e.g. for tremor and long-period events observed in volcanoes and geothermal areas. In this paper, we investigate the guided waves propagation in a fluid layer lying between two viscoelastic vertically transversely isotropic media. The viscoelastic mechanism models the attenuation due to the presence of fluid saturation in the rock. A model based on the superposition of three inhomogeneous partial plane waves: one in the fluid and two heterogeneous waves in the solid is developed. The dispersion equation is obtained for this case. A numerical solution is carried out to obtain the guided wave velocity and attenuation coefficient. The results of this investigation show that there is a strong correlation between the velocity dispersion and attenuation of Stoneley wave and the anisotropic parameters of the medium especially in a sandstone (fast) medium.  相似文献   

Mineral and chemical composition of Karak mudstone, Kohat Plateau, Pakistan: implications for smectite-illitization and provenance   总被引：1，自引：0，他引：1  

Akhtar Ali Saleemi  Zulfiqar Ahmed   《Sedimentary Geology》2000,130(3-4):229-247

The Karak mudstone interbedded in an Eocene evaporite sequence, is dominated by R-1 ordered illite-smectite with a 20 to 30% expandable component. Minor phases include kaolinite, chlorite, illite/muscovite, plagioclase, potash feldspar, quartz, dolomite and pyrite. The present illite-smectite was probably originally smectite or highly expandable illite-smectite which underwent conversion to illite-smectite with a low expandable component in a comparatively low-temperature (ca. 100°C) closed-system sedimentary basinal diagenetic environment at a depth of ca. 5 km. Al³⁺ and K⁺ necessary for the conversion reaction were provided through the breakdown of potash feldspar. Burial under a 5 km thick pile of sediments produced some of the observed structures. Whole-rock chemistry presented here suggests that the mudstone formed by severe weathering of acidic source rocks. The influx of freshwater probably flushed out Ba, Rb, Ca and Mn from the restricted basin.  相似文献   

Using a naive Bayes classifier to explore the factors driving the harmful dinoflagellate Karenia selliformis blooms in a southeastern Mediterranean lagoon     

Feki-Sahnoun  Wafa  Njah  Hasna  Hamza  Asma  Barraj  Nouha  Mahfoudi  Mabrouka  Rebai  Ahmed  Bel Hassen  Malika 《Ocean Dynamics》2020,70(7):897-911

Ocean Dynamics - The blooms of the toxic dinoflagellate Karenia selliformis can be predicted with accuracy derived from knowledge of the main forcing variables. A naive Bayes classifier modeling...  相似文献   

VARIATIONS IN KAMILA AMPHIBOLITES FROM SOUTHEASTERN PART OF THE KOHISTAN ISLAND-ARC TERRANE,PAKISTAN     

M.Ahmed Khan    M.Asif Khan    M.Qasim Jan  WT  ”BX 《地学前缘》2000,(Z1)

VARIATIONS IN KAMILA AMPHIBOLITES FROM SOUTHEASTERN PART OF THE KOHISTAN ISLAND-ARC TERRANE,PAKISTAN  相似文献   

The Dichotomy of the Mass-radius Relation and the Number of Globular Clusters     

Abdullah  Ahmed H.  Kroupa  Pavel 《Astronomy Letters》2021,47(3):170-174

Astronomy Letters - Globular clusters $$(GCs)$$ are among the oldest stellar systems in the early universe. We present galaxies formed with two different stellar populations, which depend on the...  相似文献   

Modeling Ground Water Flow and Radioactive Transport in a Fractured Aquifer     

Greg Pohll  Ahmed E. Hassan  Jenny B. Chapman  Charalambos Papelis  Roko Andricevic 《Ground water》1999,37(5):770-784

  相似文献   

Origin of spheroidal chert nodules, Drunka Formation (Lower Eocene), Egypt     

McBride  Antar Abdel-Wahab  & Ahmed Reda M. El-Younsy 《Sedimentology》1999,46(4):733-755

Chert nodules of the Drunka Formation (Lower Eocene) are mostly spherical, have diameters from 40 to 120 cm, are quasi-uniformly spaced 2–3 m apart in the plane of bedding, have concentric internal structure and, except for rare small (<6 cm) solid chert nodules, are less than 85% chertified. Nodules formed after moderate alteration of limestone by meteoric water (δ¹⁸O_calcite = –4 to –8‰) at shallow (<100 m) burial depths; more extensive alteration of limestone (δ¹⁸O = –10 to –16‰) by meteoric water followed nodule growth. Chertification was by low-temperature meteoric water (δ¹⁸O_quartz = +18‰ in margins to +22‰ in nuclei) at shallow burial depths. Meteoric water may have invaded the Drunka Formation in association with shelf progradation during the Early Eocene, or during the development of a Middle Eocene unconformity. Replacement of carbonate mud by microcrystalline quartz was the dominant chertification process, but fossils were replaced in part by fine-grained equant megaquartz, quartzine and chalcedony; the last of these occurs in places as beekite. Opal A-secreting marine organisms are the inferred source of silica, but none are preserved. There is no compelling evidence of an opal-CT precursor, so quartz may have formed by direct precipitation. Self-organization processes of enigmatic character established the spacing pattern of the nodules and also the Liesegang-banded internal structure of the chert nodules. Nodules grew chiefly by diffusive supply of silica, although one locality has elongate nodules that grew when there was some porewater advection. Chertification patterns and δ¹⁸O values of both calcite and quartz indicate that nodule growth was complex and variable. Some nodules probably grew from the centre outwards. Many nodules, however, initially grew simultaneously across the entire nodule, but late-stage growth was predominantly at the outer margins or at selective internal sites.  相似文献   

Simulation of free surface flow around a VLCC hull using viscous and potential flow methods     

Y. Ahmed  C. Guedes Soares 《Ocean Engineering》2009,36(9-10):691-696

Numerical simulations have been carried out to determine the incompressible free surface flow around a VLCC hull form for which experimental results are available. A commercial viscous flow finite volume code using the two-phase Eulerian–Eulerian fluid approach and a potential flow code based on the Rankine source method have been used in this study. The simulation conditions are the ones for which experimental results exist. The shear stress transport (SST) turbulence model has been used in the viscous flow code. A tetrahedral unstructured grid was used with the viscous flow code for meshing the computational domain, while quadrilateral structural patches were used with the potential flow code for meshing the VLCC hull surface and the water surface around it. The results compare well with the available experimental data and they allow an understanding of the differences that can be expected from viscous and potential flow methods as a result of their different mathematical formulations, which make their complementary application useful for determining the total ship resistance.  相似文献   

Managing excess capacity in small-scale fisheries: Perspectives from stakeholders in three Southeast Asian countries   总被引：3，自引：1，他引：2  

Nerissa Salayo  Len Garces  Michael Pido  Kuperan Viswanathan  Robert Pomeroy  Mahfuzuddin Ahmed  Ida Siason  Keang Seng  Awae Masae 《Marine Policy》2008,32(4):692-700

The management of fishing capacity—in both inland and marine fisheries—is a major policy concern in most countries in Southeast Asia. Excess capacity leads to a number of negative impacts, such as resource use conflicts, overfishing, environmental degradation, economic wastage, and security threats. This paper presents the results of a regional study that examined various approaches to managing excess fishing capacity in small-scale fisheries in Southeast Asia. More specifically, the paper presents an analysis of perceptions of stakeholders in Cambodia, Philippines and Thailand regarding preferred solutions to addressing excess capacity. The paper concludes with a discussion of policy guidance for addressing excess fishing capacity based on the stakeholder-preferred solutions.  相似文献   

Chemical and oxygen isotopic properties of ordinary chondrites (H5, L6) from Oman: Signs of isotopic equilibrium during thermal metamorphism          下载免费PDF全文

Arshad Ali  Sobhi J. Nasir  Iffat Jabeen  Ahmed Al Rawas  Neil R. Banerjee  Gordon R. Osinski 《Meteoritics & planetary science》2017,52(10):2097-2112

Mean bulk chemical data of recently found H5 and L6 ordinary chondrites from the deserts of Oman generally reflect isochemical features which are consistent with the progressive thermal metamorphism of a common, unequilibrated starting material. Relative differences in abundances range from 0.5–10% in REE (Eu = 14%), 6–13% in siderophile elements (Co = 48%), and >10% in lithophile elements (exceptions are Ba, Sr, Zr, Hf, U = >30%) between H5 and L6 groups. These differences may have accounted for variable temperature conditions during metamorphism on their parent bodies. The CI/Mg‐normalized mean abundances of refractory lithophile elements (Al, Ca, Sm, Yb, Lu, V) show no resolvable differences between H5 and L6 suggesting that both groups have experienced the same fractionation. The REE diagram shows subtle enrichment in LREE with a flat HREE pattern. Furthermore, overall mean REE abundances are ~0.6 × CI with enriched La abundance (~0.9 × CI) in both groups. Precise oxygen isotope compositions demonstrate the attainment of isotopic equilibrium by progressive thermal metamorphism following a mass‐dependent isotope fractionation trend. Both groups show a ~slope‐1/2 line on a three‐isotope plot with subtle negative deviation in ?¹⁷O associated with δ¹⁸O enrichment relative to δ¹⁷O. These deviations are interpreted as the result of liberation of water from phyllosilicates and evaporation of a fraction of the water during thermal metamorphism. The resultant isotope fractionations caused by the water loss are analogous to those occurring between silicate melt and gas phase during CAI and chondrule formation in chondrites and are controlled by cooling rates and exchange efficiency.  相似文献