Effects of fractures on seismic-wave velocity and attenuation   总被引：1，自引：0，他引：1  

Fred Kofi Boadu  Leland Timothy Long 《Geophysical Journal International》1996,127(1):86-110

The effects of fractures on the seismic velocity and attenuation of a rock are investigated using theoretical results and experimental data. Fractures in a rock mass influence the traveltimes and amplitudes of seismic waves that have propagated through them. The displacement discontinuity model, recently employed in fracture investigations, is modified to describe the effect of fractures on seismic-wave velocity and attenuation. This new model, the modified displacement discontinuity model (MDD), is formulated in a way analogous to transmission-line analysis. The fractures are treated as transmission lines for the passage of seismic waves. The MDD takes into consideration realistic fracture parameters which include the fracture length, the fractional area of a fracture surface in contact, and the nature of the infilling material. A single fracture of varying geometric and material properties is shown to affect dramatically the transmission properties of a propagating waveform, and hence the seismic velocity and attenuation. These effects have been shown to result in a frequency-dependent velocity and attenuation. The sensitivity of the fracture parameters to seismic-wave velocity and attenuation was investigated and interesting results were obtained. Fracture parameters used in designing experimental models consisting of synthetically manufactured cracks were fed into the MDD and a well-known crack model, Hudson's model, for comparison. Velocities as a function of the incident-wave angle were obtained from both numerical models and were compared with the results from the experimental modelling. For P waves, the MDD model results show better agreement with those of the experimental model for all crack densities investigated than those from Hudson's model.  相似文献   

Optimal Basis from Empirical Orthogonal Functions and Wavelet Analysis for Data Assimilation: Optimal Basis WADAi     

Takuji Waseda  Leland Jameson  Humio Mitsudera  Max Yaremchuk 《Journal of Oceanography》2003,59(2):187-200

Wavelet Analysis provides a new orthogonal basis set which is localized in both physical space and Fourier transform space. Empirical Orthogonal Functions (EOFs), on the other hand, provide a global representation of data sets. Here we investigate the various ways in which one can combine these basis sets for optimal representation of data. EOFs represent the global large scale information and wavelet analysis are used to supplement this large scale information with local fine scale information. Here we begin to explore the application of these two basis sets for outputs from an Ocean General Circulation Model and we explore various applications, including data assimilation. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Axisymmetric Modeling Using MODFLOW-USG     

Vivek Bedekar  Leland Scantlebury  Sorab Panday 《Ground water》2019,57(5):772-777

Axisymmetric groundwater models are used for simulating radially symmetric conditions. Groundwater simulators built specifically to model axisymmetric conditions are most commonly used for simulating aquifer tests. Although some numerical models capable of simulating flow and solute transport that are developed in the cartesian coordinate system framework offer flexibility to simulate axisymmetric conditions, most of the numerical groundwater models, such as the MODFLOW family of codes, are based on structured grids in which axisymmetric flows cannot be directly simulated. Researchers in the past have provided methods to manipulate aquifer properties to mimic axisymmetric conditions. This study presents a methodology that takes advantage of the unstructured grids of MODFLOW-USG to simulate axisymmetric models within the MODFLOW framework. To develop axisymmetric models, the intercell interface area arrays of MODFLOW-USG were calculated to accurately represent coaxial cylindrical model cells. Three examples are presented to demonstrate the application of MODFLOW-USG for axisymmetric modeling: a pumping well with delayed yield effects, a vadose zone flow model simulating an infiltration basin, and a density-dependent saltwater intrusion problem for a circular island. Results were verified against analytical solutions and published numerical codes.  相似文献   

New Praesepe white dwarfs and the initial mass–final mass relation     

P. D. Dobbie  R. Napiwotzki  M. R. Burleigh  M. A. Barstow  D. D. Boyce  S. L. Casewell  R. F. Jameson  I. Hubeny  G. Fontaine 《Monthly notices of the Royal Astronomical Society》2006,369(1):383-389

We report the spectroscopic confirmation of four further white dwarf members of Praesepe. This brings the total number of confirmed white dwarf members to 11, making this the second largest collection of these objects in an open cluster identified to date. This number is consistent with the high-mass end of the initial mass function of Praesepe being Salpeter in form. Furthermore, it suggests that the bulk of Praesepe white dwarfs did not gain a substantial recoil kick velocity from possible asymmetries in their loss of mass during the asymptotic giant branch phase of evolution. By comparing our estimates of the effective temperatures and the surface gravities of WD0833+194, WD0840+190, WD0840+205 and WD0843+184 to modern theoretical evolutionary tracks, we have derived their masses to be in the range  0.72–0.76 M_⊙  and their cooling ages ∼300 Myr. For an assumed cluster age of 625 ± 50 Myr, the inferred progenitor masses are between 3.3 and  3.5 M_⊙  . Examining these new data in the context of the initial mass–final mass relation, we find that it can be adequately represented by a linear function  ( a ₀= 0.289 ± 0.051,  a ₁= 0.133 ± 0.015)  over the initial mass range 2.7–6  M_⊙  . Assuming an extrapolation of this relation to larger initial masses is valid and adopting a maximum white dwarf mass of  1.3 M_⊙  , our results support a minimum mass for core-collapse supernovae progenitors in the range  ∼6.8–8.6 M_⊙  .  相似文献   

Brown dwarfs in the Pleiades and the initial mass function across the stellar/substellar boundary     

N. C. Hambly  S. T. Hodgkin  M. R. Cossburn  R. F. Jameson 《Monthly notices of the Royal Astronomical Society》1999,303(4):835-844

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Dust-obscured star formation and AGN fuelling in hierarchical models of galaxy evolution     

A. W. Blain  Allon Jameson  Ian Smail  M. S. Longair  J.-P. Kneib  R. J. Ivison 《Monthly notices of the Royal Astronomical Society》1999,309(3):715-730

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Soild and quaternary geology of the Southwestern United States : Edited by D.L. Weide. Special Paper 203, The Geological Society of America, Boulder, Colorado, 1985, 150 pp., $20.00     

Leland H. Gile 《Quaternary Research》1987,27(3)

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Time-dependent chemistry in expanding circumstellar envelopes     

P. V. Jameson  D. A. Williams 《Astrophysics and Space Science》1981,75(2):341-351

The rate equations of a restricted set of gas-phase chemical reactions occuring in an expanding circumstellar envelope are integrated numerically on the assumption that no chemical evolution has occurred in the stellar atmosphere. Abundances of all species are found to peak at a time on the order ofr ₀/u ₀, wherer ₀ is the initial radius andu ₀ the expansion velocity. After this time geometrical dilution dominates. For an initial density of 10⁸ cm^–3, on the order of 1% of hydrogen is converted to H₂, and CH and CO have comparable densities of 10^–6 relative to H, OH and O₂ remain very low in abundance. For higher initial densities, H and H₂ are more nearly comparable, and nearly all carbon is in CO, CH, OH and O₂ remain low in abundance. The relevance of these results to M giants and other objects is discussed.  相似文献   

Proper motions of field L and T dwarfs     

R. F. Jameson  S. L. Casewell  N. P. Bannister  N. Lodieu  K. Keresztes  P. D. Dobbie  S. T. Hodgkin 《Monthly notices of the Royal Astronomical Society》2008,384(4):1399-1413

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ULMSF conference summary     

R. F. Jameson 《Astronomische Nachrichten》2005,326(10):874-877

Ten years on from the discovery of the first brown dwarf and the first exoplanet, how well have we progressed in our understanding of these low‐mass objects? In particular how well do we understand their formation? The strong impression from this conference was that the formation of brown dwarfs was just a continuation of the star formation process, no special additional mechanism is indicated. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献