38 research outputs found
Multiple Multipole Expansions For Elastic Scattering: An Aid To Understanding The Problems In "No-Record" Areas
This paper presents a new approach to solving scattering of elastic waves in two dimensions. Wavefields are often expanded into an orthogonal set of basis functions.
Unfortunately, these expansions converge rather slowly for complex geometries. The
new approach enhances convergence by summing multiple expansions with different
centers of expansion. This allows irregularities of the boundary to be resolved locally
from a nearby center of expansion. Mathematically, the wavefields are expanded into
a set of non-orthogonal basis functions. The incident wavefield and the fields induced
by the scatterers are matched by evaluating the boundary conditions at discrete matching
points along the domain boundaries. Due to the non-orthogonal expansions, more
matching points are used than actually needed, resulting in an overdetermined system
which is solved in the least squares sense.
Since there are free parameters such as the location and number of expansion centers
as well as the kind and orders of expansion functions used, numerical experiments
are performed to measure the performance of different discretizations. An empirical
set of rules governing the choice of these parameters is found from these experiments.
The resulting algorithm is a general tool to solve relatively large and complex two-dimensional scattering problems.United States. Air Force Office of Scientific Research (Contract F49620-93-1-0424DEF
Scattering Of Acoustic And Elastic Waves Using A Hybrid Multiple Multipole Expansions-Finite Element Technique
In this paper, two different methods to solve scattering problems in acoustic or elastic
media are coupled to enhance their usefulness. The multiple multipole (MMP) expansions
are used to solve for the scattered fields in homogeneous regions which are possibly
unbounded. The finite element (FE) method is used to calculate the scattered fields
in heterogeneous but bounded scatterers. As the MMP method requires, the different
regions and methods are coupled together in the least squares sense. For some examples,
the scattered fields are calculated and compared to the analytical solutions. Finally, the seismograms are calculated for a scattering problem with several scatterers, and complex geometries. Thus, the hybrid MMP-FEM technique is a very general and useful
tool to solve complex, two-dimensional scattering problems.United States. Air Force Office of Scientific Research (Contract F49620-94-1-0282)Massachusetts Institute of Technology. Earth Resources Laboratory. Reservoir Delineation
Consortiu
Scale And Frequency Dependence Of Reflection And Transmission Coefficients
Well-logs show that heterogeneities occur at many different spatial scales. In this paper, we want to characterize how waves are affected by these heterogeneities, and we study how reflection and transmission coefficients depend on temporal frequency and spatial scale. We use wavelet transformations to filter certain spatial scales from the velocity logs. The scale-filtered logs serve as input for a numerical layer-stack model to calculate reflection and transmission coefficients as functions of frequency and scale. We find that transmission coefficients are largely independent of frequency or scale. They depend mostly on average slowness. Contrarily, reflection coefficients are extremely sensitive to the perturbations of the slownesses, even at low frequencies.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumMassachusetts Institute of Technology. Earth Resources Laboratory. Reservoir Delineation
Consortiu
Near-Surface Scattering From High Velocity Carbonates In West Texas
Seismic data acquired directly over near-surface limestone formations are commonly
observed to be of inferior quality. A possible cause for this degradation is scattering
in the near-subsurface by, e.g., the weathering layer, rough free-surface topography,
or heterogeneities such as cavities or clusters of vugs. We applied different numerical
scattering schemes to study the effects of each of these three scattering mechanisms.
For a particular dataset acquired in West Texas, we find that a weathering layer is
the dominant cause of noise on records acquired in valleys. However on mesas, nearsubsurface heterogeneity is the primary cause of scattered wave-energy. Topography
turned out to be of only secondary importance.
As additional attributes, we use energy-density and energy-flux vectors to study
the frequency dependence of the different scattering models. These attributes allow
us to study where energy concentrates and in which direction it flows. For example,
we observed that near sub-surface heterogeneities build up waveguides which efficiently
trap seismic energy near the surface.Massachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu
Blockade but not overexpression of the junctional adhesion molecule C influences virus-induced type 1 diabetes in mice
Type 1 diabetes (T1D) results from the autoimmune destruction of insulin-producing beta-cells in the pancreas. Recruitment of inflammatory cells is prerequisite to beta-cell-injury. The junctional adhesion molecule (JAM) family proteins JAM-B and JAM–C are involved in polarized leukocyte transendothelial migration and are expressed by vascular endothelial cells of peripheral tissue and high endothelial venules in lympoid organs. Blocking of JAM-C efficiently attenuated cerulean-induced pancreatitis, rheumatoid arthritis or inflammation induced by ischemia and reperfusion in mice. In order to investigate the influence of JAM-C on trafficking and transmigration of antigen-specific, autoaggressive T-cells, we used transgenic mice that express a protein of the lymphocytic choriomeningitis virus (LCMV) as a target autoantigen in the β-cells of the islets of Langerhans under the rat insulin promoter (RIP). Such RIP-LCMV mice turn diabetic after infection with LCMV. We found that upon LCMV-infection JAM-C protein was upregulated around the islets in RIP-LCMV mice. JAM-C expression correlated with islet infiltration and functional beta-cell impairment. Blockade with a neutralizing anti-JAM-C antibody reduced the T1D incidence. However, JAM-C overexpression on endothelial cells did not accelerate diabetes in the RIP-LCMV model. In summary, our data suggest that JAM-C might be involved in the final steps of trafficking and transmigration of antigen-specific autoaggressive T-cells to the islets of Langerhans
Simulation study of Non-ergodicity Transitions: Gelation in Colloidal Systems with Short Range Attractions
Computer simulations were used to study the gel transition occurring in
colloidal systems with short range attractions. A colloid-polymer mixture was
modelled and the results were compared with mode coupling theory expectations
and with the results for other systems (hard spheres and Lennard Jones). The
self-intermediate scattering function and the mean squared displacement were
used as the main dynamical quantities. Two different colloid packing fractions
have been studied. For the lower packing fraction, -scaling holds and
the wave-vector analysis of the correlation function shows that gelation is a
regular non-ergodicity transition within MCT. The leading mechanism for this
novel non-ergodicity transition is identified as bond formation caused by the
short range attraction. The time scale and diffusion coefficient also show
qualitatively the expected behaviour, although different exponents are found
for the power-law divergences of these two quantities. The non-Gaussian
parameter was also studied and very large correction to Gaussian behaviour
found. The system with higher colloid packing fraction shows indications of a
nearby high-order singularity, causing -scaling to fail, but the
general expectations for non-ergodicity transitions still hold.Comment: 13 pages, 15 figure
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Seismic Determination of Reservoir Heterogeneity: Application to the Characterization of Heavy Oil Reservoirs
The objective of the project was to examine how seismic and geologic data could be used to improve characterization of small-scale heterogeneity and their parameterization in reservoir models. The study was performed at West Coalinga Field in California
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Seismic Determination of Reservoir Heterogeneity: Application to the Characterization of Heavy Oil Reservoirs
The objective of the project was to examine how seismic and geologic data can be used to improve characterization of small-scale heterogeneity and their parameterization in reservoir models. The study focused on West Coalinga Field in California. The project initially attempted to build reservoir models based on different geologic and geophysical data independently using different tools, then to compare the results, and ultimately to integrate them all. We learned, however, that this strategy was impractical. The different data and tools need to be integrated from the beginning because they are all interrelated. This report describes a new approach to geostatistical modeling and presents an integration of geology and geophysics to explain the formation of the complex Coalinga reservoir
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