377 research outputs found
Generalized optical theorems for the reconstruction of Green's function of an inhomogeneous elastic medium
This paper investigates the reconstruction of elastic Green's function from
the cross-correlation of waves excited by random noise in the context of
scattering theory. Using a general operator equation, -the resolvent formula-,
Green's function reconstruction is established when the noise sources satisfy
an equipartition condition. In an inhomogeneous medium, the operator formalism
leads to generalized forms of optical theorem involving the off-shell
-matrix of elastic waves, which describes scattering in the near-field. The
role of temporal absorption in the formulation of the theorem is discussed.
Previously established symmetry and reciprocity relations involving the
on-shell -matrix are recovered in the usual far-field and infinitesimal
absorption limits. The theory is applied to a point scattering model for
elastic waves. The -matrix of the point scatterer incorporating all
recurrent scattering loops is obtained by a regularization procedure. The
physical significance of the point scatterer is discussed. In particular this
model satisfies the off-shell version of the generalized optical theorem. The
link between equipartition and Green's function reconstruction in a scattering
medium is discussed
A Semi-classical calculus of correlations
The method of passive imaging in seismology has been developped recently in
order to image the earth crust from recordings of the seismic noise. This
method is founded on the computation of correlations of the seismic noise. In
this paper, we give an explicit formula for this correlation in the
"semi-classical" regime. In order to do that, we define the power spectrum of a
random field as the ensemble average of its Wigner measure, this allows
phase-space computations: the pseudo-differential calculus and the ray theory.
This way, we get a formula for the correlation of the seismic noise in the
semi-classcial regime with a source noise which can be localized and non
homogeneous. After that, we show how the use of surface guided waves allows to
image the earth crust.Comment: To appear in a special issue "Imaging and Monitoring with Seismic
Noise" of the series "Comptes Rendus G\'eosciences", from the French
"Acad\'emie des sciences
Clinical Experience Using a Real Time Autofluorescence Endoscopy System in the Gastrointestinal Tract
Autofluorescence spectra of neoplastic tissues have been reported to be significantly different
from those of normal tissues when excited by blue or violet light. From this concept, a light-induced
autofluorescence endoscopic imaging system for gastrointestinal mucosa (LIFE-GI;
Xillix, Canada and Olympus, Japan) has been newly developed and the clinical evaluation of
the prototype system has been conducted in hospitals in Canada, Netherlands and Japan
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