32 research outputs found
Multiple scattering of ultrasound in weakly inhomogeneous media: application to human soft tissues
Waves scattered by a weakly inhomogeneous random medium contain a predominant
single scattering contribution as well as a multiple scattering contribution
which is usually neglected, especially for imaging purposes. A method based on
random matrix theory is proposed to separate the single and multiple scattering
contributions. The experimental set up uses an array of sources/receivers
placed in front of the medium. The impulse responses between every couple of
transducers are measured and form a matrix. Single-scattering contributions are
shown to exhibit a deterministic coherence along the antidiagonals of the array
response matrix, whatever the distribution of inhomogeneities. This property is
taken advantage of to discriminate single from multiple-scattered waves. This
allows one to evaluate the absorption losses and the scattering losses
separately, by comparing the multiple scattering intensity with a radiative
transfer model. Moreover, the relative contribution of multiple scattering in
the backscattered wave can be estimated, which serves as a validity test for
the Born approximation. Experimental results are presented with ultrasonic
waves in the MHz range, on a synthetic sample (agar-gelatine gel) as well as on
breast tissues. Interestingly, the multiple scattering contribution is found to
be far from negligible in the breast around 4.3 MHz.Comment: 35 pages, 11 figures, final version, contains the appendix of the
original articl
Radiative transfer of acoustic waves in continuous complex media: Beyond the Helmholtz equation
Heterogeneity can be accounted for by a random potential in the wave
equation. For acoustic waves in a fluid with fluctuations of both density and
compressibility (as well as for electromagnetic waves in a medium with
fluctuation of both permittivity and permeability) the random potential entails
a scalar and an operator contribution. For simplicity, the latter is usually
overlooked in multiple scattering theory: whatever the type of waves, this
simplification amounts to considering the Helmholtz equation with a sound speed
depending on position . In this work, a radiative transfer
equation is derived from the wave equation, in order to study energy transport
through a multiple scattering medium. In particular, the influence of the
operator term on various transport parameters is studied, based on the
diagrammatic approach of multiple scattering. Analytical results are obtained
for fundamental quantities of transport theory such as the transport mean-free
path , scattering phase function and anisotropy factor .
Discarding the operator term in the wave equation is shown to have a
significant impact on and , yet limited to the low-frequency regime
i.e., when the correlation length of the disorder is smaller than or
comparable to the wavelength . More surprisingly, discarding the
operator part has a significant impact on the transport mean-free path
whatever the frequency regime. When the scalar and operator terms have
identical amplitudes, the discrepancy on the transport mean-free path is around
in the low-frequency regime, and still above for
no matter how weak fluctuations of the disorder are.
Analytical results are supported by numerical simulations of the wave equation
and Monte Carlo simulations
Fluctuations of correlations and Green's function reconstruction: role of scattering
Correlations of ambient seismic or acoustic vibrations are now widely used to
reconstruct the impulse response between two passive receivers as if a source
was placed at one of them. This provides the opportunity to do imaging without
a source, or \textsl{passive imaging}. Applications include terrestrial and
solar seismology, underwater acoustics, and structural health monitoring, to
cite only a few. Nevertheless, for a given set of data, correlations do not
only yield the Green's function between the sensors. They also contain residual
fluctuations that result from an imperfect time or source averaging that might
eventually blur the images. In this article, we propose a heuristic model to
describe the level of fluctuations of the correlations in the case of
non-stationary wavefields, and more particularly in the case of scattering
media. The work includes theoretical derivations and numerical simulations. The
role of multiple scattering is quantitatively evaluated. The level of
fluctuations decreases when the duration and intensity of the diffuse waves
increase. The role of absorption is also discussed: absorption is properly
retrieved by correlation, but the level of fluctuations is greater, thus
degrading the Green's function reconstruction. Discrepancies of our simple
model in the case of strong multiple scattering () are
discussed
Recurrent scattering and memory effect at the Anderson localization transition
We report on ultrasonic measurements of the propagation operator in a
strongly scattering mesoglass. The backscattered field is shown to display a
deterministic spatial coherence due to a remarkably large memory effect induced
by long recurrent trajectories. Investigation of the recurrent scattering
contribution directly yields the probability for a wave to come back close to
its starting spot. The decay of this quantity with time is shown to change
dramatically near the Anderson localization transition. The singular value
decomposition of the propagation operator reveals the dominance of very intense
recurrent scattering paths near the mobility edge.Comment: 5 pages, 4 figure
Detection and imaging in a random medium: A matrix method to overcome multiple scattering and aberration
We present an imaging technique particularly suited to the detection of a
target embedded in a strongly scattering medium. Classical imaging techniques
based on the Born approximation fail in this kind of configuration because of
multiply scattered echoes and aberration distortions. The experimental set up
we consider uses an array of programmable transmitters/receivers. A target is
placed behind a scattering medium. The impulse responses between all array
elements are measured and form a matrix. The core of the method is to separate
the single-scattered echo of the target from the multiple scattering
background. This is possible because of a deterministic coherence along the
antidiagonals of the array response matrix, which is typical of single
scattering. Once this operation is performed, target detection is achieved by
applying the DORT method (French acronym for decomposition of the time reversal
operator). Experimental results are presented in the case of wide-band
ultrasonic waves around 3 MHz. A 125-element array is placed in front of a
collection of randomly distributed steel rods (diameter 0.8mm). The slab
thickness is three times the scattering mean free path. The target is a larger
steel cylinder (diameter 15 mm) that we try to detect and localize. The quality
of detection is assessed theoretically based on random matrix theory and is
shown to be significantly better than what is obtained with classical imaging
methods. Aside from multiple scattering, the technique is also shown to reduce
the aberrations induced by an heterogeneous layer.Comment: 48 pages, 18 figures, corrected typos, v