22,883 research outputs found
On Time-Reversal Imaging by Statistical Testing
This letter is focused on the design and analysis of computational wideband
time-reversal imaging algorithms, designed to be adaptive with respect to the
noise levels pertaining to the frequencies being employed for scene probing.
These algorithms are based on the concept of cell-by-cell processing and are
obtained as theoretically-founded decision statistics for testing the
hypothesis of single-scatterer presence (absence) at a specific location. These
statistics are also validated in comparison with the maximal invariant
statistic for the proposed problem.Comment: Reduced form accepted in IEEE Signal Processing Letter
Role of scattering in virtual source array imaging
We consider imaging in a scattering medium where the illumination goes
through this medium but there is also an auxiliary, passive receiver array that
is near the object to be imaged. Instead of imaging with the source-receiver
array on the far side of the object we image with the data of the passive array
on the near side of the object. The imaging is done with travel time migration
using the cross correlations of the passive array data. We showed in [J.
Garnier and G. Papanicolaou, Inverse Problems {28} (2012), 075002] that if (i)
the source array is infinite, (ii) the scattering medium is modeled by either
an isotropic random medium in the paraxial regime or a randomly layered medium,
and (iii) the medium between the auxiliary array and the object to be imaged is
homogeneous, then imaging with cross correlations completely eliminates the
effects of the random medium. It is as if we imaged with an active array,
instead of a passive one, near the object. The purpose of this paper is to
analyze the resolution of the image when both the source array and the passive
receiver array are finite. We show with a detailed analysis that for isotropic
random media in the paraxial regime, imaging not only is not affected by the
inhomogeneities but the resolution can in fact be enhanced. This is because the
random medium can increase the diversity of the illumination. We also show
analytically that this will not happen in a randomly layered medium, and there
may be some loss of resolution in this case.Comment: 22 pages, 4 figure
Illumination strategies for intensity-only imaging
We propose a new strategy for narrow band, active array imaging of localized
scat- terers when only the intensities are recorded and measured at the array.
We consider a homogeneous medium so that wave propagation is fully coherent. We
show that imaging with intensity-only measurements can be carried out using the
time reversal operator of the imaging system, which can be obtained from
intensity measurements using an appropriate illumination strategy and the
polarization identity. Once the time reversal operator has been obtained, we
show that the images can be formed using its singular value decomposition
(SVD). We use two SVD-based methods to image the scatterers. The proposed
approach is simple and efficient. It does not need prior information about the
sought image, and guarantees exact recovery in the noise-free case.
Furthermore, it is robust with respect to additive noise. Detailed numerical
simulations illustrate the performance of the proposed imaging strategy when
only the intensities are captured
Coherent backscattering of ultrasound without a source
Coherent backscattering is due to constructive interferences of reciprocal
paths and leads to an enhancement of the intensity of a multiply scattered
field near its source. To observe this enhancement an array of receivers is
conventionally placed close to the source. Our approach here is different. In a
first experiment, we recover the coherent backscattering effect (CBE) within an
array of sources and a distant receiver using time correlation of diffuse
fields. The enhancement cone has an excellent spatial resolution. The dynamics
of the enhancement factor is studied in a second experiment using correlation
of thermal phonons at the same ultrasonic frequencies, without any active
source
Locating and extracting acoustic and neural signals
This dissertation presents innovate methodologies for locating, extracting, and separating multiple incoherent sound sources in three-dimensional (3D) space; and applications of the time reversal (TR) algorithm to pinpoint the hyper active neural activities inside the brain auditory structure that are correlated to the tinnitus pathology. Specifically, an acoustic modeling based method is developed for locating arbitrary and incoherent sound sources in 3D space in real time by using a minimal number of microphones, and the Point Source Separation (PSS) method is developed for extracting target signals from directly measured mixed signals. Combining these two approaches leads to a novel technology known as Blind Sources Localization and Separation (BSLS) that enables one to locate multiple incoherent sound signals in 3D space and separate original individual sources simultaneously, based on the directly measured mixed signals. These technologies have been validated through numerical simulations and experiments conducted in various non-ideal environments where there are non-negligible, unspecified sound reflections and reverberation as well as interferences from random background noise. Another innovation presented in this dissertation is concerned with applications of the TR algorithm to pinpoint the exact locations of hyper-active neurons in the brain auditory structure that are directly correlated to the tinnitus perception. Benchmark tests conducted on normal rats have confirmed the localization results provided by the TR algorithm. Results demonstrate that the spatial resolution of this source localization can be as high as the micrometer level. This high precision localization may lead to a paradigm shift in tinnitus diagnosis, which may in turn produce a more cost-effective treatment for tinnitus than any of the existing ones
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