5,527 research outputs found
Study of noise effects in electrical impedance tomography with resistor networks
We present a study of the numerical solution of the two dimensional
electrical impedance tomography problem, with noisy measurements of the
Dirichlet to Neumann map. The inversion uses parametrizations of the
conductivity on optimal grids. The grids are optimal in the sense that finite
volume discretizations on them give spectrally accurate approximations of the
Dirichlet to Neumann map. The approximations are Dirichlet to Neumann maps of
special resistor networks, that are uniquely recoverable from the measurements.
Inversion on optimal grids has been proposed and analyzed recently, but the
study of noise effects on the inversion has not been carried out. In this paper
we present a numerical study of both the linearized and the nonlinear inverse
problem. We take three different parametrizations of the unknown conductivity,
with the same number of degrees of freedom. We obtain that the parametrization
induced by the inversion on optimal grids is the most efficient of the three,
because it gives the smallest standard deviation of the maximum a posteriori
estimates of the conductivity, uniformly in the domain. For the nonlinear
problem we compute the mean and variance of the maximum a posteriori estimates
of the conductivity, on optimal grids. For small noise, we obtain that the
estimates are unbiased and their variance is very close to the optimal one,
given by the Cramer-Rao bound. For larger noise we use regularization and
quantify the trade-off between reducing the variance and introducing bias in
the solution. Both the full and partial measurement setups are considered.Comment: submitted to Inverse Problems and Imagin
Nanowire Growth for Sensor Arrays
A design concept for nanowire-based sensors and arrays is described. The
fabrication technique involves electrodeposition to directly grow nanowires
between patterned thin film contact electrodes. To prove our concept, we have
electrodeposited 1-mm diameter Pd single wires and small arrays. To demonstrate
nanowire sensors, we have electrochemically grown metal (Pd, Au, Pt), metal
oxide (Sb2O3), and conducting polymer (polyaniline) bundled nanowires. Using Pt
bundled nanowires surface modified with glucose oxidase, we have demonstrated
glucose detection as a demonstration of a biomolecular sensor.Comment: To appear in Nanofabrication Technologies, Ed. E. A. Dobisz, SPIE
Proceedings 5220, pp. xxx (2003, in press
Dissipative Dynamics of Collisionless Nonlinear Alfven Wave Trains
The nonlinear dynamics of collisionless Alfven trains, including resonant
particle effects is studied using the kinetic nonlinear Schroedinger (KNLS)
equation model. Numerical solutions of the KNLS reveal the dynamics of Alfven
waves to be sensitive to the sense of polarization as well as the angle of
propagation with respect to the ambient magnetic field. The combined effects of
both wave nonlinearity and Landau damping result in the evolutionary formation
of stationaryOA S- and arc-polarized directional and rotational
discontinuities. These waveforms are freqently observed in the interplanetary
plasma.Comment: REVTeX, 6 pages (including 5 figures). This and other papers may be
found at http://sdphpd.ucsd.edu/~medvedev/papers.htm
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