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Data Inversion (data + inversion)

Distribution by Scientific Domains

Earth and Environmental Science	80%

Selected Abstracts

3124: Algorithms and instrumentation for quantified retinal oximetry in a snapshot

ACTA OPHTHALMOLOGICA, Issue 2010
AR HARVEY
Purpose To develop an instrument and techniques for useful absolute and relative clinical oximetry of the retina. Methods A novel snapshot multispectral imaging system has been optimised for retinal oximetry. Eight monochromatic images of the retina are recorded with a field of view of 24 degrees. Algorithms calculate the absorption of light at each point along delineated blood vessels and for each of the eight wavebands. Data inversion using an analytical model for light propagation enables oximetry at along the blood vessels. Spectral inversion has been refined using ray tracing and Monte Carlo modelling of light propagation using a realistic phantom eye. Results A comparison of Monte Carlo modelling of light propagation in the phantom retina with recorded images for blood of various oxygenations indicates the influence of several unknowns, including scatter from optical surfaces within the ophthalmoscope and eye, the geometry of the blood vessels and eye, the optical constants of the ocular media and the complexity of light propagation in the retinal structure. Relative oximetry within the retina is possible with repeatability of about 1% in the phantom and 5% in a real eye but the influence of various systematic effects can introduce systematic differences between actual and calculated oxygenation that can significantly exceed these values. Conclusion Although oximetry using only a very small number of spectral bands is possible, this is prone to systematic errors that can effect both absolute and relative oximetry. The ability to record eight spectral images in a single snapshot offers promise to provide an enhanced clinically useful and validated oximetry technique. [source]

Physics-based GPS data inversion to estimate three-dimensional elastic and inelastic strain fields

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2010
Akemi Noda
SUMMARY The Earth's crust is macroscopically treated as a linear elastic body, but it includes a number of defects. The occurrence of inelastic deformation such as brittle fracture at the defects brings about elastic deformation in the surrounding regions. The crustal deformation observed through geodetic measurements is the sum of the inelastic deformation as source and the elastic deformation as effect. On such a basic idea, we created a theory of physics-based strain analysis with general source representation by moment tensor, and developed an inversion method to separately estimate 3-D elastic and inelastic strain fields from GPS data. In this method, first, the optimum distribution of moment density tensor is determined from observed GPS data by using Akaike's information criterion. Then, the elastic and inelastic strain fields are obtained from the optimum moment tensor distribution by theoretical computation and direct conversion with elastic compliance tensor, respectively. We applied the inversion method to GPS horizontal velocity data, and succeeded in separately estimating 3-D elastic and inelastic strain rate fields in the Niigata,Kobe transformation zone, central Japan. As for the surface patterns of total strain, the present results of 3-D physics-based inversion analysis accord with the previous results of 2-D geometric inversion analysis. From the 3-D patterns of the inverted elastic and inelastic strain fields, we revealed that the remarkable horizontal contraction in the Niigata,Kobe transformation zone is elastic and restricted near the surface, but the remarkable shear deformation is inelastic and extends over the upper crust. [source]

Two-dimensional inversion of magnetotelluric data with consecutive use of conjugate gradient and least-squares solution with singular value decomposition algorithms

GEOPHYSICAL PROSPECTING, Issue 1 2008
M. Emin Candansayar
ABSTRACT I investigated the two-dimensional magnetotelluric data inversion algorithms in studying two significant aspects within a linearized inversion approach. The first one is the method of minimization and second one is the type of stabilizing functional used in parametric functionals. The results of two well-known inversion algorithms, namely conjugate gradient and the least-squares solution with singular value decomposition, were compared in terms of accuracy and CPU time. In addition, magnetotelluric data inversion with various stabilizers, such as L2-norm, smoothing, minimum support, minimum gradient support and first-order minimum entropy, were examined. A new inversion algorithm named least-squares solution with singular value decomposition and conjugate gradient is suggested in seeing the outcomes of the comparisons carried out on least-squares solutions with singular value decomposition and conjugate gradient algorithms subject to a variety of stabilizers. Inversion results of synthetic data showed that the newly suggested algorithm yields better results than those of the individual implementations of conjugate gradient and least-squares solution with singular value decomposition algorithms. The suggested algorithm and the above-mentioned algorithms inversion results for the field data collected along a line crossing the North Anatolian Fault zone were also compared each other and results are discussed. [source]

Simple relative space,time scaling of electrical and electromagnetic depth sounding arrays: implications for electrical static shift removal and joint DC-TEM data inversion with the most-squares criterion

GEOPHYSICAL PROSPECTING, Issue 4 2005
Max A. Meju
ABSTRACT A simple scaling relationship is shown to facilitate comparison, correlation and integration of data recorded using the common experimental configurations in electrical and electromagnetic depth sounding. Applications of the scheme to field data from typical geological and landfill environments show that it is robust and, where transient electromagnetic (TEM) data are available, enables easy identification and quantification of electrical static shift (galvanic distortion) in magnetotelluric and direct current (DC) sounding curves. TEM-based procedures are suggested for both the direct removal of static shift in DC sounding curves and effective joint data inversion with the most-squares criterion in the presence of static shift. A case study of aquifer characterization using sounding data from borehole sites in the Vale of York in England shows that static shift is a common problem in this glacial-covered terrain and demonstrates the effectiveness of the proposed joint DC-TEM inversion strategy in handling distorted soundings. [source]