Ultrasonic scattering from volumetric flaws in structural materials and their characterisation

This thesis is a theoretical and experimental study of ultrasonic scattering from volumetric flaws in structural materials and ultrasonic inversion techniques for nondestructive characterisation of such flaws. For forward scattering problems, the Method Of Optimal Truncation (MOOT) is studied. A large general purposed computer model is developed based on MOOT. The computer model can be used to simulate ultrasonic scattering from different shapes and sizes of voids, with only minor changes. Numerical results for a number of voids are presented in both the frequency and time domains to provide understanding of basic physical mechanism of scattering by volumetric flaws. The simulated forward scattering data are also used to test a new inversion technique developed in this study. A new ultrasonic inversion technique is developed for determining the geometrical features of a volumetric flaw in structural materials, by the inversion of the backscattered ultrasonic signal using the area function formula. The area function formula is derived from a weak scattering approximation, the Born approximation, but it is shown that the area function sizing technique works well for voids which are clearly strong scatterers. The technique extracts the flaw size from the shape of the area function which is evaluated from the backscattering signal. Unlike most of other ultrasonic inversion schemes, this technique has the advantage that it does not require the determination of the flaw centroid (zero-of-time problem). The technique is tested by the inversion of the numerical and experimental scattering data for estimating the sizes of a number of flaws. The results show very good agreement between the true sizes and the estimated sizes. The experimental work is carried out on simulated defects in the immersion and contact modes. Several techniques for processing experimental signals are investigated, including deconvolution techniques

Information retrieval from spaceborne GNSS Reflectometry observations using physics- and learning-based techniques

This dissertation proposes a learning-based, physics-aware soil moisture (SM) retrieval algorithm for NASA’s Cyclone Global Navigation Satellite System (CYGNSS) mission. The proposed methodology has been built upon the literature review, analyses, and findings from a number of published studies throughout the dissertation research. Namely, a Sig- nals of Opportunity Coherent Bistatic scattering model (SCoBi) has been first developed at MSU and then its simulator has been open-sourced. Simulated GNSS-Reflectometry (GNSS-R) analyses have been conducted by using SCoBi. Significant findings have been noted such that (1) Although the dominance of either the coherent reflections or incoher- ent scattering over land is a debate, we demonstrated that coherent reflections are stronger for flat and smooth surfaces covered by low-to-moderate vegetation canopy; (2) The influ- ence of several land geophysical parameters such as SM, vegetation water content (VWC), and surface roughness on the bistatic reflectivity was quantified, the dynamic ranges of reflectivity changes due to SM and VWC are much higher than the changes due to the surface roughness. Such findings of these analyses, combined with a comprehensive lit- erature survey, have led to the present inversion algorithm: Physics- and learning-based retrieval of soil moisture information from space-borne GNSS-R measurements that are taken by NASA’s CYGNSS mission. The study is the first work that proposes a machine learning-based, non-parametric, and non-linear regression algorithm for CYGNSS-based soil moisture estimation. The results over point-scale soil moisture observations demon- strate promising performance for applicability to large scales. Potential future work will be extension of the methodology to global scales by training the model with larger and diverse data sets

Goddard Visiting Scientist Program for the Space and Earth Sciences Directorate

Progress reports of the Visiting Scientist Program covering the period from 1 Jul. - 30 Sep. 1992 are included. Topics covered include space science and earth science. Other topics covered include cosmic rays, magnetic clouds, solar wind, satellite data, high resolution radiometer, and microwave scattering

Investigation of light scattering in highly reflecting pigmented coatings. Volume 3 - Monte Carlo and other statistical investigations Final report, 1 May 1963 - 30 Sep. 1966

Monte Carlo methods, Mie theory, and random walk and screen models for predicting reflective properties of paint film

Refractive index sensing with localized plasmonic resonances – Theoretical description and experimental verification

In this thesis the sensing properties of plasmonic resonators for changes in the surrounding refractive index are investigated. A self-consistent and general sensing theory is developed. This theory connects the electrodynamic properties of plasmonic resonators like resonance wavelength and electric field distribution to the sensitivity for refractive index changes. A figure of merit (FOM) is derived which includes the effects of noise and in its general form directly states if a certain change in refractive index will be measurable or not. For the FOM in the quasi-static limit absolute bounds and scalings are derived. These bounds are based on the localization of electromagnetic energy for which analytic expressions were known before. The important result of the quasi-static considerations is that the sensitivity is determined completely by the choice of material and resonance wavelength for refractive index changes that cover the whole sensing volume while for smaller analytes the energy confinement to the analyte volume is important. To confirm the developed theory numerical calculations and an experiment with crescent shaped plasmonic resonators is carried out and good agreement is found. In this experimental verification, local refractive index changes were introduced close to the crescent shaped particles and their resonance wavelength change was measured. As a model analyte polystyrene colloids were used and manipulated with an atomic force microscope. This approach leads to a very defined and controllable model system which allowed the theoretical predictions to be verified without parasitic effects. The proposed theoretical model predicts the measured wavelength changes with high accuracy and allows to extrapolate the result to the response of the resonator to the binding of a single molecule to its surface. From the theory together with the experiment it follows, that single molecule sensitivity will be possible by increasing the signal to noise ratio of the measurement

Analysis of Images of Comet 67P/Churyumov-Gerasimenko Obtained by Osiris/Rosetta: Dust Particles Investigation

The aim of this thesis is to study and characterize the dust in the coma of the comet 67P/Churyumov-Gerasimenko in order to increase our knowledge about the nature and behavior of comets. These objects are the most pristine of our solar system and their investigation allows to understand the physical and chemical conditions that took place in the primordial environment where they formed. In turn, dust is a fundamental constituent of comets and provides valuable information on their composition, structure and evolution. The importance of the dust lies in the possibility to use it as a proxy to derive properties of the interior of the cometary nuclei. Indeed, the activity produced when the comet approaches the Sun releases both surface material and pristine matter from the inside of the nucleus that cannot be reached otherwise. The ESA Rosetta mission to the comet 67P/Churyumov-Gerasimenko revealed a new insight of the cometary dust providing precious information about its composition, dimension, size distribution and structure. For the rst time it was possible to acquire images of single dust aggregates and to give a comprehensive description of their properties, thanks to the information obtained by all the instrumentation on board the spacecraft. The overall processes that act on the comets are not yet fully understood and there are still open questions that need to be claried. It is therefore essential to continue to investigate this complex scenario to unveil the real nature of these primordial objects. The rst work of this thesis concerns the photometric analysis of images of dust grains, taken by the OSIRIS Narrow Angle Camera (NAC). The large amount of data forced us to develop an automatic pipeline to identify and to analyze the particle tracks present in the images. We measured the spectral slope of about 2000 grains in 555 images taken in 18 days of observation from July 2015 to January 2016. Then we evaluate their variability with respect to the distance from the nucleus and from the Sun. To better characterize the grains composition we perform also a spectrophotometric analysis of a subgroup of 339 grains. To check their nature, we compared their three point spectra with the ones of several nucleus terrains identifying dierent types of grains spectra. The second work of this thesis treats the scattering properties of cometary dust analogs, in collaboration with the Instituto de Astrofsica de Andalusa, Granada at the Cosmic Dust Laboratory. Laboratory experiments are of fundamental importance to interpret the data since they allow to study complex phenomena in a monitored environment, favoring the evaluation of the eects due to specic physical and chemical properties of the dust. We measure the phase function and the linear polarization curve of 7 samples, consisting on 4 meteorites, 2 silicates and 1 organic compound with the aim to compare our results with the observational data of 67P and other comets. Our last work, which is still in progress, consists in the analysis of dust motion with the aim to provide observational constraints to the dynamical models of cometary dust in the inner coma as well as to the shape of the single grains. We focus our investigation on the 15 groups of images taken from July 2015 to January 2016 and we measure the rotational periods, the axis ratio and the direction of motion on a dataset of more than 500 grains

Development of a new velocity measurement technique : the laser bessel velocimetry

The present thesis describes the design, construction and testing of a new velocity measurement optical technique system. The technique has similarities with the laser Doppler velocimetry (LDV) in that it uses scattered light detection, in order to measure one component of the velocity vector of moving flows or solid surfaces. It uses the fringes of a Bessel beam produced by an axicon to generate the measurement volume. This technique, which we call Laser Bessel velocimetry (LBV), is noninvasive and permits continuous velocity measurements of moving particles. The experimental measurement set-up including the laser source, the optical devices, a moving stage with known velocities, a photodetector to capture scattered light and signal processing and data acquisition components, was developed and used to provide a proof of concept of this new technique. The set-up was also tested with a commercial LDV system. Two types of refractive linear axicons have been used to generate a Bessel type beam by illuminating the axicons with blue and red collimated and coherent laser light of dissimilar wavelengths, λ. The linear axicons offer the advantage of simplicity. The software tools for measurements, acquisition and analysis of the data are developed using NI Labview and MATLAB. Results from both theoretical simulation and experimental measurements are presented and compared.Master's These