Accueil › Animation Scientifique › Astrid DUFAURE, PhD

Astrid DUFAURE soutiendra sa thèse, intitulée "Imagerie de la structure interne des petits corps du système solaire" le mardi 31 octobre 2023 à 14h30 en amphi Rouard, Faculté des Sciences St Jérôme à Marseille.
La présentation ainsi que les slides seront en anglais.

Le jury sera composé de :

 Mark HAYNES, Radar system engineer, JPL, NASA, USA - Rapporteur (en visio)
 Lorenzo CROCCO, Director of research,IREA, CNR, Italy - Rapporteur
 Pierre VERNAZZA, Chargé de recherche, LAM, France - Examinateur
 Gilles MICOLAU, Professeur, EMMAH, France - Président
 Sampsa PURSIAINEN, Professeur, Tampere University, Finland - Examinateur
 Amélie LITMAN, Professeur, Institut Fresnel, France - Directrice de thèse
 Christelle EYRAUD, Maître de conférence, Institut Fresnel, France - Co-directrice de thèse

Titre : Imaging the internal structure of small Solar system bodies

Résumé : The intemal structure of small Solar System Bodies is still poorly understood, although it can provide important information about the formation process of asteroids and comets. Space radars can offer direct observations of this structure.
In this study, I firstly investigate the possibility to infer the intemal structure with a simple, fast and low memory consuming inversion procedure applied to radar measurements. Secondly, I am going a step further by retrieving quantitative information on the target. In particular, a non linear inversion algorithm is proposed to retrieve the 3D permittivity map of the object.
We consider a multiple quasi-monostatic configuration with a measurements over a wide frequency band, which is the most cornrnon configuration for space radars. We carried out an experiment in the laboratory equivalent to the probing of an asteriod using the microwave analogy (multiplying the wavelength and the target dimension by the same factor). Two analogues based on the shape of asteroid 25143 ltokawa were constructed with different interiors. The electromagnetic interaction with these analogues was mea­sured in an anechoic chamber using a multi-frequency radar and a quasi-monostatic configuration. I then inverted these data with two classical imaging procedures, allowing to reach the structural information of the analogues interior. Internal structural differ­ences are distinguishable between the analogues. I also worked on the reducing of the number of radar measurements used in the imaging procedures, that is bath the nurnber of transmitter-receiver pairs and the number of frequencies. Similar results can be achieved even with the optimised configuration of a reduced number of measurements. A principal component analysis was also performed on the measured scattered field. The results show that this analysis does not offer better quality reconstructions then that of the optimised configuration.
A non linear inversion algorithm was adapted allowing to retrieve the 3D relative permit­tivity map of a target via the iterative search of the porosity by using the Looyenga mixing law. The algorithm is tested on a canonical object with the goal to apply it to ltokawa analogue in the future. The quantitative imaging results obtained on the canonical object show that this porosity reconstruction approach is interesting because it allows to obtain a better reconstruction of the target shape and permittivity.

Mots Clés : electrornagnetic waves, 3D scattering, numerical methods, image processing technics, asteroids, microwave analogy, inverse problems, structural imaging, quantitative imaging