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H/F PhD Thesis. Probing the 3D Alpine lithosphere by Full Waveform Inversion of the AlpArray teleseismic data

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Date Limite Candidature : mercredi 19 mai 2021

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General information

Reference : UMR7329-STEOPE-006
Workplace : VALBONNE
Date of publication : Wednesday, April 28, 2021
Scientific Responsible name : Stéphane Operto & Vadim Monteiller
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 September 2021
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly

Description of the thesis topic

The AlpArray project (http://www.alparray.ethz.ch/home) is a European initiative dedicated to the deployment of a large and dense array of land and marine broadband stations covering the entire Alpine orogenic system (Alps-Apennines-Carpathians-Dinarides). The objective of this project is to develop new high-resolution geophysical images of the lithosphere and upper mantle of the Alpine belt with the aim to draw new inferences on past and present geodynamical processes that shape it. This PhD thesis first aims at building new P&S-wave velocity, density and Qp&Qs models of the lithosphere and upper mantle located below the station array with an unprecedented resolution by full waveform inversion of the teleseismic data collected during the deployment. In a second stage, we will focus the imaging on a complex area centered on the Ligurian node at the junction between the Ligurian basin, the Alpine arc and the Apennines, with the aim to improve the imaging of the three coexisting Mohos. Some breakthrough is expected in our insight of the complex deep Alpine structures and in our understanding of the underlying lithospheric and mantle dynamics, and their relationship with well-documented surface observations. In the southern termination of the Alpine belt, where some of our efforts will be more specifically focused, some improved understanding of the complex interplay and structural junction with the Apennine belt and Ligurian basin are sought through this work, in particular as to the origin of the notable, yet poorly understood, seismic activity recorded there.

Full Waveform Inversion (FWI) is a seismic imaging method that seeks to estimate several physical properties of the Earth's interior with a spatial resolution of the order of one wavelength, by minimizing a distance between the seismological measurements and some numerically-modeled synthetic counterparts. Although this seismic imaging technique has been initially developed by the exploration geophysics community, recent applications on teleseismic data have opened new perspectives for high-resolution lithospheric and upper mantle imaging. Indeed, compared to well-established approaches of traveltime tomography and migration of receiver functions, FWI aims at exploiting the full information content of seismological records within a given time window, hence leading to an improved spatial resolution and a quantitative estimate of several physical properties (at first order, P and S wave velocities as well as density).

The applicant will take advantage of an existing FWI code developed during a preceding PhD thesis that will need to be further developed and extended (source signature estimation, data preconditioning, regularization, accounting for second-order effects such as attenuation) from the experience gained on the AlpArray data set.
The final part of the thesis will be dedicated to an in-depth interpretation of the geophysical images and their input in our understanding of the complex Alpine geodynamics.

This PhD thesis covers a broad spectrum of expertise in the field of internal geophysics, from seismological data processing to geodynamic interpretation through the handling of leading-edge numerical methods for seismic wave propagation and inversion on large-scale numerical problems requiring high-performance computing. The applicant will have a strong background in geophysics or geophysical engineering with good skills in applied mathematics, computer sciences and signal processing, whilst showing commitment for geophysical applications. The applicant will benefit from a favourable scientific environment with daily contact with a multi-disciplinary team of Alpine geologists, geophysicists and research engineers and access to a large corpus of computer codes for seismic modeling and inversion, that will be further developed and extended, and computational ressources provided by the local SIGAMM senter hosted by the Observatoire de la Côte d'Azur and the GENCI national centers (CINES, IDRIS).

Work Context

The PhD will be carried out at Geoazur (Sophia-Antipolis) with several short periods at LMA (Marseille).
This thesis is funded by ANR – AAPG2020.

The thesis will be directed by Stéphane Operto (https://geoazur.oca.eu/en/stephane-operto) and Vadim Monteiller (Laboratoire de Mécanique et d'Acoustique de Marseille. email: monteiller@lma.cnrs-mrs.fr) with a strong involvement of Stephen Beller (Université de Princeton). This thesis work will be carried out within the framework of the LISALPS project funded by the ANR as part of the AAPG 2020 call for tenders. This project brings together Geoazur, LMA and ISTerre as partner laboratories.

Geoazur (https://geoazur.oca.eu/en/acc-geoazur) is a research laboratory in Earth and Universe Sciences with multidisciplinary activities in the fields of seismology, geodesy, natural hazards, rock mechanics, imagery of the Earth, the study of active margins and astrophysics. It has the Université de la Côte d'Azur, the CNRS, the IRD and the Observatoire de la Côte d'Azur as its guardians. It is located on the technopole of Sophia-Antipolis about twenty kilometers from the Niçoise agglomeration. The candidate will be attached to the Imaging & Waves team of Geoazur specialized in the development of imaging methods of the interior of the Earth while benefiting from interactions with seismologists and seismotectonicians of the Seismes team.
The candidate will benefit from the computing infrastructures provided by the laboratory's own resources, the SIGAMM mesocentre hosted by the Côte d'Azur Observatory and the national centres grouped around the GENCI.

Additional Information

candidate profil = Engineer, mathematician, (geo)physicist

demand skills = Numerical resolution of partial differential equations (finite-element method for the elastodynamic equations), numerical optimization (gradient method), signal processing, high-performance computing (parallel programming, Fortran90, MPI, OpenMP), basic geology.

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