[quest-info] PhD position in laboratory scale/computational seismology

Yann Capdeville yann.capdeville at univ-nantes.fr
Fri Mar 1 09:14:30 CET 2019


Dear Colleagues,

We are looking for a PhD student in a seismic  reduced scale experiments 
/ full wave form inversion / homogenisation   context.
We would be grateful if you could forward this announcement to 
potentially interested students.
The deadline for application is short (April 7th)

Contacts: Donatienne Leparoux (donatienne.leparoux at ifsttar.fr), Yann 
Capdeville (yann.capdeville at univ-nantes.fr)

Best regards

Yann  Capdeville

https://www.ifsttar.fr/offres-theses/sujet.php?num=2163&num_session=1&ver=an

*Detailed form :*
Title : Near-surface seismic imaging by waveform inversion and 
downscaling: feasibility using a combined numerical and experimental 
approach at laboratory and field scales
Main host Laboratory - Referent Advisor     GERS-GeoEND  -  LEPAROUX 
Donatienne  donatienne.leparoux at ifsttar.fr    tél. : 02 40 84 56 69
Director of the main host Laboratory     ABRAHAM Odile  - 
odile.abraham at ifsttar.fr
PhD Speciality     Géophysique
Axis of the performance contract     2 - COP2017 - More efficient and 
resilient infrastructure
Main location     Nantes
Doctoral affiliation     UNIVERSITE DE NANTES
PhD school     EGAAL - Ecologie, Géosciences, Agronomie et Alimentation
Planned PhD supervisor     LEPAROUX Donatienne  -  Ifsttar  - GERS-GeoEND
Planned PhD co-supervisor     CAPDEVILLE Yann  -  CNRS  -  LPG Nantes
Planned funding     Contrat doctoral  - Ifsttar

*Abstract*

*Context*
In climatic change context and energy transition, the need for imaging 
and characterizing the underground media and infrastructures is 
increasing, for land planning dedicated to new technologies (wind 
turbine in off-shore or on-shore contexts) or for monitoring sites and 
structures or anthropogenic zones (sea dikes, natural hazard assessment 
in urban areas ..etc).
Among the non destructive methods developed in applied geophysics, 
seismic imaging techniques provide a mechanical information of the 
described targets. Usually based on arrival times of Body Waves, their 
adaptation to the near surface needs to take into account Surface Waves. 
The latter have long been identified as noise for deep investigations 
such as oil exploration.
At the same time, the last decades have seen the development of the 
so-called "Full Wave Form Inversion". These methods are based on the 
entire seismic signal to finely reconstruct the underground parameters 
(Virieux and Operto, 2009). This approach has been successful in deep 
exploration for hydrocarbons and some recent studies propose to 
integrate Surface Waves for near-surface auscultation. The interest is 
based on taking into account complex propagation phenomena and in 
particular mode conversions. However, this type of method is currently 
based on a local optimization approach by linearization of the inverse 
problem for computation time reasons. To be successful, such approach 
requires the knowledge of an initial model close to the solution, which 
is difficult to define in complex subsurface and civil engineering 
environments. Moreover, the non-unique nature of the solution makes it 
difficult to interpret the results.

*Objective and Approach*
To overcome these difficulties, an innovative alternative approach 
proposes a 2-step process "by inversion of the homogenized model and 
downscaling" (Capdeville and Methivier, 2018): the first stage is based 
on the result of the inversion of the waveform in terms of a homogenized 
model at the scales of the propagated wavelengths (Capdeville et al., 
2010) and the second stage is based on a “interpretation inversion”. 
This second inversion is a global optimization of the reconstructed 
parameters designed to obtain a model distribution that can be 
interpreted in terms of the parameters needed for the targeted 
applications. The theoretical developments of this new approach as well 
as the first experimental 2D laboratory tests for non-attenuating media 
are being developed within the ANR HIWAI project (directed by Y. 
Capdeville, LPG). The prospects of this entirely new approach allow us 
to consider unsurpassed imaging capabilities for complex subsurface 
media if the information carried by surface waves can be integrated.
In this context, the proposed PhD topic aims to develop the feasibility 
of the approach of inversion by "homogenization and downscaling" for the 
problems of subsurface environments, generally very attenuating, 
including many heterogeneities with strong contrasts at surface wave 
propagation depths. This dual approach makes it possible to go beyond 
the traditional inversion of S wave velocities and assess the complete 
elastic tensor for then using anisotropy to provide more accurate 
information on the targeted targets. Here, the latter correspond to 
areas of alteration of the subsoil or the anthropogenic structures it 
contains and underground cavities. The objective is to define the 
potential for detection, localization and geometrical characterization 
of damage areas and cavities.
The proposed approach focuses on the theoretical analysis of the 
methodology by introducing superficial heterogeneities affecting surface 
waves and on the analysis of reduced-scale experimental laboratory data 
by taking into account not only the vertical but also the horizontal 
component (Bretaudeau et al., 2010 ; Pageot et al., 2017). To this end, 
reference models typical of the subsurface, of gradual complexity, will 
be defined and developed numerically and experimentally by reduced-scale 
resin models.

*Scientific issues :*
The main scientific issues of the thesis include the above-mentioned 
integration elements necessary for subsurface imaging that have not been 
explored or resolved to date, namely:
• integration of the horizontal component: contribution in numerical and 
experimental context
• the integration of surface heterogeneities into non-periodic 
homogenization
• the integration of attenuating media (involving a short propagation 
distance)
• experimental validation for subsurface applications in anthropogenic media

*Modeling tools :*
The numerical and experimental modeling tools are those developed at the 
LPG at the University of Nantes and at the ifsttar respectively:
• The 2D and 3D numerical codes based on the Spectral Element Method 
(SEM) developed in the LPG lab provide a fine simulation of the complex 
structures of underground media with a non-structured meshing.
• The MUSC bench developed in ifsttar provide a quantitative simulation 
of seismic recordings in a controlled environment. It provides both the 
vertical and the horizontal components.
The complementarity of both the numerical and experimental modeling has 
been already successfully tested in regional projects VIBRIS, PROSE and 
the ANR project, through the partnership of this thesis supervision teams.
Multi-component field data provided by the industrial project R2S 
managed by IFSTTAR will contribute to test the method in this PhD thesis.

*Required profile :*
Wave propagation (acoustic, seismic, ultrasonic)
Physics of continuous media
Inverse problem
Signal and data processing
Python, Matlab or Scilab, C, fortran - or other tools for scientific 
calculation
An experience in numerical modeling (finite differences, finite 
elements, spec- elements, etc.)
traux, ...) is recommended.

Thesis supervision: Donatienne Leparoux and Yann Capdeville (thesis 
co-director)

Bibliography
Capdeville Y. & Métivier L. (2018) Elastic FWI based on the 
homogenization method... illustrations. Geophys.J.Int. 213 (2), 1093-1112.
Pageot D. et al. (2017) Improving the seismic small-scale modelling ... 
numerical methods. Geophys.J.Int. 211(1), 637-649.
Virieux J, OpertoS, 2009, An overview of full waveform inversion in 
exploration geophysics, GEOPHYSICS,VOL.74,NO.6,NOVEMBER 
DECEMBER2009;P.WCC127WCC152
Bretaudeau F. et al. (2011) Small-scale modeling of onshore seismic 
experiment... methods. Geophysics, 76(5), T101-T112.
Capdeville Y. et al. (2010) 2D nonperiodic homogenization to upscale 
elastic media for P-SV waves. Geophys. J. Int. 182, 903-922.
Keywords :     Seismic Imaging, Invers Problem, Subsurface, non périodic 
homogeneization, Waveform, Numerical Modeling, experimental Modeling

-- 
LPGNantes - UMR CNRS 6112
Laboratoire de Planétologie et Géodynamique de Nantes
Département des Sciences de la Terre et de l'Univers
Université de Nantes - 2 rue de la Houssinière BP 92205
44322 Nantes Cedex 3 (FRANCE)
Tél: +33 (0)2 51 12 54 66 / Fax: +33 (0)2 51 12 52 68

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.swapbytes.de/pipermail/quest-info/attachments/20190301/6b1f2ed1/attachment.html>


More information about the quest-info mailing list