Benjamin MARY (OT-Med, IRSTEA, CEREGE) defended his PhD thesis on 16 December 2015.
It was the first defense of the PhD thesis funded by OT-Med.
Supervisors: Ginette Saracco (CEREGE), Laurent Peyras (IRSTEA)
Collaborating institutions: LMA (AMU), METIS (UPMC), IFSTTAR Nantes et le CIRAD Montpellier.
Involved IRSTEA teams and researchers: RECOVER, EMAX (Michel Vennetier) and G2DR (Patrice Meriaux and Laurent Peyras ).
Subject: Detection of woody roots growing in earth dikes using geophysical investigations
Benjamin did the presentation of a high scientific quality and replied with precision to the jury's questions. He is PhD of Aix-Marseille University with the specialty in geosciences of environment and a very honourable distinction.
Abstract
Woody vegetation from earth dikes or dams is a fragility factor which can promote mechanisms of degradation such as erosion. An accurate assessment of root system structure, of root position into the embankment (depth, extension), and a good knowledge of soil conditions are critical in order to anticipate the consequences of vegetation development for the hydraulic structure's safety. Geophysical non-destructive methods are used for soil exploration and 3D imaging by tomography. However, locating roots in earth embankments is not a trivial issue since roots systems are complex and diversified, and the constitution of hydraulic embankments are rarely known homogeneous. Thus, geophysical methods must be adapted and improved to detect and locate woody roots in specific conditions.
Laboratory experiments allowed determining intrinsic acoustical and electrical root properties leading to identify relevant signatures and discriminate anomalies related to roots in the field. The establishment of adapted experimental devices led us progressively to assess different parameters (roots mass, water content…) under controlled conditions. Experiments in semi-controlled conditions with trees planted into a homogenous loamy soil, were conducted to assess the relevance of different methodologies, such as the use of temporal induced polarization in complex resistivity tomography or the geometry of sensors for acoustical tomography. Beyond the validation of bibliographic references, innovative data processing such as wavelet analysis were used to valorize the rich database. The results were validated by the determination of actual root position. Despite of some limits, the electrical resistivity tomography offers an adequate correlation with the location of root induced anomalies. Finally, field investigations into an embankment have been performed to highlight a spatial variability of dike structures associated with trees presence. A methodology adapted to the geophysical diagnostic of vegetation roots in embankments was developed. The coherence of the results improves for each step of the study from the acquisition strategy to data processing and result interpretation when expert's knowledge is considered together with a multimethod approach.
Key words : Embankment hydraulic structures, Electrical tomography, Acoustical prospecting, Root detection.
French title : « Développement de l'imagerie des systèmes racinaires dans les ouvrages hydrauliques en remblai par tomographie électrique et acoustique ».
JURY:
- M. Loic BRANCHERIAU Chargé de recherche, CIRAD Rapporteur
- M. Christian CAMERLYNCK Maître de conférence, UPMC, Paris Examinateur
- M. Nicolas FLORSCH Professeur des universités, UPMC, Paris Rapporteur
- Mme. Laurence JOUNIAUX Directrice de recherche, CNRS-IPGS Examinateur
- M. Philippe LASAYGUES Ingénieur de recherche, LMA-CNRS Examinateur
- M. Laurent PEYRAS Ingénieur des Ponts HDR, IRSTEA Co-Directeur de thèse
- Mme. Ginette SARACCO Directrice de recherche, CNRS-CEREGE Directeur de thèse
- M. Michel VENNETIER Ingénieur IRSTEA Encadrant scientifique
- M. Patrice MERIAUX Ingénieur IRSTEA Encadrant scientifique
