Doctoral contracts 2017

Doctoral contracts for this year. You can apply for to scientific persons in charge.

1) Uranium isotopes as proxies for investigating land-to-sea sediment transfer response to Late Quaternary climate changes

Supervisor: Pr. Jean-Alix Barrat (barrat@univ-brest.fr)
Co-supervisor: Dr. Samuel Toucanne (stoucann@ifremer.fr), Anthony Dosseto (tonyd@uow.edu.au)

Studying how catchment erosion has responded to past climate change can help us better understand not only how landscape evolution and source-to-sink processes operate, but also predict the consequences of future climate change on soil resource availability and sediment discharge to the ocean. Variations in the Uranium isotopic composition of the
silicate fraction of marine sediments have been used to infer 'transport time' (i.e. measure of catchment erosion; the shorter the transport time, the greater the erosion from source to sink). However, much remains to be understood about the factors that control their distribution in terrigenous sediments, and whether long-term sedimentary records. The PhD
project proposed here aims at filling this gap, by proposing to further explore the use of uranium isotopes in marine sediments as a proxy for past erosion processes on continents. A particular emphasis will be placed on the application of Uranium isotopes to the study of past relationships between abrupt climate change (including Dansgaard-Oeschger
variability) and erosion.

Key words: Uranium series isotopes, erosion, transport time, climate change

2) Kinetic and thermodynamic study of gas hydrates in porous media: application to hydrate-bearing sediments and energy storage processes

Supervisor: Anthony Delahaye (anthony.delahaye@irstea.fr)
Co-supervisor: Livio Ruffine (livio.ruffine@ifremer.fr)

Gas hydrates have been studied for many years in various fields. Originally considered as a flow assurance issue by the hydrocarbon industry due to their ability to form plugs within natural gas pipelines, today gas hydrates are being explored in many different angles: from geoscience to the field of process engineering for gas and liquid treatment, seawater desalinization, carbon capture applications, as well as in the refrigeration field by using salts and CO2 hydrates. The latter, like in geoscience, involves in-depth knowledge of the formation and destabilization of hydrates in porous medium. The proposed PhD thesis aims to study the hydrate formation and destabilization processes in porous media, by merging knowledges developed in both the field of Geosciences at Ifremer and Process Engineering at Irstea.
Thus, the objectives of the thesis are to couple both experiments and modeling to get insight into the mechanisms of formation/destabilization of gas hydrates in porous media by considering the kinetic aspects and thermodynamic phase equilibria, as well as the hydrate morphologies. Phase quantification will also be performed to estimate the level of
saturation of the medium as a function of the hydrate-forming fluid properties (water, brine, gases) and the nature of the porous media (natural sediment, artificial porous media with or without variable porosity). This work will be based on a set of experiments, which will be carried out from several high-pressure devices. The resulting dataset will be used to
develop a numerical model that describes hydrate behavior during nucleation and growth in porous media.

Key words: gas hydrates, experiments, formation and destabilization, porous media, modeling

3) Modelling and quantification of micro-bubbling processes at the deep seabed

Supervisor: Louis Géli (geli@ifremer.fr)
Co-supervisor: Carla Scalabrin (Carla.Scalabrin@ifremer.fr)

Non-seismic microevents have been shown to be commonly recorded by Ocean Bottom Seismometers (OBS). These microevents are characterized by short durations of less than 0.8 s, by frequencies ranging between 4 and 30 Hz, and by highly variable amplitudes. In addition, no correlation between OBSs is observed, except for located 10 m apart. In addition,
SDEs appear to occur ubiquitously on the ocean floor. The presence of gas in superficial sediments, together with analogies with laboratory experiments, suggest that gas migration followed by the collapse of fluid-filled cavities or conduits could be the source of the observed microevents. If the hypothesis that SDEs are due to methane- or carbon dioxide- related processes is proven, then a rapid, on-the-back-of-the-enveloppe, estimation shows that the total mass of carbon relased by microbubbling processes at the seabed could be of the same order of magnitude than the amount of C captured at the ocean-atmosphere interface. It is therefore of utmost importance to test the hypothesis that SDEs are produced by micro-bubbling. For that purpose, an exploratory experiment was conducted in the Sea of Marmara in 2014 during the MARSITECRUISE of R/V Pourquoi pas?. An acoustic bubble detector (BOB) and 2 OBS were carefully deployed with ROV/Victor on the seafloor, in order to monitor micro-bubbling along with SDEs, for a duration of 11 days. The work proposed within the present PhD is to model the data collected during the experiment in order to decipher the relations between SDE and microbubbling.

Key words: Seafloor, Micro-bubbling, Ocean-Bottom Seismometers, Bubble detector

4) Deep structure of the Demerara plateau offshore French Guyana and Surinam

Supervisor: Frauke Klingelhoefer (fklingel@ifremer.fr)
Co-supervisor: Walter Roest (wroest@ifremer.fr), David Graindorge (David.Graindorge@univ-brest.fr)

This project aims to provide new answers on the dynamics and the evolution of transform passive continental margins. More precisely, the objective is to characterize the crustal structure and evolution of marginal plateaus, sub-marine plateaus that are associated to 30 % of the transform margins in the world (Mercier de Lépinay et al., sous presse) . These
plateaus are systematically observed at the junction of oceanic crust of different ages in accommodation zones between divergent and transform segments. Their formation remains enigmatic although as they represent reliefs established in geodynamical nubs that are keys to understand the distribution of transform margins. This project MARGATS aims to image the deep structure of the Demerara plateau offshore French Guyana and Surinam using wide angle seismic and multichannel seismic data. The experimental site is located at the junction between Central Atlantic which here opened as a divergent margin and the Equatorial Atlantic opened mainly as a transform margin in this region. Hence, this plateau has recorded at its borders the divergent opening of the central Atlantic and the transform opening of the Equatorial Atlantic. It may be this situation that leads to the formation of the plateau, but to confirm this more precise information on its deep internal structure is required. Finally, the Demerara plateau is one of the most studied regions by petroleum seismic data and high resolution seismic data. Therefore, it is the ideal place to conduct a high resolution wide-angle deep seismic survey.

Key words: marginal plateau, transform margin, Crustal structure

5) Crustal Nature, thermicity and salt deformation in the Occidental Mediterranean Sea

Supervisor: Marina Rabineau (marina.rabineau@univ-brest.fr)
Co-supervisor: Maryline Moulin (maryline.moulin@ifremer.fr)

Because of their very young age, and their exceptional sedimentary record, most of Mediterranean basins are a natural laboratory that enables to study, in great detail, the formation and the evolution of passive margins and the influence of tectonic and thermic heritage. The thick Messinian salt layer gives an outstanding marker throughout the history of the margin that enables to study its history. Since it depositions this thick salt layer has undergone strong deformation, that are generally attributed to gravitational and buoyance salt tectonics. At the same time salt deformation is strongly dependent on temperature, but this is still poorly taken into account in models. We propose that the nature of the crust (oceanic, transitional, thinned continental, etc.) is associated with different thermal regimes and that this could have an important control on the deformation and resulting geometries of the salt layers in the overlying sediment column. New data on heat flow, shallow sediment acoustics and pore fluid geochemistry obtained during the WestMedFlux cruise (April/May 2016) gives the opportunity to test this hypothesis of the relation between crustal nature, thermal regime and salt deformation.
Some of our concerns are: 1) What is the rheological behaviour of halite under changing temperature conditions and constant pressure? What is the relation between the surface heat flow expression and the geometries of the deformed salt layers, and does fluid migration play a role in heat redistribution? Can we use the morphology of the salt as a window to understand deep margin structure ?

Keywords: Passive Margins, Western Mediteranean Sea, Salt Deformation, Thermicity, Fluids

6) Characterization of free gas/gas hydrates systems in the Black sea based on deep towed high resolution
multichannel seismic imaging (SYSIF)

Supervisor: Pascal TARITS (tarits@univ-brest.fr)
Co-supervisor: Bruno MARSSET (Bruno.Marsset@ifremer.fr)

The thesis aims at a quantitative characterization of gas/ gas hydrates systems in the Black sea. Ifremer (French Institute for Research at Sea) has recently developed a High Resolution deep towed seismic system (SYSIF) which aims at increasing seismic resolution in deep waters. SYSIF was successfully deployed during the GHASS cruise (Black sea,
2015), together with a multichannel streamer (52 traces @ 2m). The GHASS project aims at characterizing the sedimentary deformations associated with the presence of gas and gas hydrates in the superficial layers (< 200m). A key point of the thesis will be the development of seismic imaging techniques tailored to the particular geometry of deep
towed seismic acquisition. Prior to the imaging process itself, the instantaneous shape of the streamer has to be carefully reconstructed and the seismic data has to be pre-processed in order to allow preserved amplitude processing algorithm. The expected final lateral resolution is equal to the mean wavelength of the seismic signal (3 m) which will allow the correlation with in situ geotechnical measurements.

Keywords: Deep towed seismic, High Resolution seismic imaging

7) Appliance of the electromagnetic probe for the in situ density measurement

Supervisor: Gérard Tanné (Gerard.Tanne@univ-brest.fr)
Co-supervisor: Christian GAC (CGAC@Ifremer.fr), Eric MENUT (Eric.Menut@ifremer.fr)

The study of marine sedimentary systems and assessment of geological hazards associated with different external mechanisms (earthquake, tsunami ...) require a characterization of the physical and mechanical properties of the medium (such as conductivity and density of the sediment). This can be achieved in laboratories on sediment samples obtained by coring, often with significant disturbances. In situ measurements can overcome this problem. These measurements require a vector able of sinking and placing the sensor into a defined depth below the sea bottom. Few types of equipment are able to accomplish these measurements with such a constraint. In IFREMER, only one instrument can be deployed and operated to perform these measurements, PENFELD penetrometer. The penetrometer, in its design, was equipped with sensors from the existing equipment technology. These sensors offer several disadvantages: an acquisition that can’t be achieved in real time and the fact that they do not allow the simultaneous knowledge of all the parameters essential for various applications. The aims of these studies concern the feasibility and implementation of a new electromagnetic sensor used for calculating the volume density of the medium and provides a size appropriate to the tip of the penetrometer in order to use in a deeply aquatic environment. The solution should allow the bearing to theaforesaid disadvantages.

Keywords: mass density, conductivity, sensors, microwave devices, wave-matter interaction, immerged antenna.