Description

The earth's mantle is by far the largest reservoir of material and heat in the earth system, and it convects much more slowly than the adjoining layers (oceans, atmosphere, iron core). With these characteristics, the mantle shapes the appearance and evolution of the near-surface layers, for example in the form of plate tectonics, or volcanism. The central method of this Ph.D. project is 3-D seismic tomography, the primary geophysical imaging method for inferring structures and processes in the mantle, from the Moho to the core-mantle boundary. The signal sources are P- and S-waves generated by earthquakes, which traverse the deep interior of the solid earth, are recorded by seismometers that the surface, and are used to compute the interior 3-D structure by the method of seismic tomography.

While the continents have been increasingly instrumented with digital broadband seismometers since the 1990's, this is not the case for the oceans (two thirds of the earth's surface), except for sparse stations on islands. This has started to change with the temporary deployment of ocean-bottom seismometers (e.g., Barruol & Sigloch 2013) and with the development and deployment of seismo-acoustic 'MERMAID' floats for tomography (www.earthscopeoceans.org), where Géoazur lab has been a leader.

The primary objective of this Ph.D. project is to incorporate existing data from oceanic seismological instruments more systematically into ray-based and finite-frequency tomography models in order to improve their image resolution. Using signal processing and waveform modelling methods, we will properly account for the characteristics of the novel sensors, like the fact that they are hydrophones rather than ground-seismometers, or reflections within the water column obscuring the useful waveforms. We will also explore the scientific potential of such data in synthetic tests, for different spatial scales and geological settings.

There is latitude for the Ph.D. student to shape this project in a more technical versus a more tectonic-geodynamic direction. Due to recently increased data coverage, natural areas of application are whole-mantle imaging in the Pacific Ocean, southern Indian Ocean, and/or in the European/Mediterranean realm.

Skills required

The candidate must hold a Master's or equivalent degree in geosciences or in a physical sciences or engineering discipline. We are looking for a curious and highly motivated early-career scientist with a track record of excellent academic results and excellent potential for independent scientific inquiry. The required skills are: - theoretical and practical skills in signal processing methods for time series - good programming/scripting skills for numerical methods software, preferably in Python or Matlab. - very good oral and written communication, including in English for communicating with our international partners and for scientific publications. Knowledge of French is a plus (most everyday communication in the lab is in French). - The following skills would be a plus: - experience working in a Unix/Linux environment; experience working with seismograms or hydroacoustic time series; numerical waveform modelling methods; prior scientific publications; other programming languages.

Bibliography

Barruol, G., & Sigloch, K. (2013). Investigating La Réunion hot spot from crust to core. Eos, Transactions American Geophysical Union, 94(23), 205-207.
https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2013EO230002

Hosseini, K., Sigloch, K., Tsekhmistrenko, M., Zaheri, A., Nissen-Meyer, T., & Igel, H. (2020). Global mantle structure from multifrequency tomography using P, PP and P-diffracted waves. Geophysical Journal International, 220(1), 96-141.
https://academic.oup.com/gji/article/220/1/96/5571093

Pipatprathanporn, S., & Simons, F. J. (2024). Waveform modelling of hydroacoustic teleseismic earthquake records from autonomous Mermaid floats. Geophysical Journal International, 239(1), 136-154.
https://academic.oup.com/gji/article/239/1/136/7715032

Simon, J. D., Simons, F. J., & Irving, J. C. (2022). Recording earthquakes for tomographic imaging of the mantle beneath the South Pacific by autonomous MERMAID floats. Geophysical Journal International, 228(1), 147-170.
https://academic.oup.com/gji/article/228/1/147/6324568

Tsekhmistrenko, M., Sigloch, K., Hosseini, K., & Barruol, G. (2021). A tree of Indo-African mantle plumes imaged by seismic tomography. Nature Geoscience, 14(8), 612-619.
https://www.nature.com/articles/s41561-021-00762-9
https://hal.science/hal-03384259/file/Tsekhmistrenko_Plume_tree_RHUM-RUM_Nat_Geosc_2021.pdf

Keywords

geophysics, seismology, geodynamics, seismic tomography, signal processing, hydrophones