Imaging the multi-scale topography of the Earth’s core

Project Details

This project is 50% funded by The Royal Society

Key Questions

How large is the topography on the Earth’s core and how does this vary spatially on different scales?

Background

Nearly hallway to the centre of our planet, the core-mantle boundary (CMB) marks a change in temperature and composition that is even larger than at the Earth’s surface. Dynamic processes that are vital for life on Earth interact in this region, with mantle flow driving plate tectonics and core convection sustaining the magnetic field that protects life on Earth from harmful extra-terrestrial radiation. Seismological imaging of the CMB provides critical insights into these dynamic processes that have shaped the evolution of our planet. However, at present no consensus exists on the amplitude or pattern of the topography of the core (Koelemeijer, 2021). This project aims to develop models of CMB topography that are essential for investigating the coupling between Earth’s mantle and core and provide valuable information on lower mantle density (Deschamps et al., 2017).

Aims of the Project

The aim of this project is to characterise the topography on the Earth’s core, on a range of scales. Using normal modes, we will constrain the longest wavelengths of topography, while body wave data will be used to constrain the short-scale topographic variations. Comparisons with geodynamic models will provide insights into the causes of the dynamic core topography and the density structure of the lowermost mantle.

Project Description

Similar as on the Earth's surface, intriguing landscapes are present deep inside our planet that can be imaged with seismological data. Particularly, it is crucial to image the landscape of the core-mantle boundary (CMB). Improved observational constraints on the topography of this boundary provide critical insights into dynamic processes in the mantle and core (Koelemeijer, 2021), leading ultimately to better constraints on the history of our planet.

 

The topography on the Earth’s core-mantle boundary is inherently linked to the density structure of the deep mantle through isostasy and dynamic flow effects. Accurate observations of CMB topography therefore help to constrain mantle viscosity and density (e.g. Deschamps et al., GJI, 2018). These two quantities are vital for determining the driving forces of mantle flow. In addition, lateral variations in CMB topography break the symmetry of the dynamic regime in the outer core, influencing core flow and the geodynamo, topics of active research in the deep Earth community.

 

Accurately constraining the topography of the Earth’s core remains an outstanding issue in deep Earth research. The recent review by Koelemeijer (2021) identified a discrepancy between models based on observations of high-frequency travelling waves (body waves) and long-period standing waves (normal modes), which provide information on different scale lengths. This review also indicated ways in which progress may be made, specifying methods and data that are most suitable to be used. These recommendations form the basis of this PhD project, partly funded by the Royal Society, with the main objective to develop a model of CMB topography that is consistent with a wide range of seismological data.

 

Methodology

The student will first assemble a data set of body wave phases that interact with the core-mantle boundary in different ways (i.e. reflecting, refracting). The raw waveform data will be downloaded from the IRIS data management centre (freely accessible online). Using open-source python tools within Obspy (Krischer et al., 2015), the student will make multi-frequency traveltime measurements of these travelling waves, which will constrain Earth structure on a range of wavelengths.

 

Subsequently, the student will use the SOLA method (Zaroli, 2016; Zaroli et al., 2017) to invert these data for feasible CMB topography models. SOLA allows us to obtain models with unbiased amplitudes, rather than damped models of real Earth structure and to focus on the region of interest rather than having to invert for the entire mantle. The suite of models, with their uncertainties, will be tested against independent normal mode data measured by the primary supervisor and others (freely available with the relevant publications). These data provide strong constraints on long wavelengths and have global coverage. Combining these two data types within one framework is vital to resolve existing discrepancies.

 

Finally, the produced CMB topography models will be compared to the results of geodynamic simulations, particularly from runs with different density structures in the lowermost mantle, as part of existing collaborations of the primary supervisor. Interactions with geodynamicists will be crucial for the interpretation of the final models and also provide the student with important networking opportunities.

 

Training & skills gained

The successful candidate will join the seismology group at the University of Oxford, and benefit from interactions with existing PhD students, postdocs and faculty who work on similar topics. Through the project, they will develop a comprehensive understanding of the structure and dynamics of Earth’s interior.

 

They will receive training in computational methods, including data analysis of big seismic data sets, as well as inverse methods and numerical modelling, all useful skills to help secure a future career as a research scientist in academia or elsewhere.

 

The student will also be mentored on how to prepare scientific results at (inter)national conferences, how to write manuscripts for publication in international journals and how to communicate their science to a general audience.

Besides receiving training in research and transferable skills, the student will be actively encouraged to undertake personal development courses and benefit from career support and advice on funding applications by the primary

Methods to be used

The successful candidate will assemble seismic data sets of new and existing observations of both travelling and standing waves. Combining insights from both data types, which provide information on different scale lengths, is crucial for building a consistent CMB topography model (Koelemeijer, 2021). The student will perform tomographic inversions for CMB topography using the SOLA method (Zaroli, 2016; Zaroli et al., 2017), which ensures that the developed models truly reflect Earth structure. Finally, the student will compare the topography models to predictions based on geodynamic simulations of mantle flow, with the aim to constrain the origin of lower mantle structures.

Specialised skills required

This project is suitable for a numerate candidate with an interest in global seismology and deep Earth structure. They should have a background in Physics or Geophysics, preferably with knowledge of seismology and programming experience. 

supervisor.

Contact

Please contact Dr Paula Koelemeijer (at Oxford from May 2022: paula.koelemeijer@rhul.ac.uk) or Prof Tarje Nissen-Meyer (tarje.nissen-meyer@earth.ox.ac.uk) if you are interested in this project.