PhD Research: Seismological studies of the lowermost mantle and core-mantle boundary region

Supervisor: Dr. Arwen Deuss

The core-mantle boundary (CMB) region at 2891 km depth is the largest thermal and compositional interface in the Earth separating the solid silicate mantle from the liquid iron-nickel outer core. A more detailed understanding of the velocity structures in the lowermost mantle (the D'' region) and of the CMB topography is a prerequisite for the integration of results from seismology with mineral physics and convection modelling. The seismic structures found in the D'' layer are comparable in complexity with the structures found in the lithosphere and are still not well understood. Some of the these features may be due to the existence of partial melting or compositional heterogeneity in the lowermost mantle.

My research is aimed at obtaining a better understanding of the D'' region and the CMB using the Earth's free oscillations (normal modes) in combination with body waves. In this way, I hope to derive a more detailed picture of the possible processes occuring in the lower mantle in relation to the evolution of the Earth.

This project is funded by the Nahum Graduate Studentship in Physics and a College Research Studentship, both awarded by Pembroke College Cambridge, and an ERC studentship. I am a member of the Deep Earth Structure and Global Seismology group in Cambridge.

Normal mode observability

Several complex seismic structures are found in the D'' region, including topography on the CMB, ultra low velocity zones (ULVZ), an anticorrelation between bulk sound velocity and shear wave velocity heterogeneity, anisotropy and the existence of discontinuities. Many of these small scale structures have been found using short period body wave data, and we therefore wanted to determine whether long wavelength normal mode data are also sensitive to these structures. To this purpose, we calculate synthetic models for various features in the lowermost mantle and compare the results with splitting function measurements from the normal mode data set of Deuss et al. (2013). We make use of the uncertainties in the data to test whether the structures are observable - e.g. the signal is larger than the uncertainties. We demonstrate that normal modes are sensitive to most of the features in the D'' region, including small scale ULVZs and layers less than 100 km thick.

KOELEMEIJER, P. J., A. DEUSS and J. TRAMPERT, (2012), Normal mode sensitivity to Earth's D'' layer and topography on the core-mantle boundary: What we can and cannot see, Geophys. J. Int., 190(1), 553-568, doi: 10.1111/j.1365-246X.2012.05499.x.

CMB Stoneley mode observations

The occurrence of several large magnitude earthquakes over the last decades has allowed to make new observations of many modes sensitive to mantle P-wave velocity structure and inner core structure (Deuss et al. (2013)). We use this data set to make observations of modes with extreme sensitivity to the D'' region and core-mantle boundary - so called CMB Stoneley modes.

Normal mode data for testing mantle and CMB topography models

Normal modes are extremely useful to test the large scale structure of geodynamic and seismic models of mantle structure and CMB topography due to their long wavelength character and global data coverage.We calculate splitting function predictions, misfits and correlations to test the effect of various parameters and assumptions in existing models on normal mode data.

SOLDATI, G., P. J. KOELEMEIJER, L. BOSCHI, and A. DEUSS (2013), Constraints on core-mantle boundary topography from normal mode splitting, G-cubed, doi: 10.1002/ggge.20115.

Previous work: Humidity dependent surface diffusivity of salt crystals

With Prof. Chris Spiers and Dr. Colin Peach of Utrecht University

We studied the effect of humidity on diffusion through adsorbed aqueous films on cleaved NaCl crystals. For a wide humidity range from almost zero humidity up to the deliquescence point, we find that diffusive crack healing rates are reduced by six-to-seven orders of magnitude with increasing humidity. This affects the permeability evolution in rocksalt and has important implications for geological storage applications.

KOELEMEIJER, P. J., C. J. PEACH, and C. J. SPIERS, (2012), Surface diffusivity of cleaved NaCl crystals as a function of humidity: Impedance spectroscopy measurements and implications for crack healing in rocksalt, J. Geophys. Res., 117, B01205, doi:10.1029/2011JB008627.

Background

I studied for a Bachelor of Science degree in Earth Sciences from Utrecht University from 2005 to 2008, specialising in geology, geophysics and geochemistry. From 2008 to 2010, I continued with a Master of Science degree in Geophysics with a focus on seismology. During this master, I spend a year on a research project on normal modes at the University of Cambridge and graduated cum laude in 2010 with my thesis "Normal mode study of the D'' region and core-mantle boundary topography".