High-resolution probing of the Earth’s lowermost mantle
BACKGROUND AND MOTIVATION
The core-mantle boundary separates a slowly-convecting silicate mantle on its upper side from a vigorously-convecting iron-alloy outer core on its lower side. The outer core is a place where the Earth’s geodynamo is operating, and due to the magnetic field that it generates, life on Earth’s surface is possible, namely, the geomagnetic field protects us like a global shield from harmful cosmic radiation. The heat that is transferred from the core to the mantle across this boundary (as the Earth is getting cooler), and the way it affects both dynamics of the mantle (e.g. the plate tectonics cycle) and of the inner core (e.g. the nature and the rate of its growth), is still not well understood. A part of the answer to these questions lies in the understanding of the internal structure and dynamics of the lowermost mantle, just atop the core-mantle boundary, and this is where global seismology with its various toolboxes becomes invaluable.
One of the seismological toolboxes is high-resolution imaging. Among many techniques, some of which developed in the RSES Seismology & Mathematical Geophysics group, there are various geophysical inversion tools, ranging from sophisticated parameter searches using digital seismic waveform data to various flavours of seismic tomography – imaging of the Earth’s interior in a similar way medical tomography works. However, in the lowermost mantle, where spatial coverage by seismic waves is sparse, we are facing an ill-posed problem. To illustrate this problem, a parallel could be drawn with medical imaging: it would be as if medical physicists attempted to image the human brain by placing only a few sources and receivers on one side of the patient’s head, and radiologists then had to interpret those imprecise images. Indeed, we don’t have the luxury of having sources and receivers everywhere, therefore, we need to be innovative and industrious in our approaches. One of the approaches that we have recently developed in imaging of the lowermost mantle includes a rigorous treatment of uncertainty through a probabilistic (Bayesian) framework – the Hamiltonian Monte Carlo algorithm.
We have successfully applied the above-mentioned to the datasets of long-period S-waves and short-period P-wave travel times sensitive to the lowermost mantle. Although long-period S-wave tomography finds strongly concordant tomograms of large slow velocity provinces (LLSVPs) underneath Africa and the Pacific Ocean, we also find the existence of substantial inhomogeneity in the lowermost mantle at multiple spatial scales – prominent medium and short-scale features superimposed on long-scale domains. On one hand, this is further observational evidence that the lowermost mantle contains a large amount of the recycled material from the Earth’s lithosphere, which agrees well with the shorter-wavelength structures found in thermally-driven geodynamical models. On the other hand, there is evidence of heterogeneous chemical compositions, and the debate on the composition and thermal conditions is still ongoing.
Ongoing research on the Earth's lowermost mantle also investigates topics such as ultra-low velocity zones, anisotropy, the thickness and nature of the upper boundary of D’’ layer, and the core-mantle boundary topography. The following approaches can be pursued to achieve a better spatial sampling and imaging resolution of the lowermost mantle:
- Installation of seismic stations at extreme geographic latitudes and ocean floor. The Australian deployments of the seismographic stations across Australia and in the surrounding regions, including Antarctica and Southern Ocean help in achieving this objective;
- Development and application of new seismological and mathematical geophysics techniques (e.g. array signal processing, Bayesian inversion using rigorous treatment of uncertainty or waveform measurements through machine learning instead of hand picking), which will allow us to use large amounts of data that were previously not available.
NATURE OF PROJECT(S): computational (e.g. running software on Terrawulf or National Computational Infrastructure computers), numerical, observational, interpretational (e.g. see a couple of our recent papers on this topic here or here).
ESSENTIAL BACKGROUND: PHYS3070 (Physics of the Earth)
POSSIBLE FUTURE RESEARCH DIRECTIONS: Any of the approaches mentioned above could become student projects. Highly motivated students with backgrounds in geophysics, physics, astronomy, computer sciences, engineering or mathematics will find the project and work on this topic challenging and satisfying.