Laura Cobden

Seismic detection of post-perovskite inside the Earth

Since 2004, we have known that perovskite, the most abundant mineral in the lower mantle , has the capacity to transform to a denser structure, post-perovskite, if subjected to sufficiently high temperature and pressure . But does post-perovskite exist inside the Earth? And if it does, do we have the resources to locate it seismically? In this chapter, we present an overview of what we know about the perovskite-to-post-perovskite phase transformation from mineral physics, and how this can be translated into seismic structure. In light of these constraints, we evaluate the current lines of evidence from global and regional seismology which have been used to indicate that post-perovskite is likely present in the deep mantle.

INFLUENCE OF ACOUSTIC SOURCE DENSITY ON CROSS-CORRELATED SIGNALS: IMPLICATIONS FOR AMPLITUDE-BASED TOMOGRAPHY IN HELIOSEISMOLOGY

We present new results from the numerical simulations of cross-correlated helioseismic data in a two-dimensional quiet non-magnetic solar subphotosphere for studying the effects of spatiotemporal density of solar acoustic sources on signal quality. Although the signal-to-noise level of helioseismic signals can be improved by data averaging, our results show that there exists a threshold above which this change in data quality is largely independent of the density of the acoustic sources. We also demonstrate that if the source density is below this threshold, significant data artifacts may arise depending on the source location relative to the two observation points at which cross-correlation is performed. Our numerical results therefore show that the density of the acoustic sources is a key, previously unidentified parameter that influences the quality and amplitude of cross-correlated helioseismic signals. We conclude that the identification of this source parameter is important for developing helioseismic tomography beyond travel-time-based methods, namely by modeling amplitudes.

The Challenge of Combining Research and Teaching: A Young Geoscientist’s Perspective

I present an overview of the working environment of the early-career geoscientist who performs both teaching and research duties. I outline what motivates these scientists to undertake optional teaching duties and the challenges faced in integrating teaching into a research-only job contract. Young scientists have a range of teaching possibilities at their disposal, and we consider strategies for optimum time management. Specific examples drawn from the author’s own experience are used to discuss the benefits of combining geoscience teaching with research, which has advantages for both students and the researcher.

Inversion of full acoustic wavefield in local helioseismology: A study with synthetic data

We present the first results from the inversion of full acoustic wavefield in the helioseismic context. In contrast to time-distance helioseismology, which involves analyzing the travel times of seismic waves propagating into the solar interior, wavefield tomography models both the travel times and amplitude variations present in the entire seismic record. Unlike the use of ray-based, Fresnel-zone, Born, or Rytov approximations in previous time-distance studies, this method does not require any simplifications to be made to the sensitivity kernel in the inversion. In this study, the acoustic wavefield is simulated for all iterations in the inversion. The sensitivity kernel is therefore updated while lateral variations in sound-speed structure in the model emerge during the course of the inversion. Our results demonstrate that the amplitude-based inversion approach is capable of resolving sound-speed structures defined by relatively sharp vertical and horizontal boundaries. This study therefore provides the foundation for a new type of subsurface imaging in local helioseismology that is based on the inversion of the entire seismic wavefield.