First results for experiments on inner core attenuation

The Earth’s inner core is made of a solid iron alloy. Seismic observations suggest a structure and an anisotropy which leads to variations in both the velocity and the attenuation of the seismic waves. Attenuation is the loss of energy during the propagation of the seismic waves. Whether this attenuation arises from intrinsic properties of the iron alloys or extrinsic origins remains an open question. In this context, studying attenuation in metallic alloys could help improving our knowledge about the physical properties and the geodynamic of the inner core.

Extrinsic attenuation is linked to external environment that impact the wave propagation, such as scattering or heterogeneities. Intrinsic sources are related to the properties of the material itself such as its viscoelastic behavior. This work focuses on the latter and particularly on the anelastic relaxation, which is one of the sources of internal friction.

In this work, we seek to understand attenuation mechanisms in metals at high temperature. The experiments are conducted on a dynamic mechanical analysis (DMA) instrument with control of temperature and oxygen fugacity albeit at ambient pressure. We use a Mg alloy as analogous material to that of the inner core, which presents similar crystallographic structure and is expected to behave the same way.

Here, we will present some results and hypotheses derived from temperature, frequency, and strain sweeps realized with DMA. These analyses allow us to investigate viscoelastic values like internal friction, storage and loss modulus at different conditions. Results show a temperature-dependent behavior that can be related to the underlying mechanisms. Scanning electron microscopy analyses (electron back scattered diffraction) were performed to further assess the attenuation mechanisms involved in our experiments. Grain size, texture or grain boundaries were analyzed to understand our analogous material. These experiments are led in conditions which could allow us to discuss attenuation in the inner core.