Towards thermo-chemical full-waveform inversion: integrating mineral physics and seismology

Knowledge of planetary interior structure is essential for understanding the origin and evolution of the Solar System. Seismic full-waveform inversion (FWI) is currently the state-of-the-art method for imaging Earth's interior and has revealed mantle heterogeneities at increasingly finer scales. However, FWI solely images variations in seismic properties and, as a result, the thermal or/and chemical origin of such anomalies is poorly constrained. Mineral physics provides complementary constraints by linking seismic properties to temperature, pressure and composition through laboratory measurements. Over the past two decades, significant work has been done to integrate seismic observations with mineral physics predictions. Nevertheless, direct integration of FWI and thermodynamic calculations has not yet been achieved, limiting our ability to fully exploit the information contained in seismic waveforms to image changes in temperature and composition.

In this contribution, we present a novel framework that integrates state-of-the-art FWI techniques with mineral physics to directly invert seismic waveforms for mantle temperature and chemical composition. The forward model is based on pre-calculated tables of mantle properties, which provide the seismic properties to carry out wave propagation. The inverse problem is formulated as an optimisation problem where gradients of the objective function with respect to temperature and composition are required. For the seismic component, we employ the adjoint method. For the thermodynamic component, we developed a formalism accounting for the pressure–density coupling which is combined with the seismic part by exploiting the chain rule. This approach is tested on 2-D synthetic models containing thermal and compositional anomalies where P–SV elastic wave propagation is simulated. The optimisation problem is solved using the L-BFGS algorithm. The proposed framework successfully recovers anomalies in temperature and composition, while revealing a strong trade-off. Such non-uniqueness reflects the importance of taking into account both thermal and compositional variations and a priori information about them. The proposed framework enables the integration of diverse geophysical datasets as well as the incorporation of additional information on the potential origin of certain mantle anomalies based on petrological constraints, which are crucial to tackle the non-uniqueness of the inverse problem.