Dynamic topography and satellite gravity data joint inversion using Reduced Order Models (DYGIRO)

Abstract

Geophysical observables, such as surface elevation, gravity field anomalies, seismic data, surface heat flow, etc, are essential pieces of information used to make inferences about the structure and dynamics of the Earth’s interior. Simultaneously fitting different observable datasets is crucial in order to obtain consistent models. Among geophysical data, gravity data from ESA’s GOCE satellite mission provides key information in properly constraining the Earth’s density distribution. WINTERC-G is a new global thermochemical model of the lithosphere and upper mantle (currently being extended into the transition zone and lower mantle) based on terrestrial and satellite gravity data (Fullea et al., 2021). The inversion procedure behind WINTERC-G has two main steps. In step 1, a 1D column-wise inversion of surface wave tomographic, surface elevation (isostasy) and heat flow data is performed. Then, in step 2, the output model from step 1 is used as prior information for the inversion of the gravity field data (filtered geoid anomalies and gravity gradients from GOCE at satellite height) to refine the 3D crustal density and upper mantle composition. The model predicts a residual, non-isostatic topography that can be considered as a proxy for dynamic topography. However, within a rigorous framework, dynamic topography cannot be simply taken as a non- isostatic residual, but it should be explicitly computed (i.e. solving the Stokes equation for a given rheological and density distribution) and consistently integrated into the joint inversion of the gravity field and the terrestrial observation with feedback from both the static and dynamic parts. The goal of DYGIRO project is to add a third step into the global WINTERC-G inversion scheme that consistently integrates dynamic topography as an additional model constrain. We present here the first steps of such integration at global scale. To do that, the dynamic topography is computed by solving the Stokes flow problem associated with the current WINTERC-G model down to the transition zone. The dynamic topography thus obtained is coupled with the static thermochemical model constrained by gravity and seismic data within an iterative scheme where the observed surface elevation coincides with the model’s isostatic plus dynamic elevation contributions. The high computational cost associated with the large- scale 3D flow computations will be alleviated by means of Reduced Order Models. Such models are based on the idea of creating surrogate models that approximate the solution at a much lower computational cost.   Fullea, J. Lebedev, S., Martinec, Z., Celli, N. L. (2021). WINTERC-G: mapping the upper mantle thermochemical heterogeneity from coupled geophysical-petrological inversion of seismic waveforms, heat flow, surface elevation and gravity satellite data, Geophysical Journal International, 226(1), 146–191.