We present here the results obtained from 3-D tomographic inversion of synthetic data in order to investigate the ability of tomography to reveal the detailed thermal structures created by mantle convective processes. The synthetic input velocity models are based on the density heterogeneities obtained from a 3-D spherical-shell model of thermal convection. P and pP arrival-times are used in the inversion. We study the effect of the length-scale of the input anomalies characterized by the Rayleigh number of convection simulations (Ra=3 × 10 5, 8 × 10 5, 10 6) and asses the sensitivity of the inverse problem to parametrization and explicit regularization (damping). We concentrate mainly on the effect of projection error and we show that its role is substantial with increasing chaotic convective behaviour. We use both irregular cell parametrization and regular cell parametrization (equisurface area). Due to the uneven distribution of sources and receivers, we found substantial differences between inversion output for models with regular and irregular cells. For the irregular parametrization, the structures located in the well-covered regions are resolved quite successfully, although the resolving power decreases with depth. In the poorly covered regions, where the ray sampling demands large parametrization cells, the correspondence between the input and output is rather low due to high parametrization error. An explicit regularization is not needed if only the projection error is assumed. On the other hand, for the regular parametrization, the inverse problem is very unstable and oscillations occur, unless an explicit regularization is applied. However, if an optimum damping is applied, the global correlation between the input and output anomalies is substantially higher than in irregular case. Further, we compare the power spectra of input seismic velocity anomalies and the inversion output. In case of irregular parametrization, the spectra of input model and inversion output differ especially in the uppermost part of the lower mantle and in the lowermost mantle, again due to the high parametrization error associated with the large parametrization cells. Much better agreement between the spectra of input and output is obtained in regular parametrization model, provided that a proper damping is applied. Therefore, we conclude that on a global scale, the regular parametrization gives better results than the irregular one, while the inversion with the irregular parametrization is able to resolve much more detailed structures in well-covered regions with the same number of model parameters and thus with the same computational costs.
- Resolution tests
- Synthetic inversion