Imaging the Earth using seismic surface waves: lateral heterogeneities, mode conversion and the path average approximation

Seismic surface waves are routinely used to image the Earth’s upper mantle structure. Surface wave
tomography based on great-circle ray theory has certain limitations which become increasingly
significant with increasing frequency of waves used in the analysis. One such limitation is the
assumption of different surface wave modes propagating independently from source to receiver,
valid only in case of smoothly varying media. In the real Earth, strong lateral gradients can cause
significant interconversion among modes, thus potentially wreaking havoc with ray theory based
tomographic inversions that make use of multimode information. The issue of mode coupling (with
either normal modes or surface wave modes) for accurate modelling and inversion of body wave
data has received significant attention in the seismological literature, but its impact on inversion of
surface waveforms themselves remains much less understood.
This talk presents an empirical study with synthetic data, to investigate this problem with a two-fold approach. In the first part, 2D forward modelling using a finite difference method that allows modelling a single mode at a time, is used to build a general picture of energy transfer among modes as a function of strength and sharpness of lateral heterogeneities. In the second part, a multimode waveform inversion technique based on the path average approximation is used to invert the synthetic data and assess how mode conversion can affect the process of imaging the Earth. Special attention is paid to ensuring that any biases or artefacts in the resulting inversions can be unambiguously attributed to mode conversion effects. This study helps pave the way towards the next generation of (non-numerical) surface wave tomography techniques geared to exploit higher frequencies and mode numbers than are typically used today.