Waveform inversions and the significance of surface-wave mode coupling

If body-wave phases in laterally heterogeneous media are modelled by surface-wave mode summation, mode coupling is required to bring out the ray character of the body-wave phases. However, it is frequently assumed that in laterally heterogeneous media surface-wave modes propagate without interacting with other modes. The effects of neglecting surface-wave mode coupling on the results from waveform inversions including body-wave arrivals have been studied by performing inversions with and without taking surface-wave mode coupling into account. The two inversion schemes are similar to the Partitioned Waveform Inversion (Nolet 1990) but differ in the approximations used to compute the synthetics: WKBJ, which neglects mode coupling, or SEA (Marquering & Snieder 1995), which does take mode coupling into account. Synthetic inversion experiments show that neglecting mode coupling can lead to biases in the deeper part of the model. Their most striking feature is that for 'real' shallow lateral heterogeneities, artificial anomalies with an opposite sign may show up in the deeper part of the model. When mode coupling is taken into account, as in the SEA inversion scheme, these biases do not occur. We have also studied possible bias effects as the result of using WKBJ in the EUR-S91 model of Zielhuis & Nolet (1994), which is characterized by strong velocity anomalies extending as deep as the transition zone. One of the most striking features in this model is the region of low velocities at larger depths beneath the fast Russian platform on the eastern side of the Tornquist-Teisseyre Zone (TTZ). Nolet & Zielhuis (1994) suggested that these low velocities are an indication for injection of water in the mantle by an earlier subduction. Our results show that the neglect of mode coupling indeed does bias the model towards a larger reversal of velocities with depth, and heterogeneities show up even where the S ray does not sample the mantle. However, low velocities deep below the eastern part of the TTZ persist even when mode coupling is taken into account.