H. Marquering et al., WAVE-FORM INVERSIONS AND THE SIGNIFICANCE OF SURFACE-WAVE MODE-COUPLING, Geophysical journal international, 124(1), 1996, pp. 258-278
If body-wave phases in laterally heterogeneous media are modelled by s
urface-wave mode summation, mode coupling is required to bring out the
ray character of the body-wave phases. However, it is frequently assu
med that in laterally heterogeneous media surface-wave modes propagate
without interacting with other modes. The effects of neglecting surfa
ce-wave mode coupling on the results from waveform inversions includin
g body-wave arrivals have been studied by performing inversions with a
nd without taking surface-wave mode coupling into account. The two inv
ersion schemes are similar to the Partitioned Waveform Inversion (Nole
t 1990) but differ in the approximations used to compute the synthetic
s: WKBJ, which neglects mode coupling, or SEA (Marquering & Snieder 19
95), which does take mode coupling into account. Synthetic inversion e
xperiments show that neglecting mode coupling can lead to biases in th
e deeper part of the model. Their most striking feature is that for 'r
eal' shallow lateral heterogeneities, artificial anomalies with an opp
osite sign may show up in the deeper part of the model. When mode coup
ling is taken into account, as in the SEA inversion scheme, these bias
es do not occur. We have also studied possible bias effects as the res
ult of using WKBJ in the EUR-S91 model of Zielhuis & Nolet (1994), whi
ch is characterized by strong velocity anomalies extending as deep as
the transition zone. One of the most striking features in this model i
s the region of low velocities at larger depths beneath the fast Russi
an platform on the eastern side of the Tornquist-Teisseyre Zone (TTZ).
Nolet & Zielhuis (1994) suggested that these low velocities are an in
dication 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 hete
rogeneities show up even where the S ray does not sample the mantle. H
owever, low velocities deep below the eastern part of the TTZ persist
even when mode coupling is taken into account.