Advancements in Long-Offset Seismic Imaging: A Blind Test of Traveltime and Waveform Tomography

In 2003 a realistic long-offset synthetic seismic dataset was made available to the community for the purpose of testing modelling, inversion and imaging algorithms. Here we present the results of 2-D traveltime and 2-D waveform tomography applied by workers who, at the time, did not know what the true model was. The synthetic wide-angle dataset consisting of 51 shots was calculated for a realistic crustal model using a 2-D visco-elastic code; these data are still available at terra.rice.edu/department/faculty/zelt/ccss/. The model is 250 km long, and the shot and receiver spacings are 5 km and 90 m, respectively. The center frequency of the source is 5 Hz, with energy between 2-11 Hz. The true model contains large-scale features such as laterally-varying sediment thickness, a basement outcrop, a low-velocity zone, and regions where the crust-mantle boundary is sharp and smooth. Superimposed on this are non-stationary intermediate to wavelength-scale stochastic features. Both first arrival and simultaneous PmP/Pn traveltime tomography were applied to obtain a smooth velocity model with a sharp Moho. The traveltime model compares favorably with the large-scale features of the true model, although it does not capture the details of the low-velocity zone or the smooth crust-mantle transition zone. However, the model obtained from first-arrival traveltime tomography was essential as a starting model for the 2-D acoustic, frequency-domain waveform tomography method we have applied. Data windowing in time, re-weighting in offset, and model smoothing were applied, and a relatively low starting frequency was used, 0.8 Hz, progressing up to 7 Hz. The final model from waveform tomography predicts the input data to a high degree of accuracy for each chosen frequency, and a comparison of the original time domain data with time-domain forward modelling through the final model also reveals a good match. The final model from waveform tomography matches the large and intermediate-scale (down to ~1 km) features of the true model, including the recovery of the low-velocity zone and the structure of the crust-mantle transition. The combined results from traveltime and waveform tomography show the complementary nature of these approaches and the potential for the analysis of real wide-angle crustal data in the future.