Waveform Inversion of Seismic Refraction Data and Applications to Young Pacific Crust

摘要

We present a method for constraining the velocity-depth structure in the Earth using seismic refraction waveform data. We test the method with synthetic 'data' from known models, and apply it to a set of data collected in June 82 from the East Pacific Rise at 13 deg N. In this iterative process WKBH seismograms are computed for a starting model; the difference between these and the observed seismograms is used to update the model subject to physical constraints. An important first step in the inverse scheme is the linearization of the WKBJ seismogram equation, allowing us to compute 'differential seismograms', partial derivatives of the synthetic seismogram with respect to specific model parameters. This linearization provides the means for estimating required model perturbations, based on the misfit in the seismograms. The choice of a suitable numerical strategy for computing an updated model is a crucial second step in formulating a working algorithm. Because the data contain noise, synthetic seismograms can only fit the data to this noise level. In this case, infinitely many models fit the data to this tolerance, and some of these estimates are non-physical, involving negative layer thicknesses. A successful strategy must choose a well-defined, physically resonable new model from among the possibilities.