David J. Secord

A method of computing generalized ray theory (GRT) synthetic seismograms suitable for marine seismic refraction OBS studies has been established. The method employs FORTRAN programs to compute and evaluate raypaths, to compute the impulse response for each worthwhile raypath by Cagniard-de Hoop method, to sum the impulse responses and to convolve the summed impulse responses with source and receiver operators to arrive at the synthetic waveform.

Vertical component (uz) synthetic seismograms were computed for the same parameters by the GRT and reflectivity methods. Differences in the results are attributed, in part, to different boundary conditions at the sea floor where the OBS resides. Other causes for the differences have not been resolved.

A simple layered model for oceanic crust to the northwest of the Explorer Ridge off western Canada was used as a starting model for the computation of record sections of vertical component synthetic seismograms. Alterations to the starting model brought the synthetic seismograms into better agreement with the observed traces. There is evidence for a gradual transition between crustal layers 2 and 3 rather than a sharp discontinuity. A large PPS event, in evidence on the observed seismograms, is not observed on the synthetic traces. This occurrence is probably due to an incorrect approximation to velocity variation within the thin sediment layer.

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Supervisor: M. J. Keen