Guide to the Interpretation of Crustal Refraction Profiles

Summary Using the reflectivity method, synthetic, vertical-component seismograms have been computed for simple velocity transitions appropriate to crustal models. Record sections of the synthetic seismograms including refracted and reflected phases aid in elucidating the fine structure of the crust since they illustrate the effects of various velocity models. The computed record sections indicate the importance to interpretation of secondary arrivals such as near-vertical incidence and wide-angle reflections and illustrate the following: (1) an upper crustal low velocity layer (LVL) can best be recognized from reflections from the top of the LVL; (2) wide-angle reflections are nearly always of larger amplitude than head waves and have apparent velocities indicative of the layer velocities; (3) head wave amplitudes are greatly affected by velocity gradients within the refracting layer; (4) near-vertical incidence reflections are of significant amplitude principally for reflections from first-order velocity discontinuities; and (5) comp!ex wave patterns result from relatively simple upper crustal models which contain an LVL and serve to emphasize the need for detailed amplitude studies so that amplitudes of head waves and near-critical reflections may be considered. Synthetic seismograms which closely approximate observed seismograms for a detailed refraction line in the eastern Basin and Range province are used to infer fine structure of the crust. Synthetic record sections computed for a Basin and Range and an eastern United States crustal model illustrate different amplitude-timedistance features which are characteristic of observed seismic data from these regions.