(a) Observed record section (top) and synthetic seismograms (bottom) for the vertical geophone of OBS063 on SPr10. The position of the OBS is shown by an arrow in Figs. 1 and 3. A gain factor proportional to distance has been used to enhance the distant seismograms. The reduction velocity is 8 km/s. (b) Record section (top) and ray diagram (bottom) for OBS050 on line SPr11. Calculated travel time curves are also plotted on the record section.