(a) Record section for the vertical geophone of OBS0106 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. P
: crustal arrivals, P
: refractions from the uppermost mantle, P
P: mantle reflections. (b) Calculated travel time curves superimposed on the observed record section (top) for the final velocity model (bottom). Ray diagram is also shown in the bottom. (c) Calculated travel time curves (top) for the test model with high V
at the base of the crust (bottom). On the ENE side of the OBS, the calculated later arrivals could not reach over an offset of 50 km.