Picking travel times.

Figure. This plot shows only the associated arrivals, with all sources corrected to 0 km depth. The predicted travel time curves are from the IASPEI tables, for a source depth of 0 km. Note that the 'ghosting' visible preceding the P-arrivals could be due to a single uncataloged preshock. From: Albuquerque Seismological Laboratory Data Collection Center

How to obtain consistent and high-quality picks (of large volumes of seismograms) is still an active area of research in Seismology. Also check out the following site of the University of Washington. A group there is picking global P phases in order to improve the precision of P phase travel time curves.

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