Drill-bit seismics have been used on land and in shallow-marine environments for many years. The technique uses acoustic energy radiated during drilling to pro,ide time/depth and formation-velocity information at the well site, allowing the position of the bit to be plotted on the surface-seismic section as drilling progresses. By generating look-ahead vertical-seismic-profile (VSP) images, the approach. to critical horizons, such as casing points or overpressured zones, can be monitored.