The basic task of a zero-offset VSP is to match seismic events to specific interfaces. Because the depth of the well geophone is accurately known, time-depth relationships are established precisely, and thus reliable reflection identification and subsurface seismic velocities are obtained. Zero-offset VSPs are also used to identify multiples and other events, and to study reflections from below the bottom of the borehole (because of the increased resolution) to aid in deciding if a well should be deepened ("looking ahead of the bit"). Payne claims 15-25m resolution when looking ahead 600-1,200m.