Abstract: Technique of seismic while drilling uses the vibrations produced by a drill bit while drilling as a downhole seismic energy source. The continuous signals generated by the drill bit are recorded by a pilot sensor attached to the top of the drillstring and by receivers located around the borehole. Signals from the reference sensor are used as a pilot signal for cross-correlation with signals from the surface geophones. The cross-correlation method accomplishes two important objectives. First, the cross-correlation function is a measure of the
travel time of drill-bit generated energy up through the strata to the surface geophones minus the travel time from the drill bit up the drill string to the reference sensor. Even though the drill bit vibrations are continuous, in the cross-correlation function they are compressed into pulse-like events. Second, the cross-correlation process also enhances low-level, drill-bit generated signals from weak reflectors. The pilot signal is cross-correlated with the receiver signals to compute travel times of the arrivals and to attenuate incoherent noise.

Key words: Correlation, Seismic while drilling, Seismic wave, Signal