Seismic-While-Drilling (SWD), also termed TomexR is a technique that monitors the
seismic signals created during rotary drilling using a pilot sensor located near the top of
the drillstring along with 'earth' sensors deployed near the surface of the earth. Cross-correlations
between the pilot sensor signal and each of the earth sensors are used to
attenuate incoherent noise and to compute traveltime differences between signals
received at the earth sensors and signals received at the pilot sensor. By compensating for
the delay and multipath filtering created by propagation in the drillstring, the correlated
signals can be processed to create a data set equivalent to an inverse VSP, with a point
source at the drill bit and a receivers near the earth's surface. Real-time information from
the SWD data such as interval and average velocities and reflectivity ahead of the bit are
used commonly by the oil industry to position the drill bit on the (time) seismic section,
predict overpressure, and steer the bit in the case of deviated wells. In some cases,
logging runs cannot be made after the hole is drilled (due to stability problems), and
SWD data may be the only borehole geophysical data recorded. The attractive
characteristics of SWD make it a natural for ODP cruises, where hole conditions
sometimes make it impossible to perform wireline logging.
