Vertical Seismic Profiles While Drilling
A vertical seismic profile can be created by lowering a geophone into the wellbore and using surface energy sources, as described above. A measurement while drilling technique, commonly called a TOMEX survey, uses the vibrations from the drill bit as a downhole energy source, using surface geophones to record the signal. The significant advantages include broader spatial coverage, easier access to rugged terrain, and much more detailed recording versus depth in the borehole. Multiple offset and radial 3-D surveys are much cheaper with TOMEX than with wireline VSP.

The continuous signals from the drill bit are transformed into VSP displays using cross correlation and de-convolution. The energy source signature is obtained by continuously recording the vibrations on the drill pipe. The time lag for the reference signal to travel up the pipe is calculated from the length of the pipe and the velocity of sound in steel. Cross correlation is used to define the signature in the same way as for Vibroseis sources.

De-convolution of the recorded geophone traces with this signature produces the seismic traces. Traces are stacked continuously as drilling proceeds and an output trace is generated every 10 feet of drilled depth. By taking into account the geometry of the drill bit and geophone locations, the VSP section can be displayed. Data quality is comparable to conventional VSP data, as shown in the comparative example below. Rig noise can sometimes be a problem, but it can usually be cured.

 

VSP while drilling

The geometry of the system and the de-convolved traces versus depth are shown below. Because of the close spacing, a very accurate time - depth plot can be created. This allows continuous calculation of an interval velocity log, equivalent to a sonic log filtered over a ten foot span.

VSP while drilling - geometry and recorded traces after de-convolution

Time - depth plot from VSP while drilling