THE USE OF DRILL-BIT ENERGY AS A DOWNHOLE SEISMIC SOURCE

A new wellbore seismic technique uses the vibrations produced by a drill bit while drilling as a downhole seismic energy source. The technique is described as "inverse" VSP because the source and receiver positions of conventional VSP are reversed. No downhole instrumentation is required to obtain the data and the data recording does not interfere with the drilling process. These characteristics offer a method by which borehole seismic data can be acquired, processed, and interpreted while drilling. Interchanging the conventional VSP source and receiver positions improves the efficiency of recording multioffset surveys for imaging a 3-D data volume in the borehole vicinity. 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 at selected positions around the borehole. The pilot signal is crosscorrelated with the receiver signals to compute traveltimes of the arrivals and to attenuate incoherent noise. Deconvolution and time shifts of the pilot signal compensate for the effects of propagation from the drill bit to the top of the drillstring. By repeating this process for an interval of the well, a VSP-equivalent data set is generated. Results from a test well demonstrate that the processed drill-bit data are comparable to conventional VSP data.