Abstract
Drill-string waves can be successfully used as reference pilot signals for drill-bit seismic-while-drilling (SWD) purposes. The seismic signals obtained by correlating pilot and geophone measurements are disturbed by the drill-string reverberations because the pilot waves are reflected at each interface between string sections with different acoustic impedances. Inverse filtering of these reflections, using a reference-pilot deconvolution calculated in the presence of additional noise, may cause signal distortion. To overcome this problem, we consider using dual-sensor measurements in the drill string to remove the reflections of the drill-bit waves in the acquisition phase and to improve pilot deconvolution. We measure acceleration and strain of drill-string dual fields, which have opposite reflection coefficients and, in a string of constant elastic properties, the same transmission coefficients. These quantities are scaled to fit the amplitude of the direct arrivals, summed to remove the reflections in the drill string and in the rig, and may be deconvolved by Einstein deconvolution to characterize the reflection coefficient between the drill bit and the formation. Synthetic numerical examples and real measurements acquired downhole in a location close to the bit show that upgoing and downgoing drill-string pilots can be separated using dual fields and jointly used to improve the SWD seismograms.
