Drill bit wavefields

Abstract

The seismic body wave radiation pattern of a working rock bit can be characterized by theoretical modeling and field data examples. My model of drill-bit signal generation is a pseudo-random series of bit tooth impacts that create both axial forces and tangential torques about the borehole axis. Each drill tooth impact creates an extensional wave that travels up the drillstring and body waves that radiate into the Barth. The model predicts that P waves radiale primarily along the axis of the borehole, and shear waves radiate primarily perpendicular to the borehole axis. In a vertical hole the largest P waves will be recorded directly above and below the drill bit; whereas, the largest shear waves will be recorded in a horizontal plane containing the drill bit. In a deviated borehole, the radiation patterns should be rotated by the inclination angle of the bit. The proposed seismic body wave radiation is investigated with field data examples using multichannel arrays of vertical geophones. The modeled radiation pattern is also investigated with a three-component inverse VSP data example shown in Figure 1 . The variation in the signal-to-noise ratio witti drill bit depth and the partitioning of S-wave energy between radial and transverse components is consistent with the proposed radiation pattern. A quantitative measure of the drill bit radiation pattern, taken from calculating the rms ratio between the shear wave direct arrival on the radial component and the P-wave direct arrival on the vertical component, is also consistent with the proposed radiation pattern .