Radiation pattern and seismic waves generated by a working roller-cone drill bit

The seismic body wave radiation pattern of a working roller‐cone drill bit can be characterized by theoretical modeling and field data examples. Our 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 drill string and body waves that radiate into the earth. The model predicts that P‐waves radiate 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 drill bit. This proposed seismic body wave radiation pattern is investigated with field data examples. The presence of the drill string ...