Patent ID: 7889596

Claim:
A method for determining the position of a drill bit comprising determining a starting position ( r start ) of the drill bit, determining a position of each of a plurality of seismic wave detectors, recording, by means of the detectors, data relating to transient seismic waves generated at the drill bit, identifying, based on the seismic wave data from the detectors, events (k 1 , k 2 ) at the drill bit, determining for each of the detectors a seismic wave propagation velocity between the drill bit and the respective detector, determining a plurality of relative positions of the drill bit, based at least partly on the seismic wave propagation velocities and differences in arrival time (t d obs (i, k 1 , k 2 )), at at least some of the detectors, of seismic waves from events (k 1 , k 2 ), and determining, based at least partly on the starting position ( r start ) and a sum of the relative positions, an absolute position of the drill bit, wherein the method comprises determining for each of at least some of the detectors, a first seismic wave propagation velocity (v 1 1a , v 1 1b , v 1 1c , v 1 1d , v 1 1e ) between the drill bit and the respective detector, determining, based at least partly on first seismic wave propagation velocities relating to at least some of the detectors, a position of the drill bit at a second event (k 2 ) in relation to a position of the drill bit at a first event (k 1 ), and determining a second seismic wave propagation velocity (v 2 1a ) between the drill bit and at least one of the detectors, based at least partly on the determined position of the drill bit at the second event (k 2 ).