Patent ID: 8463548

Claim:
A subterranean position detection apparatus for detecting the position of a drill bit of a drill string, said apparatus comprising: an acoustic signal generator, said acoustic signal generator being mounted in proximity to the drill bit; said acoustic signal generator having a known acoustic signature; sensors mounted remote from said drill bit; said sensors being mounted to monitor subterranean sound in the neighbourhood of the drill string; a signal processor connected to receive observations from said sensors; said processor being operable to establish a correlation between (a) data from a first of said sensors and (b) said known acoustic signature; said processor being operable to apply said correlation to data received at others of said sensors and to measure at least one of (a) respective phase shifts, and (b) travel times, therebetween; and said processor being operable to employ at least one of (a) said phase shifts, and (b) travel times, to estimate position of said drill bit; and said apparatus is one in which one of: (i) said acoustic signal generator includes a first portion and a second portion, and said signature includes a first component emitted by said first portion and a second component emitted by said second portion; and (ii) said known acoustic signature of said acoustic signal generator is a made up of a combination that includes a first signature and a second signature.