Method and a system for determining the position of a drill bit

#The present invention relates to a method and a system for determining the position of a drill bit.

#WO01/5268A1 relates to a method for estimating the position and trajectory of a drill during drilling of boreholes in rock. WO01/75268A1 presents an adaptation to drill positioning of a method used for determining the relative position of similar microearthquake events described in Geophys. J. Int. (1995), 123, 409-419, by Slunga, Rögavaldsson and Bödvarsson. The method in WO01/75268A1 comprises the steps of determining the starting position of the drill bit and recording the received transient seismic waves generated by the drill on impact, and estimating the difference in arrival time between events. Using these estimations, the relative distances between the positions of the drill bit at different events are calculated and from these the present position is calculated.

#The method described in WO01/75268A1 is particularly suitable when the length of the bore hole to be drilled is limited. However, when the drilling process reaches further, the properties in the rock may have changed considerably, in turn changing the propagation velocities of the seismic waves generated by the drill bit. Further, these changes are impossible to predict. Also, the propagation velocity differences may increase further by other discontinuities in the rock, such as cracks.

#Below, the invention and further advantages thereof will be described in greater detail with reference to the drawings, in which

#FIG. 1 shows schematically a vertical cross-section of an inhomogeneous rock, with a drill, geophones and wave propagation through the rock,

#FIG. 2 shows in a time-domain, signals detected by four geophones,

#FIG. 1 shows a drill bit 2, its trajectory 3 from the surface 4 to its present position in the ground, and five seismic wave detectors in the form of geophones 1a-e at various positions in the ground. Computation means C, e.g. one or more computers, with computational and storage capabilities, is adapted to receive signals from the geophones. It should be noted that in the method according to the invention for determining the position of the drill bit, alternatively, only four, or more than five geophones can be used. To both determine the position of the drill bit and recalculate the wave propagation velocity, signals from at least four geophones are required. However, in practice, with a greater the number of geophones, more accurate results can be obtained.

#In order to establish an absolute position of the drill bit, a starting position rstart of the drilling is necessary, and a position of each of the geophones 1a-1e. The starting position rstart relative to the positions of the geophones can easily be calculated by the computation means C since the absolute geographical positions of the geophones and starting position rstart easily can be obtained by any suitable positioning system, such as GPS (Global Positioning System) or direct measurement. If the actual starting position is unsuitable for use as starting position rstart for the drill positioning in the drilling process, an alternative starting position can easily be determined, in a manner known in the art, by using measurement data from the geophones and/or measuring the length of the drill rod fed into the bore hole.

#By means of the geophones data relating to transient seismic waves generated at the drill is recorded during drilling. For each of the geophones 1a-e, a seismic wave propagation velocity between the drill bit and the respective geophone is determined, based on a position of the drill bit, the position of the respective geophone, and seismic wave data recorded by the respective geophone. For example, the seismic wave propagation velocity between the drill bit and a geophone can be determined by correlating seismic wave data recorded at a known position of the drill bit, for example the starting position rstart, and seismic wave data recorded by the geophone, to obtain a time of travel for the waves, and obtaining the velocity by dividing the distance between the drill bit and the geophone by the time of travel.