This invention is in the field of elastic wave generation and detection in the earth. More particularly, it is concerned with determining the position in the earth of the drilling bit during a drilling operation in a deep borehole.
In the prior art, various means have been devised for determining the position of the borehole in north-south and east-west coordinates, at selected depths during a drilling operation. This has been done by using survey instruments specially designed for introduction through the drill pipe, which by their internal mechanisms make a record of the slope of the borehole at each of a plurality of selected depths, as well as a measure, in relation to the magnetic compass, of the azimuth of the slope of the borehole.
The disadvantage of this particular system is that it requires a stopping of the drilling process so that the drill stem can be broken and the survey instrument inserted on a wire line into the drill pipe. Since the cost of operation of the drill rig runs into many hundreds or thousands of dollars a day, time lost from the drilling operation makes these measurements extremely costly.
More recently several patents have been issued which involve a seismic method of measurement involving the use of a seismic source at or near the bit and detecting the first arrivals of the seismic waves at a plurality of geophones, positioned in a two-dimensional array around the borehole, and determining the position of the source by measurement of travel times of the seismic waves to each of the geophones.
These methods hold some promise, except that they involve the need for a high-energy seismic source at the bit, to provide a useful signal from the geophones. It is extremely difficult to provide large-energy seismic waves without interrupting the drilling process.