This invention lies in the field of elastic wave generation and reception in the earth. More particularly, it is concerned with the determination of the position in three dimensions of the point at which an elastic wave is generated in the earth.
Still more particularly, it concerns the generation of a seismic wave at the bottom of a long liquid filled pipe in a deep borehole in the earth, receiving the seismic wave at a plurality of sensors in the earth, and determining the position of the bottom of the pipe.
Alternatively, the invention is concerned with initiation of a seismic wave at a selected shallow depth in a water layer overlaying the earth's surface for the purpose of determining the geologic structure of the earth to a selected depth below the water-earth interface.
In the prior art, various means have been devised for determining the position of a borehole in North-South, East-West coordinates, at selected depths during a drilling operation. This has been done by using survey instruments specially designed for introduction into the borehole, and sometimes introduced through the drill pipe, which, by their internal mechanism make a record of the slope, (or angle with the vertical) of the borehole at each of a plurality of selected depths, and a measure, in relation to the magnetic compass, of the azimuth of the slope of the borehole. Knowing the direction of slope, and magnitude of slope, at each of a plurality of selected known depths, and assuming that the borehole is straight in between the survey points at which measurements are made, the profile of the borehole in three dimensions can be determined.
A principal disadvantage of this particular system is that it requires a stoppage of the drilling process so that the drill pipe can be broken and the survey instrument inserted by a wire line into the drill pipe. Since the cost of operation of the drill rig runs into many thousands of dollars a day, time lost from the drilling operation, to make these measurements, is extremely expensive. There is understandable reluctance on the part of drillers to make these necessary measurements, so that at some future time, when this knowledge is needed, it will not be available and cannot be easily obtained.