DETAILED DESCRIPTION It is a primary object of this invention to provide a simple and inexpensive means for independent determination, at any selected time, a measurement indicative of the position of the bit and the bottom of the borehole during a drilling operation.
It is a further object of this invention to provide a method of determining the position of the bottom of the borehole without interrupting the drilling operation.
It is a still further object of this invention to provide a continuing operation, whereby the position of the bit can be determined at a plurality of selected depths, spaced a selected number of feet apart in depth, whereby a continuous log of the bit position, with time, is obtained.
These and other objects are realized and the limitations of the prior art are overcome in this invention by generating weak seismic waves at the bottom of the borehole, at selected intervals of time.
These weak seismic signals are detected at each of a plurality of geophones near the surface of the earth, positioned in at least a two-dimensional array around the anticipated position of the bit.
The geophone signals are digitized and temporarily stored, and then stacked for a great number of repetitions.
Because of the weak seismic source and the great seismic noise, the signal to noise ratio (S/N R) will be very poor, and it is anticipated that many hundreds, or thousands, of repetitions and stacking will be required to obtain a useful signal.
A number of ways have been shown by which such weak seismic waves can be generated.
Some of these are illustrated in U.
S.
Pat.
Nos.
3,817,345; 3,979,140; 3,979,724; 3,993,974; and 4,003,017.
A first way might comprise means to raise the drill pipe a selected small distance and drop it, so that the bit strikes the bottom of the borehole.
A second way involves momentarily stopping the mud flow and building up a dynamic water hammer pressure in the liquid column in the pipe, and then suddenly releasing the mud flow through the bit.
A third general method involves generating a shock wave in the liquid in the drill pipe near the surface, which shock wave will travel down the pipe to the bit, and when expelled from the bit will generate a seismic wave
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