| 摘要 |
A method for producing an inverse vertical seismic profile using a seismic source place down a borehole has been invented. The directly transmitted waves from the seismic source and the secondary reflectances thereof are detected by a sensor(s) on the earth's surface. The method depends only on knowing the relative arrival times at the sensor of the direct waves and the secondary reflectance waves. A pseudo-reflection seismogram is produced by (1) Autocorrelating a recording of a long series of random pulses, such as those produced by a drill bit, to produce a one-sided autocorrelation function containing peaks corresponding to the direct waves, primary and secondary reflectances; (2) Stacking (adding the amplitudes for equivalent time points) a plurality of autocorrelation functions obtained at a single depth in the borehole to reduce noise; (3) Producing a series of stacked one-sided autocorrelation functions for different depths in the borehole to yield an IVSP; and (4) Vertically stacking together the one-sided autocorrelation functions from the series of recordings to remove peaks due to primary reflectances. The invented method also provides means to derive the interval velocities for individual geologic strata and to determine the position of the source relative to the IVSP and to a geologic horizon. The IVSP, source positions, and interval velocities can be determined at the well site in approximately real time. A single, one-component seismic sensor is sufficient for the recordings required to produce the IVSP. Either polycrystalline diamond bits or tricone bits may be used as seismic sources. However, the method is not limited to continuous randomly generated seismic sources and may also be used with periodic or impulsive sources.
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