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The Vertical Seismic Profile (VSP) method can often be used to assist in making drilling decisions. These decisions may involve anticipation of overpressured zones, distance to the seismically determined target, and verification of geologic or geophysical interpretation.
A VSP is recorded with a seismic source on the surface and receivers in the borehole. Both up and downgoing waves are recorded and are separable. Layers beneath the borehole are recorded in the upgoing waves at every receiver position. This redundancy can be exploited to achieve a high signal-to-noise ratio and good quality time-amplitude information. The conversion of amplitude to acoustic impedance gives time and interval velocity with density held constant. Depth is then a function of time and interval velocity.
A VSP has several advantages over surface seismic data in inversion. These include knowledge of attenuation, waveform, and multiples from averaging downgoing waves. The borehole coverage allows use of a control zone for establishing optimal inversion parameters. Averaging of upgoing waves gives an unusually good signal-to-noise ratio so that deconvolution with the averaged downgoing waves yields an excellent estimate of primary reflections. These considerations, combined with the favorable geometry of the VSP, provide for considerable accuracy in estimation of velocity and depth below the bit.
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