POLARIZATION MEASUREMENT UNCERTAINTY ON THREE-COMPONENT VSP

摘要

To determine the downhole orientation of the sonde on a three-component vertical seismic profile (VSP) recorded using three offset sources at different azimuths, the azimuth of the direct P arrival for each source with respect to a horizontal geophone axis was found using particle polarization analysis. The azimuthal angles between the sources as determined by polarization analysis were compared with the actual azimuths between the sources to determine the error in polarization measurement. The least scatter in azimuthal measurements for the three sources at a given depth was found to occur for measurements made on a time window over which the particle motion had the most rectilinear behavior. This window occurred after the first break and preceded the peak of maximum amplitude. Uncertainties in azimuthal measurement and signal-to-noise (S/N) ratios were found over four depth intervals on the field VSP, with azimuthal uncertainty ranging from 1.0 degrees at a S/N ratio of 9.0 to 5.6 degrees at a S/N ratio of 3.7. Cases were found for which the azimuthal measurement was most likely in error for only one of the three sources at a given depth, and after these errors were removed, the uncertainty in polarization measurement reduced by as much as 60%. Polarization uncertainties on a model consisting of a sinusoidal three-component signal contaminated with band-limited Gaussian noise exhibited values similar to those for the field VSP at comparable SIN ratios. [References: 7]