Use of Principal Component Analysis to Determine Down-hole Tool Orientation and Enhance SH-Waves

Abstract

A common problem in down-hole shear wave surveys is the determination of the tool rotation relative to the seismic source polarization direction. This paper reports an algorithm which has been developed to solve for this angle in the horizontal plane. The method employs Principal Component Analysis (PCA) to determine the angle from the large motion of the SH-wave (not first motion). Once the angle is known, the horizontal component data may be rotated so that one component is aligned with the source polarization, and the other orthogonal to it.Significant findings include the following. First, subtraction of oppositely polarized source efforts improves the PCA formulation by enhancing the S-waves. Second, the Swave enhancement is improved by scaling the oppositely polarized source efforts by the vertical (not horizontal) component data, as this provides a better attenuation of non-Swave energy. Third, observation of the tool orientation as it exits the borehole is helpful in resolving the 180 degree azimuthal ambiguity in the PCA approach. Finally, the polarization of the source radiation may drift relative to the axis of the source during a survey, and should not be assumed invariant.

Field, laboratory, and modeling investigation of the skin effect at wells with slotted casing, Boise Hydrogeophysical Research Site

“…The drive casing was not removed easily and the considerable jerking and vibration during drive casing removal with the oversize drive shoe undoubtedly contributed to formation collapse as well. Also, geophysical testing for vertical seismic profiles of each well using a geophone pressed against the inner borehole wall with a bowspring indicated good mechanical coupling to the formation through the well screen (Michaels, 1998;2001) and generated complete profiles without intervals of ringing or highly attenuated and noisy signals that would indicate zones of poor collapse (Dr P. Michaels, 2001, personal commun.). Also, the vast majority of the aquifer is cobbles and sand (Barrash and Reboulet, 2004) with framework cobbles which are not likely to compact and cause significant decrease in K in the same sense as more 'pliable' sands or finer-grained sediments might.…”

The Influence of Wellbore Inflow on Electromagnetic Borehole Flowmeter Measurements

“…A core, drive, and drill technique was used to construct the wells at the BHRS with minimal disturbance of the formation including natural collapse of the formation against the well screen after removal of the drive casing (e.g., Morin et al 1988). Collapse of the formation against the polyvinyl chloride (PVC) screen was anticipated, then confirmed with vertical seismic profiling (Michaels 1998, 2001). The potential for small regions of incomplete collapse makes bypass flow around blockage of the borehole by a packer or skirt an issue.…”

Uncertainty in a Geophysical Survey for a Bridge Foundation Design

“…The down-hole geophone was a BH-2 3-component sidewall clamping geophone (Crice 1996). Downhole geophone orientation was done by principle component analysis of horizontal component hodograms (Michaels 2001b).…”