Orientation Azimuth Calibration of Borehole Geophones
Pietro (Peter) Giuseppe Gagliardi
Using results from field experiments and forward modelling, factors affecting the orientation calibration of borehole geophones were investigated. Well deviation, lateral raybending and anisotropy were all found to produce systematic deviations in orientation analysis. A method was developed to compensate for effects due to a deviated well, and successfully applied to a field dataset. The effects due to lateral raybending and anisotropy were characterised using analytic and finite-difference models; these produced one-cycle and two-cycle sinusoidal trends when orientation was plotted against sourcewell azimuth. Significant evidence of these trends was found in field examples, in that the magnitude of the trends was greater than the standard deviations in orientation azimuth.
Analytic and hodogram methods of orientation analysis were compared, and it was found that the analytic method produced more consistent results. The optimal size of the analysis window was related to the wavelength of the direct P-wave arrival, but should generally be determined experimentally. Finally, it was shown that the optimal source-well offset range for calibration surveys was between 1 and 2 times the receiver depth.