Modelling of systematic errors in borehole geophone orientation calibration
Pietro (Peter) Giuseppe Gagliardi, Donald C. Lawton
Using results from analytic and finite-difference 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. Effects due to errors in a well deviation survey were found, based on analytic modelling, to produce errors of a similar magnitude on orientation analysis; specifically, errors of ±2° in inclination and azimuth angle produced scatter of 2.53° over a sample of 48 geophones. The effects due to lateral raybending and anisotropy were characterised using finite-difference models; these produced one-cycle and two-cycle sinusoidal trends when orientation was plotted against source-well azimuth. In the case of a dipping layer, errors in geophone orientation were found to be 0° along the dip direction; in the case of an HTI medium, they were found to be approximately 0° along the fast and slow directions. This suggests that acquisition design of geophone orientation calibration surveys should be guided by knowledge of the geology in the area of interest.