Borehole geophone calibration experiment

Pietro (Peter) Giuseppe Gagliardi, Henry C. Bland, Donald C. Lawton

Using an Envirovibe vibrator source, calculated geophone orientation azimuths for an 18-level tool were examined in order to determine the effects of stacking and correlation on orientation analysis. Sweeps were 20 s long, and their range was either 10-80 Hz or 10-200 Hz. Using all data points, uncorrelated traces produced standard deviation in the azimuths of 7.90 prior to stacking, and 3.74 after stacking; for correlated traces this was 5.84 prior to stacking and 5.54 after stacking. When the data were split into the separate sweep frequencies, it was found that the higher frequency sweep resulted in less scatter for correlated data, while having little effect on the uncorrelated data. The best statistics were obtained using uncorrelated stacked traces, using shots recorded with the 10-200 Hz sweep, giving a standard deviation of 3.62. While this is an acceptable amount of scatter, it will still produce errors in offset of more than 6%. It is suggested that the number of shot points, which totalled 11 in this study, should be increased in order to obtain better precision.