Geophone orientation analysis in a 3D VSP survey, Alberta
Pietro (Peter) Giuseppe Gagliardi, Donald C. Lawton
Geophone orientation azimuths were found from 3D and 2D VSP data, acquired near Lousana, Alberta, in order to examine any dependence of computed geophone orientation on source-well offset or azimuth. Additionally, a comparison was made between analytic and hodogram methods. The 2D dataset consisted of three lines; the standard deviation for this survey was 0.67° for all lines, 0.45° for the east line, 0.41° for the southeast line and 0.55° for the south line. Removal of sources less than 500 m (approximately 1/2 of the geophone depth) significantly improved the scatter in this dataset. Standard deviation in orientation azimuths for all lines was found to be 0.90° using the hodogram method; thus, while the both methods performed well, the analytic method produced more consistent results. The 3D dataset was divided based on source-well azimuth into bins with centers trending 0°-180°, 45°-225°, 90°-270° and 135°-315°. There appeared to be little dependence on source-well sector azimuth, which is expected for flat, isotropic geology near the well. Standard deviation in orientation azimuths were found to be 1.74° using the full 3D dataset. Offsets were binned into ranges of 0-600 m, 600-950 m, 950-1300 m, 1300-1650 m and greater than 1650 m. Scatter in rotation angles was shown to be strongly dependent on offset, with the most constrained results in the 1300-1650 m offset bin. The optimal offset range for geophone orientation calibration was found to be between 1 and 2 times the receiver depth.