Effects of horizontal transverse isotropy on determination of the orientation of buried geophones

Peter Gagliardi and Don C. Lawton

ABSTRACT

The effects of horizontal transverse isotropy (HTI) on geophone orientation calibration were examined, under the assumption of weak anisotropy. It was found that values of the Thomsen parameters δ and ε as low as ±0.05 could produce errors in geophone orientation calculations of as much as 5°; it was also found that the maximum error in this angle was more sensitive to ε than δ. A model was created using ε = 0.1 and δ = 0.025, which resulted in a maximum polarization angle error of 6.45°. Using this model, values for apparent polarization error were found for various source locations; these values, when plotted as a function of source-receiver offset, produced distinct trends depending on the orientation of the source-well line. Further investigation of these errors revealed that they asymptotically approached a particular value as offset increased, due to the behaviour of the source-receiver azimuths. When geophone orientation angle errors were plotted as a function of source-receiver azimuth, it was found that all lines followed the same trend, regardless of their orientation. The results of this study show that HTI media should be taken into consideration when undertaking orientation calibration for buried microseismic arrays or geophones used in vertical seismic profiles.

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