Depth calibration of fibre-optic distributed vibration sensing measurements

Abstract

Depth calibration for conventional vertical seismic profile (VSP) tools is relatively straightforward. For fibre-optic distributed vibration sensing (DVS) measurements, however, depth calibration is more problematic and has been identified as a particular drawback. In this paper we describe four different methods for calibrating the depth of DVS recording channels. The end-of-fibre and freeze methods can be applied to both permanent and temporary fibre installations whereas the downhole source method is primarily applicable to permanent installations. Methods based on the raw backscatter measurements are more likely to be accurate because they do not involve any effects from the processing stages required to extract the phase. The freeze and downhole source methods both suffer from problems associated with picking an amplitude peak that has been smoothed by the gauge length. If a permanently installed fibre must be depth calibrated at a variety of depths, the downhole source appears to be the most promising method. The error associated with the hammering technique is significantly larger than the other techniques and thus we do not recommend its use.