Depth calibration of fibre-optic distributed vibration sensing measurements
T. Dean, T. Cuny, A. Constantinou, P. Dickenson and E. Hamouche
Journal name: First Break
Issue: Vol 36, No 3, March 2018 pp. 29 - 34
Info: Article, PDF ( 829.4Kb )
Price: € 30
Depth calibration for conventional vertical seismic profile (VSP) tools is relatively straightforward, typically being achieved by correlating gamma-ray logs. For fibre-optic distributed vibration sensing (DVS/DAS) measurements, however, calibration is more problematic because the location of positions along the fibre cannot be readily correlated to wellbore depth. Here we describe five methods for depth calibrating DVS data. The end-of-fibre and freeze methods can be applied to permanent and temporary fibre installations whereas the downhole-source, hammering-on-thewellhead, and symmetry-detection methods are 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, downhole-source is the most promising method. The symmetrydetection technique is straightforward and robust but relies on the fibre following the same path into and out of the well. The error associated with the hammering technique is significantly larger than the other techniques and we do not recommend its use.