DAS trace location assignment at the CaMI.FRS fibre loop
Kevin W. Hall, Donald C. Lawton
As our knowledge of the optical fibre loop at the Containment and Monitoring Institutes Field Research Station (CaMI.FRS) continues to evolve, we are able to assign x, y and z coordinates to seismic traces recorded upon the loop using various interrogators. For example, gyroscope surveys conducted on observation wells 1 and 2 (OBS1 and OBS2) in the past year confirm that neither well is perfectly vertical. Using this updated information, we have built a trace geometry model that can be easily adjusted for varying trace spacings, uncertain cable lengths, fibre indices of refraction (actual and as used in interrogator software), and other unknowns. For downhole data with up- and down-going fibre, we may exploit symmetry by coarsely locating the bottom of the well using cross-correlation, fine-tuning using stack-power in sliding windows over a small trace range (+/- 5 traces), and applying the geometry from our model. This strategy works well even for noisy shots, where cross-correlation by itself gives slightly varying answers from shot to shot. Quality control of observation well data thus far has been by inspection of interleaved up- and down-going DAS data sorted by true vertical depth (Figure 1) as well as stacks and residuals. Comparisons of straight and helical fibre data from the wells and the trench have not progressed beyond interleaving data sorted by true vertical depth or easting. Stacking will require a careful trace interpolation step to compensate for differing effective trace spacings.