Near-surface velocity characterization at Priddis and installation of fibre-optic cables at Brooks, Alberta

Donald C. Lawton, Malcolm B. Bertram, Kevin W. Hall, Kevin L. Bertram, Marie Macquet

CREWES operates a shear-wave seismic source for multicomponent near-surface seismic studies. The source component of the system is a model A200 weight drop device with a 100 kg hammer accelerated by compressed nitrogen operating at 1000 psi. In 2016, we recorded a multicomponent walk-away vertical seismic profile at the Priddis Geophysical Observatory in to characterize the near-surface P-wave and S-wave velocity structure to a depth of 141m. P-wave data were collected with the source operating in a vertical force orientation. A pivot system enables the source to generate both P-wave and S-waves. The source mast can be operated in a vertical mode for generating P-waves and it can rotate +/-45 degrees transverse to the longitudinal axis of the trailer, in order to generate down-going P-wave and S-waves simultaneously. Pure-mode down-going Swaves are generated by subtracting records taken with the mast rotated in the positive and negative tilt modes. Good-quality zero-offset P-wave and S-wave VSP data recorded into the CREWES well with a 3C receiver spacing of 3.06 m. A thin surface layer and two thicker layers are interpreted from the first arrival P-wave and S-wave travel times, with the second layer thickness of 46 m. P-wave velocities are 2450 m/s and 3260 m/s and Swave velocities are 796 m/s and 1346 m/s respectively in the two deeper layers. This yields Vp/Vs values of 3.06 in the second layer, and 2.50 in the third layer.

At the Brooks Field Research Station, being developed by the Containment and Monitoring Institutes, both straight and helical optical fibres have been installed in a 350 m deep well and in a 1.1 km trench, to assess optical fibre recording for VSP and surface seismic surveys. Recording into the fibre will be undertaken in 2017.