Velocity tomography and reflectivity imaging using crosswell seismic data from the Noel tight gas field
Joe Wong, Robert R. Stewart
We analyze data from a crosswell seismic survey that was conducted in 2004 at the Noel gas field located in northeastern British Columbia and operated by BP Canada Energy Company. The goal was to create high-resolution images of gas-bearing, but tight, sandstone channels 2400 m to 2650 m deep in the Cadomin and Nikanassin formations. The survey employed hydrophones and a piezoelectric vibrator source operating between two wells approximately 145 m apart. The dominant frequencies in the crosswell seismograms are about 1.0 kHz to 1.5 kHz, with wavelengths on the order of 4 m to 6 m. Compared to wavelengths of about 100 m that are usual for surface seismic data, the shorter wavelengths in the crosswell data provide significantly better resolution of beds 10 m to 25 m thick. A P-wave velocity tomogram was created from first arrival times using a back-projection algorithm that accounts for head-wave arrivals. Wavefield separation isolated up-going and down-going reflections. These reflections were mapped with a VSP/CDP method that used ray-tracing guided by the velocity tomogram. Seismic boundaries on the velocity tomogram and the reflectivity image correlate closely with the known geology.