Refraction analysis of the Blackfoot 2D-3C data

Jocelyn Dufour, Donald C. Lawton

A three-component 2D seismic line was acquired in the Blackfoot Field area in 1995 by the CREWES project. The area is covered of glacial deposits and shale. These low velocity near-surface deposits induce time delays for the recording of P-wave and S-wave reflection seismic data. Refraction analysis can help resolve these delays by determining near-surface models. S-wave refractions were identified on the radial component of the Blackfoot 2D-3C dataset and were used to establish a S-wave near surface model, while P-wave refraction were used to build the P-wave model. Two refraction method were used: the Plus-Minus time analysis method and the Generalized Linear Inversion method. The subweathering thickness for both P and S-wave near-surface model was established at 94 meters. The average P-wave velocity for this layer is 1968 m/s and around 3100 m/s for the second layer, while the average S-wave velocity is 465 m/s for the first layer and around 1200 m/s for the second layer. The Vp/Vs ratio of the first layer is 4.2 and 2.5 for the second layer. According to the P-S reflection raypath geometry, the shot static corrections were computed from the P-wave model and the receiver static correction from the S-wave model and were applied to the radial component data. Some improvements were noticed in term of intermediate wavelength reflector structure and reflector continuity. However, the improvement is not as clear as with the application of the static corrections on the vertical component.The results of the Plus-Minus time analysis and Generalized Linear Inversion methods are consistent in term of depths, velocities, static corrections. However, the Plus-Minus time analysis method provides a more detailed model than Generalized Linear Inversion method.