Near-surface characterization and Vp/Vs analysis of a shale gas basin
Liliana M. Zuleta Tobon
The data from a multicomponent refraction survey acquired in Northeast British Columbia was used in this thesis to carry out P-wave and S-wave near-surface description and to undertake Vp/Vs analysis of shallow and deep formations. The datasets had good quality for picking first break arrivals and to process the reflection data successfully.
Three lines from the refraction survey were used for this study. An important feature was the confirmation of a channel that was detected through a previously acquired electromagnetic survey. The near-surface model obtained from P-wave data showed two layers with P-wave velocities of ~ 1950 m/s for the first layer and ~ 2800 m/s for the second layer. The SH-wave data from one of the lines showed three layers in the west end of the line and two layers to the east end of the line. The other line showed three layers along the entire profile. The S-wave velocity of the first layer was found to be around 375 m/s, that for the second layer was ~ 727 m/s and the S-wave velocity for the third layer was ~ 1400 m/s.
The velocity-depth structure obtained for the near-surface was used for the calculation of static corrections. As expected, S-wave static corrections gave higher values that the P-wave static corrections. The datum static corrections vary from -150 ms to -250 ms for SH-wave data and about -15 ms to 15 ms for P-wave data.
Vp/Vs analysis was performed for the near-surface structure with the values of velocities obtained from the refraction analysis and also through PP-PS registration for deeper structures. Measured values of Vp/Vs showed differences of around 0.1 when compared to those from well log. Values of Vp/Vs ranged from 5 in the near-surface to 2.2 in deeper formations.