VSP processing for coal reflections
Salman K. Bubshait
Five VSP surveys were acquired in Alberta as part of a study of Mannville coals. The goal is to study the AVO variation in the Mannville coals to detect anomalies using VSP wavefields.
The zero offset VSP survey was processed using the VISTA software through to corridor stack and shows high reflection quality of P waves with no significant multiples in the data.
A log analysis and synthetic seismogram were generated to compare with the outside and inside corridor stacks obtained from the zero offset VSP survey. The top of the Mannville B coals was indicated by a sharp trough and/or decrease in impedance in all the stacks in addition to the synthetic. The synthetic seismogram proved to the best match to the outside corridor stack. Also, an analysis of the mute window proved that varying the mute from 30-70 ms gave the same approximate result possibly because of no significant multiples in the data.
Also, the three walkaway VSPs along different azimuths were processed through to the VSPCDP stage. A recommendation is suggested to have an overlap of a receiver in the borehole to minimize shot static errors. The walkaways displayed high reflection quality of both P and S waves that highlighted the Mannville coals. A slight improvement of the reflection of the coals is noticed in the SV waves over the P waves as offset increases.
An AVO study was performed on all three walkaway VSP surveys. The walkaways were converted to angle gathers and processed in the Hampson-Russell AVO and STRATA packages. Intercept versus gradient AVO crossplots were generated for the
Mannville B coals that indicated a potential gas anomaly in the southeast angle gathers. However, the east and southeast did not indicate a gas anomaly. The AVO intercept versus gradient crossplots consistently plotted the Mannville B coal with a strong AVO intercept in quadrant II.
Inversion was performed on the walkaway angle gathers to invert for the P and S wave impedances. The impedances then were utilized to generate the Lambda Rho versus the Mu Rho crossplots. All the Lambda Rho versus the Mu Rho crossplots indicated that the Mannville B coals plotted over the background trend indicating that the coals are non gas bearing. There were some physically impossible Lambda Rho values in the top of the Mannville Fm due to errors in the inversion but left the Mannville B coal target zone unaffected.