Time Imaging (PSTM)
1. Velocity Model Building (Time): Refined RMS velocity picking on an optimized grid.
2. Initial PSTM: Preliminary Pre-Stack Time Migration for QC.
3. Iterative Velocity/Anisotropy Updating: Migration velocity analysis (MVA) and residual moveout correction.
4. Final Full 3D Anisotropic PSTM: Producing the final time-migrated gathers.
5. Post-Stack Processing: Residual noise suppression, spectral whitening, and time-variant filtering.
Time Imaging Workflow (PSTM Focus)
1. PSTM Velocity Model Building
- Objective:
To create an initial RMS velocity field suitable for moving seismic events to their true spatial positions in time.
- Detailed Parameters:
1. Grid Density: Velocity picking grid (e.g., 400m x 400m or 200m x 200m).
2. Smoothing Kernels: Spatial smoothing (e.g., 500m radius) to ensure stability during migration.
- Controlling Parameters (QC):
1. Semblance Resolution: Ensure sharp energy peaks in the semblance plots.
2. Horizon Consistency: Velocity picks must follow key geological markers (horizons).
- Issues to Check:
1. Velocity Inversion: Be cautious of unrealistic velocity drops unless geologically justified (e.g., overpressure zones).
2. Lateral Variations: Strong lateral velocity changes can cause PSTM to fail; this is the signal to move to PSDM
2. Pre-Stack Time Migration (PSTM) Execution
- Objective:
To focus diffractions, collapse Fresnel zones, and image dipping events.
- Detailed Parameters:
1. Migration Aperture: The radius (in meters) the algorithm searches for energy (e.g., 5000m).
2. Maximum Dip: Limit of the angle to be imaged (typically 70°–80°).
3. Anti-Aliasing Filter: Frequency cut-offs to prevent spatial aliasing on steep dips.
- Controlling Parameters (QC):
1. Impulse Response: Check for "Migration Smiles" which indicate aperture or velocity issues.
2. Fault Definition: Sharpness of fault planes is the ultimate test for migration accuracy.
- Issues to Check:
1. Aperture Edge Effects: Insufficient aperture leads to "chopped" dipping reflectors or missing fault segments.
2. Computational Cost: Excessive aperture significantly increases run-time without always improving quality.
3. Residual Velocity Analysis (RVA / RMO)
- Objective:
To flatten the events in Common Image Gathers (CIGs) after the initial migration.
- Detailed Parameters:
1. High-Density Picking: Finer grid (e.g., 100m x 100m) for residual corrections.
2. RMO Scans: Percentage-based velocity updates (e.g., 90% to 110% of V-initial).
- Controlling Parameters (QC):
1. Gather Flatness: Visually inspect PSTM Gathers to ensure reflectors are perfectly horizontal across all offsets.
2. Stack Power Maximization: Compare the stack before and after RVA.
- Issues to Check:
1. Non-Hyperbolic Moveout: In far offsets, standard RMO may fail, requiring anisotropy correction.
4. Anisotropy Correction (ETA Estimation)
- Objective:
To correct the "Hockey-stick" effect at far offsets caused by Vertical Transverse Isotropy (VTI).
- Detailed Parameters:
1. ETA (h) Parameter: Used to adjust the non-hyperbolic moveout equation.
2. Long Offset Range: Typically requires offsets > 1.5x the target depth.
- Controlling Parameters (QC):
1. Far-Offset Alignment: Ensure events at 4000m+ offsets are aligned with near-offset events.
- Issues to Check:
1. V-ETA Ambiguity: Velocity and Eta parameters are coupled; picking one incorrectly affects the other.
5. Final Conditioning & Stack
- Objective:
To produce the final 3D volume with maximum Signal-to-Noise ratio (SNR).
- Detailed Parameters:
1. Muting Strategy: Surgical mutes to remove NMO stretch and refracted energy.
2. Stacking Method: Mean stack, Median stack, or Diversity stack to suppress noise.
3. TVF (Time-Variant Filtering): Bandpass filters that evolve with depth to maintain resolution.
- Controlling Parameters (QC):
1. Frequency Spectrum: Analyze the bandwidth to ensure no over-filtering.
2. Amplitude Preservation: Ensure the relative amplitudes are geologically meaningful (for AVO).
- Issues to Check:
1. Over-Muting: Can lead to loss of deep signal or poor SNR.
2. Artifacts: Look for acquisition footprints or processing-induced noise (smearing).