Stacking Velocity QC

Quality-check stacking (NMO) velocities from seismic velocity analysis. Identify unrealistic velocities, inversions, gradient anomalies, and picking errors before committing to processing or depth conversion. Essential QC for reliable seismic imaging.

Velocity QC NMO Velocity Dix Equation Processing QC

Velocity Picks

Expected Lithology

Sets expected velocity range for QC checks

Time-Vrms Pairs

Data Import/Export

CSV format: time(s), vrms(m/s)

QC Results

Theory & Guidelines

NMO vs Interval Velocity

NMO (Vrms) Velocity:

  • RMS velocity from velocity analysis (semblance, velocity spectra)
  • Best-fit hyperbolic moveout velocity
  • Smoothly increasing with time for normal geology
  • Used for NMO correction and stacking

Interval Velocity (Vint):

  • True layer velocity from Dix equation
  • More variable, reflects lithology changes
  • Can be negative if Vrms picks are inconsistent (unphysical)

Dix Equation

Converts RMS velocities to interval velocities:

Vint(n) = √[(Vrms(n)² × tn - Vrms(n-1)² × tn-1) / (tn - tn-1)]

QC Application:

  • Negative Vint² → picking error or velocity inversion
  • Vint > 8000 m/s → unrealistic for sedimentary rocks
  • Highly erratic Vint → poor pick quality or complex geology

Expected Velocity Ranges

Typical Vrms ranges by lithology:

  • Shale: 1500-4500 m/s (generally increases with depth)
  • Sandstone: 2000-5500 m/s (porosity and compaction dependent)
  • Carbonate: 2500-6500 m/s (can be very fast)
  • Salt: 4200-4700 m/s (nearly constant)
  • Mixed: 1500-6000 m/s (default range)

Normal gradient: 500-1500 m/s per second TWT

Common Picking Errors

Symptoms of bad picks:

  • Velocity inversions: Vrms decreasing with time → re-pick
  • Abrupt jumps: >30% velocity change → check for misidentified horizon
  • Negative Vint: Vrms inconsistency → smooth or re-pick
  • Extreme gradients: >3000 m/s/s → likely picking error

How to fix:

  1. Review velocity spectra at flagged locations
  2. Check for horizon mis-ties or cycle skips
  3. Apply spatial smoothing to velocity field
  4. Re-pick problematic locations with tighter constraints
  5. Consider geological plausibility (don't force monotonic if geology isn't)

QC Checks Performed

This tool performs:

  1. Range Check: Vrms within expected lithology range
  2. Inversion Check: Vrms should generally increase with time
  3. Gradient Check: dV/dt between picks is reasonable
  4. Dix Check: Vint is positive and realistic
  5. Smoothness Check: No abrupt velocity jumps (>30%)

Severity levels:

  • PASS: All checks OK
  • WARNING: Minor issues, acceptable for processing
  • FAIL: Critical issues, must fix before processing