4D Repeatability Calculator

Assess 4D seismic repeatability using NRMS (Normalized RMS) metric. Essential for time-lapse monitoring of reservoir changes during production.

4D Seismic Production Monitoring QC

Input Parameters

Amplitude Statistics

amplitude units
RMS amplitude of (Monitor - Base)
amplitude units
RMS amplitude of baseline survey
amplitude units
RMS amplitude of monitor survey

Expected Production Signal

amplitude units
Expected RMS amplitude change from production

Quick Scenarios

Results

Theory & References

NRMS Definition

Normalized RMS (NRMS):

NRMS = 2 × RMS(Monitor - Base) / (RMS(Base) + RMS(Monitor)) × 100%

Quality Thresholds:

  • Excellent: NRMS < 20%
  • Good: NRMS 20-40%
  • Fair: NRMS 40-60%
  • Poor: NRMS > 60%

Lower NRMS means better repeatability (less noise).

Signal-to-Noise Ratio

SNR = Expected 4D Signal / RMS Difference

Detectability Guidelines:

  • Strong: SNR > 2 (signal twice noise level)
  • Detectable: SNR 1-2 (signal above noise)
  • Marginal: SNR 0.5-1 (signal near noise)
  • Undetectable: SNR < 0.5 (buried in noise)

Applications

  • Pre-drill Feasibility: Assess if 4D is viable
  • Survey Design: Determine acquisition requirements
  • QC Monitoring: Track repeatability during acquisition
  • Post-processing: Validate data quality
  • Interpretation: Distinguish real changes from noise

Improving Repeatability

  • Match acquisition parameters (same vessel, source, geometry)
  • Time surveys for similar environmental conditions
  • Apply cross-equalization processing
  • Use permanent receiver arrays (OBC, OBN)
  • Increase fold for better noise suppression