Capstone: 4D repeatability challenge

Most of the easy 4D cases use dedicated baselines and monitors acquired with matched equipment. This capstone is the hard case: an 18-year gap between a 2006 legacy streamer baseline and a 2024 OBN monitor, with different sources, different geometries, different bandwidths, and different processing environments. Every trick in Part 8 has to work, and the NRMS target is correspondingly lower.

Project setup

Legacy field. Baseline: 2006 narrow-azimuth streamer, 8 km cables, 12 Hz peak frequency. Monitor: 2024 OBN, 2 km node spacing, multi-component, 5 Hz peak frequency. 18 years of production have caused compaction, fluid-contact movement, and pressure changes that the 4D processing should reveal. Primary deliverable: a compaction map and a fluid-movement map.

The pipeline

Why this is the hardest 4D case

• Geometry mismatch. Streamers move; nodes are stationary. No direct trace-to-trace pairing possible; geometry harmonisation is the first and biggest hurdle.
• Wavelet mismatch. 2006 air-gun arrays vs 2024 calibrated source arrays produce different wavelets. Wiener cross-equalisation is mandatory.
• Bandwidth mismatch. Legacy data has no signal below 5–6 Hz; monitor has signal down to 1.5 Hz. Can't match spectra below the legacy cutoff; must be honest that these frequencies are absent from the baseline.
• Processing environment mismatch. Legacy processed in 2006 tools with 2006 conventions (possibly with AGC somewhere); modern processed with 2024 tools. Joint reprocessing from raw gathers is essential; no shortcut through "just compare the archived outputs".

Realistic NRMS targets

• Dedicated 4D (matched acquisition + joint processing): < 15 %.
• Legacy vs modern, joint-reprocessed: 25–40 %. This project.
• Legacy vs modern, best-effort matching only: 50–70 %. Too noisy for quantitative interpretation.

What this project can deliver

• Large-scale compaction maps. 18 years of production causes 10–20 m time-structural changes in the overburden; these are visible through the noise floor.
• Gross fluid-contact movement. Oil-water and gas-oil contact migrations of hundreds of metres are detectable.
• Pressure depletion (qualitative). Regional velocity decreases in depleted reservoirs; visible on 4D but with ~20 % quantitative uncertainty.

What this project CANNOT deliver

• Detailed fluid fronts with sub-100-m precision.
• Quantitative saturation changes (rock-physics inversion requires repeatability this dataset does not have).
• Reliable thin-bed 4D (tuning effects dominate the residual).

Part 10 closes here

Six capstones, six project archetypes: land vibroseis, OBN deep-water salt, WAZ FWI, CCS monitoring, geotechnical near-surface, legacy-to-modern 4D. Each uses techniques from Parts 0–9 in different combinations, with different stages on the critical path. The skill of a seismic processor is not knowing a single flow but recognising which combination of techniques is right for each project.

What Part 11 provides

Part 11 is the quiz bank — one set of ~30 MCQs per previous part plus a final exam drawing from all ten. Use it to self-check your understanding section by section before you commit to a career stake in processing.

References

• Yilmaz, Ö. (2001). Seismic Data Analysis (2 vols.). SEG.
• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge UP.
• Claerbout, J. F. (1976). Fundamentals of Geophysical Data Processing. McGraw-Hill.
• Virieux, J., Operto, S. (2009). An overview of full-waveform inversion in exploration geophysics. Geophysics, 74, WCC1.