Capstone: Land vibroseis through a weathered layer

Part 10 turns abstract techniques into project narratives. Each capstone walks a real project archetype from raw data to deliverable, highlighting which stages are critical, which are routine, and where ML replaces or augments classical methods. The widget below shows the stage-by-stage pipeline with a quality indicator and a click-to-expand detail panel.

Project setup

A 2D land survey on Mesozoic carbonate targets under a variable weathered layer (30–50 m thick, with velocity ranging 300–1000 m/s between locations). Vibroseis source, 12 s sweep 10–80 Hz. 240-channel receiver spread, 25 m group interval. The weathered layer introduces static shifts of up to ±15 ms between adjacent shot points — more than half a wavelet period at the 40 Hz target frequency. Without a good near-surface correction, the stack is incoherent and no migration will save it.

The pipeline

Click any stage to see what it contributes and why it is (or is not) critical for this project type.

Why refraction statics is the central step

In marine processing the near-surface is the water column — uniform, predictable, negligible statics. On land the near-surface is whatever soil, regolith, weathered bedrock, and water-table variation exists between each shot and receiver. Static shifts of 10–30 ms are routine and destroy stack coherence because adjacent traces arrive at subtly different times. The solution is first-break picking (ML-assisted, §9.4) + refraction tomography to build a near-surface velocity model, then apply the time shifts that would have resulted if the near-surface were replaced with a uniform datum velocity. Get this step right and the stack snaps into focus; get it wrong and nothing downstream works.

Surface-consistent deconvolution + amplitude balancing

The weathered layer distorts the source wavelet differently at each shot-receiver position. Surface-consistent decomposition (SCD) separates the observed wavelet variations into shot, receiver, offset, and CMP terms, removes the shot and receiver components (acquisition-related), and keeps the offset and CMP components (geology-related). Without this, AVO and post-stack amplitudes reflect weathering, not structure.

What this project does NOT get

• Depth migration. Post-stack time migration is adequate; lateral velocity variation below the weathered layer is moderate, so PSDM would be overkill.
• FWI. Land 2D data has limited azimuthal coverage; FWI struggles. Tomography + post-stack migration is the production standard.
• Full QI. Post-stack structural imaging is the primary deliverable; AVO analysis is considered advisory only because SCA imperfection leaves residual amplitude biases.
• 4D. Weathered-layer variation between repeat surveys is large; NRMS under the reservoir is typically 40–50 %, not 4D-grade.

Where this goes next

§10.2 contrasts this with a deep-water OBN project. The near-surface problem goes away; the sub-salt imaging problem takes its place.

References

• Yilmaz, Ö. (2001). Seismic Data Analysis (2 vols.). SEG.
• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge UP.
• Robinson, E. A., Treitel, S. (2008). Digital Imaging and Deconvolution. SEG.
• Claerbout, J. F. (1976). Fundamentals of Geophysical Data Processing. McGraw-Hill.