Multiple classification

Part 4 is all about multiples — reflections that bounce more than once before reaching the receiver. They look like primaries on the gather but live at wrong times, wrong velocities, or both. Every one you leave in is a false reflector on the final image, a misleading AVO anomaly, or an artifact that wraps through your interpretation. The tools for killing them each target a different multiple type, so the first job is learning to tell the types apart.

1. Why multiples happen

A primary is a reflection that travels straight down, bounces once off a subsurface interface, and comes straight back. A multiple is a reflection that has bounced more than once. The free surface (sea surface for marine, ground surface for land) is a near-perfect reflector, so anything traveling up and hitting the surface has a 95–100 % chance of bouncing back down and taking a second trip. That is where most multiples come from.

2. Four families you must recognize

The widget above has one primary water bottom at 0.3 s (1500 m/s), two sediment primaries at 0.8 and 1.4 s (2200 and 2800 m/s), plus four multiple families you can toggle on or off. Each overlay is labelled; switch them on one at a time and compare curvatures at the same t₀ — that is the diagnostic you use on real data.

• Water-bottom (WB) multiple. The water bottom reflection bouncing between surface and seafloor. Appears at exactly 2× t₀, 3× t₀, … with the same curvature (same velocity) as the primary. Most visually obvious multiple family — a whole train of identical hyperbolae stepping down at 0.3 s intervals.
• Peg-leg multiple. A sediment primary with an extra short bounce — typically between the sediment reflector and the sea surface or seafloor. Appears at t₀(primary) + 2× t₀(peg) with the primary’s velocity (deeper). Looks like a delayed duplicate of the primary.
• Surface-related internal multiple. A sediment reflection that bounced once off the sea surface before being recorded. Appears at 2× t₀(sediment primary) with the sediment primary’s velocity.
• Pure inter-bed multiple. A reflection that bounces only between subsurface reflectors, never reaching the surface. Appears at the sum of travel times with overburden velocity — LOWER than the primary at the same depth. Hardest to attenuate; no surface-based method sees it.

3. The kinematic tells

Each family has a signature that, once you have seen enough gathers, you recognize instantly:

• Same velocity as some primary? → WB multiple (same as WB primary) or peg-leg (same as sediment primary).
• At exactly k·t₀ of WB primary? → WB multiple train.
• Lower velocity than any primary at that time? → pure inter-bed multiple.
• Slightly delayed and similar velocity? → peg-leg.

4. Which tool kills which

• Predictive deconvolution (§2.7): WB multiple trains. Cheap, reliable for periodic water-bottom multiples.
• SRME (§4.2): ALL surface-related multiples — WB, peg-legs, surface-related internals. Data-driven, no subsurface model needed. Standard for marine processing.
• Radon demultiple (§4.3): Any multiple whose residual NMO differs from the primary at the same t₀. Fast, offline, works on CMP gathers.
• Adaptive subtraction (§4.4): Combines a PREDICTED multiple train (from any method above) with a least-squares matched filter to produce a better subtraction. Almost always the final clean-up step.
• Inter-bed modelling (§4.5): Pure inter-bed multiples. Requires a subsurface model; significantly more expensive than surface-related tools.

5. When multiples dominate

Multiples are most damaging when:

• Hard seafloor. Limestones, chalks, and cemented seafloors reflect almost as strongly as the sea surface. Multiple amplitudes approach primary amplitudes.
• Shallow water with deep target. WB multiples fold on top of the target reflector; NMO differential is small, so Radon struggles.
• Strong shallow reflector (e.g., volcanic). Peg-legs from the reflector bury primaries directly below it.
• Carbonate platform. Multiple cycle lengths shorter than the underlying section — everything below is multiple-contaminated.

In each case, multiple attenuation moves from “clean up” to “make or break.” Skip or under-apply and the imaging fails.

6. When multiples are tolerable

For shallow-water deep-target work where multiples are clearly separated in time from the primary, or for land data where the surface reflector is weak and the WB train is minor, a moderate SRME pass + Radon cleanup is enough. You do not always need the heaviest tools — but you do always need to know which family you are fighting before picking the tool.

Where this goes next

§4.2 introduces SRME, the data-driven method that predicts every surface-related multiple from the data itself. It does not distinguish the sub-families — it treats them all as one problem — which is exactly why it is the most widely used multiple attenuation tool in marine processing.

References

• Yilmaz, Ö. (2001). Seismic Data Analysis (2 vols.). SEG.
• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge UP.
• Verschuur, D. J., Berkhout, A. J., Wapenaar, C. P. A. (1992). Adaptive surface-related multiple elimination. Geophysics, 57, 1166.