From 2D lines to 3D volumes

Part 1 — Foundations of Seismic

Learning objectives

Understand why the shift from 2D to 3D was a transformation in interpretation, not just an increase in data
Recognize the problems that 2D data cannot solve (sideswipe, misties, out-of-plane reflections)
Use inlines, crosslines, time slices, and arbitrary lines as complementary views of a volume
Adopt a volume-centric interpretation mindset

Through the 1970s, seismic interpretation was done almost exclusively on 2D lines — individual cross-sections recorded along a survey track, displayed side-by-side on a light table, and correlated manually. 2D lines still have uses (reconnaissance, frontier areas, long regional profiles), but for any serious interpretation of a prospect or field, they have been replaced by 3D volumes since the 1980s. The shift was not just a numerical upgrade; it changed what was even possible to interpret.

Why 2D is not enough

Three problems that plague 2D interpretation and that 3D largely solves:

Misties. Two intersecting 2D lines should show the same reflector at the same time at their intersection. In practice they rarely do — processing differences, small navigation errors, or 3D geology that projects differently onto the two line directions all cause depth/time discrepancies. Interpreters spent large fractions of their careers reconciling misties.
Sideswipe. A 2D line records energy not only from directly below the line but also from geology off to the side. A steep fault 200 m off the line can produce a coherent reflection that appears to lie beneath the line — misleading. 3D acquisition samples the surface densely enough to resolve where the energy actually came from.
Continuity assessment. A 2D line tells you what is under the line. It tells you nothing about whether a feature extends 100 m or 10 km perpendicular to the line. Interpreters extrapolated, and got it wrong often.

A 3D survey measures the full volume under the survey area densely enough that every subsurface point is sampled from many angles. The reflection coefficient, the structural geometry, and the stratigraphic continuity are all resolvable from the 3D volume in a way that grids of 2D lines cannot match.

Once you have a 3D volume, the interpretation interface changes. You no longer flip through 2D cross-sections imagining what lies between them. You slice the volume interactively — picking the cross-section or map view that best illuminates the feature you are studying. The four standard views every interpreter uses:

The four interpretation views

Inline — a vertical cross-section parallel to one horizontal axis. Usually the original acquisition line direction. Good for structural interpretation along the primary axis.
Crossline — a vertical cross-section parallel to the other horizontal axis. Perpendicular to inlines. Shows a different "face" of the same geology; crosslines often reveal faults that were parallel to inlines and hence foreshortened on inline view.
Time slice (or depth slice in depth-migrated data) — a horizontal cut at a fixed time/depth. Map-view of the subsurface at that level. Invaluable for pattern recognition: channels, faults, and other geologic features often stand out on time slices in ways that are invisible on vertical sections.
Arbitrary line — a cross-section along a user-drawn polyline that does not need to follow inline or crossline directions. Essential for aligning a section perpendicular to a dipping structure or along the strike of a fault.

§1.0's cube viewer exposes the first three of these directly. An interpreter flips between them constantly, building a 3D mental model from the three orthogonal 2D views. Arbitrary lines and horizon-slicing tools extend that basic set and make specific workflows (fault interpretation, channel mapping) much faster.

The volume-centric mindset

Working in 3D is not about viewing more cross-sections. It is about treating the entire volume as a single object and asking questions of it:

Is this channel continuous? (Slice at the channel's time; the answer is a picture.)
Does this fault cut higher up the section? (Follow it through a sequence of time slices.)
How does this reservoir thickness vary across the field? (Extract the time difference between top and base picks and map it.)
Where in the volume does a particular seismic signature appear? (Compute a volume attribute; threshold it; the result is a 3D object.)

None of these questions is naturally answered by a grid of 2D lines. All of them are naturally answered by a 3D volume with the right tools. The rest of this textbook assumes the volume-centric mindset: every question starts with "given this volume, what do I slice / compute / threshold to see X?"

One practical consequence. 3D data is much larger than 2D data — a typical survey is 10–100 GB for a post-stack volume, 1–10 TB pre-stack. The software, hardware, and workflow of 3D interpretation all reflect this scale. It is also why teaching material (including ours, for now) uses small teaching subsets and synthetic volumes: the volumes real interpreters work with would not fit in a browser. We will deal with full-size real data in Phase 2 when we ingest the F3 Netherlands teaching subset.

References

Brown, A. R. (2011). Interpretation of Three-Dimensional Seismic Data (7th ed.). AAPG Memoir 42 / SEG IG13.
Bacon, M., Simm, R., & Redshaw, T. (2003). 3-D Seismic Interpretation. Cambridge University Press.
Yilmaz, Ö. (2001). Seismic Data Analysis (2 vols.). Society of Exploration Geophysicists.
Sheriff, R. E., & Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.