How to look at a 3D volume

Part 2, The Interpreter's Toolkit

Learning objectives

Adopt a disciplined scanning strategy for reconnaissance of a new 3D volume
Choose the right view (inline, crossline, time slice, arbitrary line) for the geological question at hand
Recognize when a single-view interpretation is not enough and switching views would clarify
Build the habit of reading displays critically, palette, gain, and polarity choices before geology

Part 1 gave you the physics. Part 2 teaches the craft. And craft starts with one unglamorous habit: how to look at a 3D volume. Professional interpreters develop this habit over thousands of hours; we can shortcut a lot of it by making the method explicit.

The two-minute reconnaissance

When you open a new 3D volume, do not start picking. Spend two minutes doing reconnaissance. The goal is not to interpret anything yet, it is to build a mental map of the volume so that when you do start interpreting, you know what normal looks like. A disciplined reconnaissance:

Scroll through time slices top to bottom. Map-view patterns are where the obvious regional features live, drainage systems, polygonal fault networks, salt margins, major stratigraphic boundaries. Get the big picture before you zoom in.
Scroll through inlines end to end. Look for major reflectors, fault networks, changes in reflector character. Note depths where the data gets noisy or fades.
Scroll through crosslines end to end. Same survey, different axis. A feature that runs along the inlines, which an inline cuts lengthwise, is cut across here.
Spot-check a few arbitrary lines. Pick an interesting-looking structure on a time slice and cut a line perpendicular to it. This is how you catch features that are oriented wrong for the acquisition axes.

Two minutes of this saves two hours of re-work later. The interpreter who dives straight into picking a target horizon often discovers, after days of work, that the horizon they have been tracing is actually a processing artifact or a sideswipe event from a feature off to the side.

The "check the display" habit

Before you interpret anything, confirm what you are looking at. This takes 30 seconds and catches a large fraction of the errors junior interpreters make:

What polarity convention? On zero-phase data, SEG normal polarity shows a rise in acoustic impedance as a central peak; reverse polarity, often called European, shows it as a trough. On minimum-phase data the SEG standard starts a rise with a trough (Section 2.2). Which colour a peak gets is set by the colour bar, so read that too. The same rocks produce opposite visual signatures on the two conventions. If you do not know which your dataset uses, you cannot read amplitudes at all.
What phase is the wavelet? Zero-phase data has its peaks aligned with the reflectors. Minimum-phase or mixed-phase data has the peak offset from the reflector. Picking the wavelet peak on zero-phase data gives you the reflector; doing the same on minimum-phase data gives you a position that lags the reflector, because a minimum-phase wavelet starts at the reflector and peaks later.
What palette and gain? RWB makes polarity obvious; grayscale de-emphasizes polarity in favour of geometry. High gain saturates the brightest events into solid blocks; low gain makes weak events disappear. Neither setting is wrong, but you need to know which you are looking at.
Time or depth domain? Time sections are in ms TWT. Depth sections are in m or ft. Features look different in each domain because layers of different velocity have different time vs. depth ratios. Pick map contour intervals appropriate to the domain (50 ms vs 50 m are entirely different contours).

Section 1.0's F3 cube viewer lets you toggle palette and gain and see the effect on the same geology. If you have not played with it, do that now before continuing.

Never interpret from one view alone

The single most common pitfall in 3D interpretation is treating a single 2D slice as if it contained the answer. Any feature you pick on an inline should be confirmed on the crossline and on the time slice. Any fault you trace on a cross-section should be visible as a lineament on the time slice within the fault's time window. Any subtle amplitude anomaly that excites you on one slice should be compared with the surrounding slices in both directions.

The practical rhythm is: spot a feature on one view, immediately flip to the other views and confirm. If the feature is coherent across views and survives a step to the neighbouring slices, it is probably real. If it appears only on one view or disappears when you shift to a nearby slice, it is probably noise. Then look at its shape: a feature that repeats at the spacing of the acquisition lines and runs exactly along them is acquisition footprint, however many views show it. Train yourself to flip views automatically, without stopping to think about whether it is worth it. It always is.

Figure 2.1 puts that rhythm to work on four candidates in one small volume, two of them real. Each is marked at the same point on the inline, the time slice and the crossline. Step both sections away from it, watch how much of it is left, and commit to a verdict before the figure gives one.

Candidate C is the case the rule was written for. On inline 146 it reads as a short, bright reflector at 500 ms, yet two lines away none of it is left, and on the crossline it is one bin wide: it is noise on one acquisition line. Candidate D is the harder lesson. It shows on the crossline and the time slice but not on the inline, and three lines away it is back at full strength, reversed, so neither counting views nor stepping slices rejects it. What condemns it is its shape: it repeats every 6 lines, exactly along the inlines. Geology has no reason to repeat at the spacing of the acquisition lines and exactly along them; acquisition footprint does. The fault, B, is the opposite warning: a section that runs along a fault's strike shows no offset at all, so a real fault can be missing from a view that happens to parallel it. This one strikes obliquely, and offsets the reflector on both sections.

Start broad, end narrow

Interpretation workflows invariably spiral inward: start with a regional overview, identify the targets, zoom in to specific features, and refine. Inverting this order, picking a target before understanding the regional context, leads to interpretations that do not fit the larger story.

A rule of thumb: every pick should be consistent with a story that makes sense regionally. A horizon that plunges into a fault on one crossline, then jumps 300 ms on the next crossline without a corresponding fault, is telling you the horizon pick is wrong. A reservoir sand that thins to zero in one direction and thickens in the other without a depositional explanation is telling you the geometry is wrong. The volume is self-consistent; interpretations that appear internally inconsistent are almost always the interpretation's fault, not the data's.

The next section (Section 2.2) introduces the polarity-and-phase conventions so you know what you are reading before you read it. Sections 2.3 through 2.5 pair methodology with interactive picking tools on a synthetic survey whose true horizons and faults are known, so every pick you make is graded against the truth.

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.
Sheriff, R. E. (2002). Encyclopedic Dictionary of Applied Geophysics. Society of Exploration Geophysicists.
Chopra, S., & Marfurt, K. J. (2007). Seismic Attributes for Prospect Identification and Reservoir Characterization. Society of Exploration Geophysicists.