Faults in 3D: how to see them, how to pick them

What a fault is

A fault is a fracture surface across which rocks on one side have moved relative to the rocks on the other side. The movement can be mostly vertical (normal or reverse faulting, in extensional or compressional regimes) or mostly horizontal (strike-slip). The amount of movement is the throw (vertical component) and heave (horizontal component). Together they describe the slip on the fault.

On seismic, a fault appears as an abrupt offset of reflectors. Layers on one side of the fault are at one time; on the other side they jump to a different time. The bigger the throw, the more obvious the offset — a 50 ms fault is visually prominent; a 5 ms fault might be at the limit of detection on a typical dataset.

What a fault looks like on seismic

Three visual signatures together identify a fault:

• Reflector offset. The horizon you're tracking steps abruptly up or down. Usually multiple horizons offset together — a fault cuts through a stack of layers, not just one.
• Acoustic shadow. The fault plane itself is often a diffracting surface that returns little coherent energy, so the seismic section shows a band of reduced amplitude or noisy character along the fault trace.
• Diffraction hyperbolas (unmigrated data) or migration artifacts (over-migrated data) cluster along fault planes, giving them a characteristic jagged appearance.

A good fault interpretation identifies the fault plane itself (where the rocks are broken) not just the offset — the plane is what you pick, because that's what the subsurface actually contains. The offsets on either side are the evidence, not the target.

Three fault types, three signatures

• Normal faults — the hanging wall (the block above the fault plane) moves DOWN relative to the footwall. Most sedimentary basins have lots of these: extensional tectonics stretches the crust and produces down-dropped blocks. On seismic, normal faults dip at 55–70 degrees typically and show reflectors on the downthrown side clearly lower than those on the upthrown side.
• Reverse (thrust) faults — the hanging wall moves UP relative to the footwall. These are compressional features, common in mountain belts and foreland basins. Low-angle thrusts (less than 30 degrees dip) can look superficially like bedding on cross-sections and are one of the hardest fault types to interpret correctly.
• Strike-slip faults — the two sides move horizontally past each other. On a cross-section the two sides might show similar stratigraphy at similar depths, which makes strike-slip difficult to spot on isolated inlines. Strike-slip faults are typically identified on time slices where the map-view offset of a stratigraphic feature is obvious.

Picking faults as polylines

Unlike a horizon (one time value per crossline), a fault trace on an inline is a line segment — typically just a few straight-line pieces that follow the fault plane from its shallow expression down to where it dies out (or leaves the volume). You pick a fault by clicking along its trace, laying down vertices to define a polyline. Each inline gets its own fault trace.

The picking workflow is simpler than horizon picking in one respect: you don't need to pick every crossline. Two or three vertices along a fault trace suffice for a straight fault; a listric (curved) fault might need five or six. The polyline between vertices is interpolated linearly, so vertex placement should sit where the fault direction changes.

On the other hand, fault picking is harder in one respect: you have to decide EXACTLY where the fault plane is, and the seismic data often underdetermines it. The acoustic shadow along a fault is frequently several traces wide; the reflector offset gives you the approximate location but not the precise angle. Experienced interpreters consistently place fault traces in the middle of the shadow zone and reconcile the geometry across inlines.

Exercise

• On the default inline, identify the fault by looking for offset reflectors. The fault is roughly vertical on the left-center part of the section.
• Click on the shallow end of the fault trace to place the first vertex. Click on the deeper end to add a second vertex. The polyline connects them.
• Press End trace (or double-click) to finish the fault polyline.
• Scroll to a different inline and pick the fault there too. The fault is the same geological feature across all inlines; the trace should look similar from inline to inline.
• After you've picked traces on at least 3 inlines, press Grade my picks to see how close your polylines are to the reference fault.
• Toggle Show reference to reveal the expert fault trace. Use it as a teaching aid — not as a crutch.

Aim for Excellent or Good. A poor grade usually means your polyline vertices sit in the offset reflectors rather than along the acoustic shadow that marks the fault plane itself.

Pitfalls to avoid

• Picking the offset, not the fault. Beginners sometimes draw a polyline along the shifted reflectors rather than the fault plane. The reflectors are the EVIDENCE that a fault exists; the fault itself is the plane cutting through them. Always pick between the offset reflectors, not on them.
• Fault looking wrong on different inlines. If your fault pick changes dramatically from one inline to the next, something is wrong. Real faults are 3D surfaces whose traces should vary smoothly across inlines. Abrupt changes mean you picked something else on one of the inlines, or the fault you identified is actually part of a fault network with separate segments.
• Acquisition footprint mimicking faults. Linear features that appear periodic and parallel to the acquisition direction are often acquisition footprint, not real faults. Check whether the "fault" corresponds to any stratigraphic offset — if reflectors pass through it without disruption, it's not a fault.
• Sideswipe masquerading as a fault. A reflector from outside the line (on unmigrated 2D) can look like a fault. Migrated 3D data mostly eliminates this, but very steep dips and edge-of-survey areas still suffer.
• Missing the small ones. Faults with throw below the seismic resolution limit (less than about λ/4) still exist but are invisible. For reservoir-scale work, sub-resolution faults can still matter for flow; extending the seismic interpretation with attributes (coherence, dip — Part 6) reveals some of them.

From 2D traces to 3D fault surfaces

A fault picked on just one inline is a line. Pick the same fault on many inlines and you have a set of lines that together outline the 3D fault surface. The quality of a 3D fault interpretation depends on picking enough inlines to constrain the surface. For a simple planar fault, 4-6 inlines evenly spaced across the fault's extent are enough; for complex fault networks (intersecting faults, changing strike, branching), you need denser coverage.

In production workflows the picks from individual inlines are triangulated into a fault plane, which is then used to constrain horizon picking (horizons stop at fault boundaries), reservoir volumetrics (fault blocks are the compartments), and flow modelling (faults can be sealing or conducting). The v1 picker here produces the polylines; connecting them into a 3D surface is a v2 feature.