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, find the faults by looking for offset reflectors. There is not one of them: about ten fault traces cut this section, which is what a real inline looks like and is why the survey ships thirty faults rather than a single tidy example. Start with the clearest. The largest here sits left of centre, roughly a third of the way across, and carries about 34 ms of throw. Note that it leans rather than standing vertical, drifting some nineteen crosslines between 300 and 1100 ms, so pick along the plane rather than dropping a vertical line through it.
• 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 Finish stick to close the fault stick. Every stick belongs to the fault named in the chips above the section, so press New fault before picking a different one, and click a chip to go back to a fault you started earlier. Naming them yourself is the point: a fault is a surface you decide to join together, and the panel will tell you if you joined two that are not one. Then move to another inline and pick the same faults again, because a fault surface is built from sticks along its length.
• Nothing you pick is final. Drag any handle on the active fault to move that vertex, alt-click a handle to delete it, shift-click a stick to remove the whole thing, and press Ctrl+Z, or Cmd+Z, to undo any of it. Real interpretation is mostly revision: you pick a trace, carry it three inlines, see what the fault does, and come back to fix the first one. Carry forward copies the current stick to the next inline and takes you there, which is how a surface actually gets built. It arrives dashed, because a copy is a hypothesis and not a pick: the fault dips, so the same trace on the next section is wrong by however far the plane moved. Drag it onto the plane and it counts. Leave it and the statistics panel will tell you how many of your sticks are still copies, since two identical sticks agree with each other perfectly whatever the data says.
• The panel under the section is the displacement of every fault in the survey plotted against inline, which is the shape a fault actually has: zero at one tip, a maximum somewhere in the middle, back to zero at the other. The faults you have mapped are drawn in your own colours with a tick at each inline you put a stick on. Watching your ticks cover only the middle of an ellipse is how you learn a surface is not finished.
• Open Fault statistics when you have a fault on two or more inlines. It measures your faults from your own picks, not from an answer key: how long each one is, how much it moves, whether the displacement dies towards the tips the way a real fault does, and where each sits on the displacement-length plot against the thirty faults in this survey and the scaling that natural populations follow. It also plots the cumulative frequency of the population, which is the curve fault populations are actually judged on: how many faults carry at least a given throw, length or displacement, on logarithmic axes where a power law is a straight line. You can fit a power law, a lognormal, an exponential or a straight line to it, and all four are scored the same way so their numbers can be compared. Read the margin between the best fit and the next one before believing any of them. On thirty faults spanning a single decade the power law leads the exponential by nine thousandths of an R squared, which is not enough to tell them apart, and the panel says so rather than naming a distribution the data cannot support. If a number is affected by something other than your picking, such as a neighbouring fault inside the measurement window adding its own offset to yours, the panel says so and by how much.
• Step to a different inline and pick the fault there too, using the arrows either side of the Inline slider or the left and right arrow keys once you have clicked the section. Moving one inline at a time is the point: the fault is the same geological feature across all of them, and the trace should change smoothly rather than jump. If it jumps, you picked something else on one of them.
• Watch the throw-along-strike panel under the section fill in as you pick. Throw rises to a maximum near the middle of the fault and dies to nothing at both tips, around inlines 1006 and 1056, and the ticks show which inlines you have actually picked. A fault picked on three inlines out of sixty is not a surface.
• After you have picked sticks on at least three inlines, press Grade. It reports how far your vertices sit from the true plane, and separately whether they scatter either side of it or sit consistently to one side. That second number is the one to read: a one-sided error is the signature of picking along the offset reflectors instead of through them, which is the mistake this section warns about above all others. The stick-to-stick smoothness check runs separately for each fault you picked, comparing a fault only against its own sticks, so mapping several faults at once costs you nothing.
• Toggle Show true fault plane to reveal every fault that crosses the inline, not just the one you are working on, with the heavier lines carrying the larger throw. Each line stops where its fault stops: a plane is drawn only over the interval where it actually offsets a reflector, so a short line means a small fault dying out, not a fault someone forgot to finish. Use it as a teaching aid, not as a crutch.
• Not every bend is a break. Where a fault reaches its tip its displacement dies to zero, so the reflectors sag across several traces instead of stepping across one. That drape is a real structure and worth noticing, but it is not a fault plane and no line is drawn on it. If you find yourself picking a feature that never resolves into a clean offset on any inline, you are almost certainly on the tip of a fault whose plane is somewhere else in the volume.

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 3D surface tab above does that joining for you, and the distinction it draws is the one that matters: your sticks are solid lines, the sheet lofted between them is translucent. One is what you measured and the other is what was assumed between your measurements. A surface built from three sticks across sixty inlines looks convincing from a distance and is mostly interpolation, which is easier to see once it is a shape you can orbit than it ever is on a section.