Flattened Section
A flattened section is a seismic display where one specific reflector has been stretched and squeezed until it appears as a perfectly horizontal line across the screen. Every other reflector above and below it is shifted by the same amount as the flattening reflector, so the structure of those other layers is shown relative to the flattened one. The technique lets a geologist read the picture as if the chosen layer was the original sea floor at the time it was deposited, removing later folding and tilting from the display. It is one of the simplest and most powerful interpretation tools in seismic geology.
Key Takeaways
- Flattening a seismic section means picking a horizon (an interpreted reflector that follows a single geological surface) and shifting every trace in time so that horizon becomes a flat line at a chosen depth or time.
- The flattened display shows the structural geometry that existed at the time the chosen horizon was deposited, before later folding and faulting deformed everything sitting above it. This is sometimes called a paleogeologic or paleostructure view.
- Flattening is most useful when the modern structural geometry hides important geological detail. A growing salt diapir, an evolving rift basin, or a buried sand channel can all be much easier to interpret in a flattened section than in the original time section.
- Flattening is purely a display technique, not a processing step. The underlying data is unchanged. The interpreter can flatten on any horizon at any time, then unflatten and try a different one. Most modern interpretation software (Petrel, OpendTect, Kingdom) supports flattening with a single click.
- The technique only works correctly when the chosen horizon was approximately flat at the time it was deposited. If the original deposition surface had significant relief, flattening it imposes a false reference and the deeper layers appear distorted.
Fast Facts
Flattening was a manual exercise in the era of paper seismic sections. Interpreters used a flexible ruler to trace the chosen horizon, then shifted every other reflector vertically by hand to remove the folding. The process took hours per section and was prone to errors. The shift to digital workstations in the 1980s reduced the work to a few seconds. Today, geologists routinely flatten a 3D seismic survey on multiple horizons in sequence to watch the structural evolution of a basin layer by layer, building up a sequence of paleo-views from the deepest interpreted horizon up to the present day.
What a Flattened Section Looks Like, Explained
Take a stack of pancakes. Pour a generous amount of syrup over the top. The syrup sinks into the pancakes and the whole stack settles unevenly. The top pancake is no longer flat. None of them are. If you tilt your head until the third pancake from the top looks flat across, you can see exactly how the pancakes above and below it sagged differently. That is the basic idea behind a flattened seismic section.
A seismic section is a vertical slice through the earth showing geological layers as wavy reflectors. Over millions of years, those layers get folded, tilted, and faulted. By the time the seismic survey is shot, the modern geometry can be a tangled mess. Picking a single layer and pinning it flat lets the interpreter see the geometry of everything else relative to that one layer. The deeper layers reveal what was already deformed by the time the flattened layer was deposited. The shallower layers show what folded after.
The technique is purely visual. The seismic data does not change. The flattening just shifts the time or depth axis at every trace by the amount needed to put the chosen horizon at a constant level. Run the same operation on a different horizon and the picture changes again, this time showing the world from that earlier (or later) layer's reference frame.
Where Flattened Sections Earn Their Keep
Salt tectonics is one of the strongest cases. A salt diapir grows over millions of years, pushing the layers above it into a dome shape. The shape of the dome at any given time depends on how much salt has moved by that time. A flattened section on the top of salt shows what the area looked like when the salt was at its current top depth. Flattening on a younger horizon shows the geometry at that later time. Comparing the two gives a movie of how the salt grew. Operators in the deepwater Gulf of Mexico, the offshore Gabon margin, and the Norwegian Continental Shelf all use this approach to map salt-related traps and predict where the next prospective trap might sit.
Channel mapping is another natural application. A buried sand channel laid down 30 million years ago is now twisted and faulted by everything that happened since. Flattening on a horizon just above the channel restores the channel to roughly its depositional geometry. The interpreter can trace the channel's path much more easily, and the geometry helps predict where the channel is wide and thick (good reservoir) versus narrow and thin (poor reservoir).
Rift basins benefit too. The fault patterns and sediment thicknesses in an active rift change over time. Flattening on a syn-rift horizon shows where the active faults were at that time, which often differs from where they ended up. Mapping the active fault locations through time helps interpret which faults charged the reservoir and which acted as later seals.
Synonyms and Related Terminology
A flattened section is also called a flattened display, a paleostructure view, or a horizon-flattened section. Some interpreters distinguish between "flattening" (visual shift of the display) and "decompaction" (a more physically rigorous restoration that removes the effect of overburden compaction in addition to the geometric flattening). Related terms include horizon (an interpreted seismic reflector that follows a single geological surface across the survey area; the basis for any flattening operation), seismic interpretation (the process of converting seismic reflection data into a geological model of the subsurface; flattening is one of the standard techniques used during interpretation), reflector (a seismic event corresponding to a contrast in acoustic impedance at a geological boundary; the basic data unit that flattening shifts and aligns), paleogeology (the geological geometry of an area at some specific time in the past; flattened sections are a quick way to approximate paleogeologic views without doing a full restoration), and 3D seismic (the volumetric subsurface imaging dataset where flattening is most commonly applied; modern 3D interpretation routinely flattens multiple horizons in sequence to build up the depositional history of a basin).
Why Pinning One Layer Flat Reveals What the Modern Picture Hides
An interpretation team working a deepwater minibasin in the Gulf of Mexico is trying to map a sub-salt prospect. The original time section shows the salt body and the layers above it clearly. The layers below the salt are present but warped, with reflectors bending around the salt and through several faults. A small four-way structural closure might or might not be present at the target depth. Two interpreters disagree about whether the closure is real or an artefact.
The team flattens the section on the top of salt. The salt becomes a horizontal line. Everything below the salt is shifted upward by the corresponding amount at every trace. The view that emerges is what the area looked like when the salt was at its current top depth. The sub-salt structure now appears as a clear anticline with definite four-way closure, perched on the flank of an older fault. The closure is real. The faulting that obscured it in the original section was post-salt deformation, irrelevant to the original trap geometry.
The flattened section settled the disagreement in a few seconds. Without it, the team would have spent weeks trying to model the salt geometry forward and would still not have had a clean answer. The technique itself is over fifty years old, but its value has only grown as deepwater prospects have moved into more structurally complex settings where the modern geometry tells you almost nothing about what the trap actually looks like.