Coherent
In seismic interpretation, "coherent" describes seismic events (reflections) that show continuity from one trace to the next across a seismic section. A coherent reflector traces smoothly across many adjacent traces, indicating a continuous geological boundary in the subsurface. An incoherent reflector breaks up, jumps around, or fails to link from trace to trace, indicating either a discontinuous geology, a faulted boundary, or noise in the data. Coherence is one of the most fundamental concepts in seismic interpretation. Modern seismic processing includes a whole class of techniques (coherency volumes, semblance attributes, geometric attributes) designed to highlight where coherence breaks down, because those breaks often correspond to faults, channels, fluid contacts, or other features the interpreter wants to map.
Key Takeaways
- A coherent seismic event is a reflection that maintains its character (amplitude, phase, frequency) across many adjacent traces, allowing it to be traced as a continuous reflector. The continuity is visual evidence that the seismic data is recording a real subsurface boundary.
- Incoherent events break the trace-to-trace continuity. Faults are the classic cause of seismic incoherence, because the reflector on one side of the fault is offset from the reflector on the other. Stratigraphic features like channels, fluid contacts, and pinchouts also produce localized incoherence.
- Coherency attribute volumes, introduced in the early 1990s by Mike Bahorich and Steve Farmer at Amoco, compute a coherence value at every point in a 3D seismic survey. The output volume highlights faults, channel edges, and other discontinuities as low-coherence features against a background of high-coherence layered sediments.
- Coherency is one of a family of seismic attributes that measure trace-to-trace similarity. Related attributes include semblance, eigenstructure, variance, and several proprietary geometric measures developed by major seismic interpretation software vendors. Each attribute is sensitive to slightly different aspects of trace similarity.
- Modern seismic interpretation routinely uses coherency volumes alongside the original amplitude data. Faults that are subtle on amplitude data often show up clearly on coherency. Channel systems, salt diapir edges, and gas chimneys often appear sharper on coherency volumes than on the original seismic.
Fast Facts
Bahorich and Farmer's 1995 paper in The Leading Edge introducing the coherence cube is one of the most cited papers in seismic interpretation history. The technique was so immediately useful that within five years it was standard practice in every major operator's interpretation workflow. Modern coherence algorithms have evolved well past the original cross-correlation method, but the basic concept (compute trace-to-trace similarity, display where it breaks down) is unchanged. The technique transformed fault interpretation in 3D seismic from a manual effort into a semi-automated process that can map thousands of faults in a survey within hours.
What Coherence Means in Seismic Practice
Imagine a striped shirt with horizontal stripes running across it. The stripes are coherent: each stripe continues smoothly from one side of the shirt to the other. Now imagine the shirt has a tear running diagonally across it. The stripes on one side of the tear no longer line up with the stripes on the other side. The tear has broken the coherence.
Seismic data behaves similarly. A reflection from a continuous geological boundary (the top of a sandstone, the base of a salt body, an unconformity) appears on the seismic section as a wavy line that traces smoothly across many adjacent traces. The line is coherent. A fault that cuts the same boundary offsets the reflection on one side relative to the other. The line is no longer coherent across the fault: it jumps abruptly, just like the stripes across the tear in the shirt.
Interpreters use coherence both qualitatively (visual recognition that a reflector is continuous or broken) and quantitatively (computed coherence attributes that produce numerical values at every point in the data). The quantitative version is what powers modern fault-mapping workflows.
Where Coherence Helps Interpretation
Fault interpretation is the largest single application. A 3D seismic survey may contain hundreds of faults, ranging from large basin-bounding structures to small ones too subtle to see on amplitude data. A coherency volume highlights every break in the layered sediment fabric as a low-coherence feature. The interpreter scrolls through coherence time slices, recognizes faults as linear or curved low-coherence streaks, and digitizes the fault traces in a fraction of the time the amplitude-only workflow would require.
Channel mapping is the second major application. A buried fluvial or turbidite channel cuts through layered sediments, breaking the layer continuity at the channel edges. The channel itself often has internal stratigraphy that differs from the surrounding rock. Both effects produce low-coherence signatures that highlight the channel against the background. Coherency volumes in the deepwater Gulf of Mexico, the Niger Delta, and the Brazilian Campos Basin have mapped hundreds of buried channel systems, many of them productive reservoirs.
Salt diapir interpretation also uses coherency heavily. The flanks of a salt body create steep dipping reflectors that look incoherent when computed against laterally adjacent flat reflectors. The coherency volume shows the salt edge as a clear ring of low coherence at every depth slice. Norwegian Continental Shelf operators, Australian operators in the Bass Strait, and Gulf of Mexico operators all rely on coherency for salt interpretation in complex deepwater settings.
Synonyms and Related Terminology
"Coherent" in seismic context is distinct from "coherent" in optics or electronics, which refers to phase-aligned waves. The seismic usage is closer in meaning to "continuous." Related terms include coherency attribute (a computed seismic measure of trace-to-trace similarity; produces a 3D volume that highlights discontinuities including faults, channels, and stratigraphic boundaries), seismic attribute (any quantitative measure derived from the seismic data, used to highlight features of interest; coherence is one of the most widely used attributes), fault interpretation (the process of identifying and mapping faults in seismic data; the application where coherency volumes provide the largest interpretation efficiency gain), seismic interpretation (the broader discipline of converting seismic reflection data into geological understanding; coherence is one of many tools the interpreter uses), and 3D seismic (the volumetric subsurface imaging dataset where coherence attributes are most commonly applied; provides the trace-to-trace continuity that coherence calculations rely on).
Why a Single Computed Volume Cuts Interpretation Time in Half
An exploration team is interpreting a 600-square-kilometre 3D seismic survey over a Western Canadian sedimentary basin prospect. The original amplitude volume is large and the structural setting is complex, with multiple intersecting fault systems. Manual fault interpretation on the amplitude data alone would take a single interpreter approximately three months of full-time work to complete.
The team computes a coherency volume from the amplitude data using their interpretation software's standard coherency algorithm. The coherency volume is generated overnight. The next day, the interpreter loads the coherency volume alongside the amplitude volume and starts reviewing time slices. Every fault in the survey appears as a low-coherence streak on the coherency time slice. Many of the faults are subtle on amplitude but obvious on coherency.
The interpretation that would have taken three months on amplitude alone takes six weeks with the coherency volume guiding the work. The fault map is more complete, particularly for the smaller faults that affect reservoir compartmentalization. The downstream reservoir simulation and well planning benefit from the more detailed fault picture.
The cost of the coherency volume was negligible (a one-time compute job on the existing data). The interpretation time saved was approximately six weeks of senior interpreter work, worth perhaps CAD 90,000 in directly attributable cost, plus some larger but harder-to-quantify gain in interpretation quality. Coherence is one of the few seismic processing techniques where the value-to-cost ratio is so obviously favourable that it is now embedded in every interpretation workflow as a standard step rather than an optional one.