How to Interpret Seismic Data: A Practical Guide
What to look at first on a 2D seismic section, how to tell signal from noise, and which enhancement filters to apply before you start picking.
1. Read the axes before the image
A 2D seismic section plots trace number (or distance along the line) horizontally and two-way travel time (TWT) vertically, with time increasing downward. TWT is not depth: a reflector at 1.5 s may sit anywhere from roughly 1.5 km to 3 km down depending on interval velocity. Always confirm the vertical unit and the trace spacing before quoting a fault throw or a layer thickness — in SeismicEnhance those live in the Display / Scale panel, where you can show the TWT axis in seconds and a distance axis in metres or kilometres.
2. Identify continuous reflectors
Reflectors are the laterally continuous bright and dark bands produced by acoustic impedance contrasts — typically bedding planes, unconformities, or fluid contacts. Work from the strongest, most continuous events downward. Useful cues:
- Continuity — genuine geology tracks smoothly across many traces; noise does not.
- Polarity and amplitude — a consistent bright event that changes polarity laterally often marks a fluid or lithology change.
- Termination geometry — onlap, downlap and truncation against a surface identify unconformities and sequence boundaries.
3. Map faults by offsets, not by dark lines
A fault is inferred where a set of reflectors is systematically offset or abruptly loses coherence along a consistent trend. Follow the same reflector on both sides and measure the vertical separation — that is the throw in time. Steeply dipping zones of low similarity, rather than any single dark streak, are the reliable indicator. Migration artefacts and diffraction tails commonly mimic faults, so require the offset to repeat across several reflectors before committing a pick.
4. Judge the noise before you filter
Noise on a seismic section splits along two independent axes, and confusing them leads to the wrong filter:
- Noise level — how much of the section is low signal-to-noise. A magnitude question, answered by an SNR map.
- Noise type — random (speckle, ambient, broadband in the amplitude spectrum) versus coherent (ground roll, multiples, acquisition footprint, which appear as organised dipping energy or narrow spectral peaks).
The diagnostics panel in SeismicEnhance reports both axes plus the amplitude spectrum, so you can see whether you are fighting speckle or structured interference.
5. Apply enhancement filters in the right order
- AGC (automatic gain control) — normalises amplitude down the trace so deep, weak reflectors become visible next to shallow strong ones. Apply first; lengthen the window if it starts flattening real amplitude contrasts.
- Median filter — removes isolated random speckle while preserving edges. The right choice for random-dominant noise; keep the kernel small so thin beds survive.
- Structure-oriented filtering (SOF) — smooths along the estimated local dip rather than across it, improving continuity without blurring faults. The best general-purpose step before picking horizons.
- Unsharp masking — sharpens reflector boundaries after smoothing. Use sparingly; it amplifies whatever noise remains.
Filtering is a display aid, not a data fix. Toggle back to the raw section regularly and confirm any feature you interpret also exists before enhancement.
6. Use attributes to confirm interpretations
Seismic attributes recast the same data to emphasise one property. Instantaneous amplitude (envelope) highlights bright spots; instantaneous phase makes weak but continuous events stand out; instantaneous frequency drops across attenuating intervals; similarity (coherence) lights up faults and channel edges; sweetness helps flag potential reservoir intervals. Treat an attribute as corroboration for a feature you already see, not as evidence on its own.
7. Pick, quality-check, export
Pick the strongest horizon across the full line before moving to weaker ones, keep picks on a consistent phase (always the peak, or always the trough), then check that horizons and faults stay geologically consistent — horizons should not cross, and throws should vary smoothly along a fault. Export picks as CSV for mapping or depth conversion elsewhere.
Try it on your own line
Open the SeismicEnhance workstation and drop in a SEG-Y file — diagnostics, filters, attributes, and horizon/fault picking all run in your browser, with nothing uploaded to a server.