Polarity, phase, and wavelet conventions

Part 2 — The Interpreter's Toolkit

Learning objectives

Distinguish SEG positive from European polarity conventions and their visual consequences
Recognize zero-phase, minimum-phase, and phase-rotated wavelet shapes on a seismic trace
Identify the convention of an unfamiliar dataset by inspection and by well-to-seismic tie
Explain why misreading convention inverts every amplitude-based interpretation

The most consequential thing about a seismic dataset is not its geology — it is the convention under which the geology is displayed. Two different processing teams can take the same raw field data and produce datasets that look inverted or shifted relative to each other, even though the underlying earth has not moved. Learning to read those conventions is prerequisite to any amplitude work.

Polarity: what positive amplitude means

Polarity convention answers one question: does a positive amplitude on the display correspond to an increase or a decrease in acoustic impedance going into the reflector? Two conventions coexist globally:

SEG (positive) polarity — used in much of the world, especially North American and European oil companies working with SEG-standard processing. Impedance increase = positive amplitude = peak on the display.
European (negative) polarity — historical convention in parts of the European hydrocarbon industry. Impedance increase = negative amplitude = trough on the display. The same rocks produce visually opposite displays under this convention.

There is no "correct" convention. Each is internally consistent. The danger is using one while thinking you are using the other. A bright trough on SEG-positive data is a gas sand signal (impedance drop); the same visual on European-convention data is a hard event (impedance rise). Invert the convention and you invert the hydrocarbon call.

Phase: how the wavelet is shaped

Phase is a description of how the wavelet arranges its energy in time. Three shapes dominate:

Zero-phase — the wavelet is symmetric around its peak. Zero-phase peaks sit exactly at the reflector times. This is the ideal for interpretation, and most modern processing tries to achieve it through phase correction.
Minimum-phase — the wavelet is causal: all its energy lies at or after the reflector time, with the peak slightly lagged and a slow decay afterwards. This is the natural shape of many seismic sources (dynamite, some airgun sources) before any phase processing. The apparent event on the trace follows the actual reflector by some fraction of a period; picking the peak gives you a location lagged behind the reflector by that fraction.
90°-rotated (or more generally, phase-shifted) — the wavelet has been rotated in phase by a quarter period. The reflector now sits on a zero crossing, with a peak on one side and a trough on the other. Pick the peak and you miss the reflector by a quarter period; pick the zero crossing and you get the reflector correctly, but noise makes zero crossings harder to pick than extrema. Phase rotations can be accidentally introduced in processing and need to be detected and corrected.

The widget above lets you hold the underlying physics constant — one reflector, one reflection coefficient — and cycle through every combination of convention and phase to see what the trace looks like. Set a positive R of 0.15 (a typical hard-over-soft boundary). Switch polarity from SEG to European. The peak becomes a trough. Same rocks, different sign on the display.

Now switch phase from zero-phase to 90°-rotated while keeping SEG positive and R at 0.15. Notice that the maximum amplitude no longer sits at the reflector time — instead there is a zero crossing at the reflector, with a peak-and-trough pair straddling it. An interpreter who picks the peak in this configuration places their horizon a quarter period above the true reflector. Over a section, that error compounds into systematic depth mis-picks across the whole survey.

Identifying the convention of an unknown dataset

When a new dataset lands on your desk without clear documentation, there are three ways to figure out the convention:

Processing report. Any competent processing team includes a polarity and phase statement in the project report. Always read this first.
Known reflector. If the section contains a feature with known physics — the water-bottom reflector on marine data (water-over-sediment is a strong impedance increase), or the top of a known regional salt body — check its sign. A water-bottom peak means SEG positive; a water-bottom trough means European.
Well-to-seismic tie. The definitive test: at a well location, compute a synthetic seismogram from the sonic and density logs, then compare it to the real seismic trace at that location. The synthetic tells you what the seismic should look like under each convention; the one that matches is the one your dataset is using. Well ties are the ground-truth calibration for every interpretation; we will build one explicitly in Part 8.

Why misreading convention is catastrophic

A misinterpreted convention inverts every amplitude-based inference you make. The top of a reservoir becomes the base and vice versa. A gas bright spot becomes an unusually quiet zone. An AVO Class III anomaly (soft sand in shale) becomes an AVO Class IV (harder sand in shale). Every conclusion flips.

The saving grace is that convention errors are usually SYSTEMATIC — if you got it wrong, you got it wrong everywhere in the same way. A quick cross-check ("does the water bottom look right?") exposes the error in 30 seconds. The interpreters who make career-ending mistakes are the ones who never check. Always check.

References

Bacon, M., Simm, R., & Redshaw, T. (2003). 3-D Seismic Interpretation. Cambridge University Press.
Sheriff, R. E., & Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.
Brown, A. R. (2011). Interpretation of Three-Dimensional Seismic Data (7th ed.). AAPG Memoir 42 / SEG IG13.
Hilterman, F. (2001). Seismic Amplitude Interpretation. SEG/EAGE Distinguished Instructor Short Course.