Polarity: SEG Standard Convention, Zero-Phase Wavelets, and Seismic Interpretation in the WCSB

Polarity describes the sign convention that ties the physical property being measured to the way it is displayed or recorded, and in oil and gas the term appears in three distinct settings: the positive and negative excursions of a seismic wavelet, the positive and negative terminals of electrical equipment such as logging tools and downhole motors, and the north-south orientation of magnets and the Earth's magnetic field used in magnetometer surveys and measurement-while-drilling (MWD) directional sensing. In reflection seismology, which is where the term carries the most interpretive weight, polarity defines whether an increase in acoustic impedance at a geologic boundary is plotted as a peak or a trough on the seismic section. The Society of Exploration Geophysicists (SEG) fixed a standard to remove ambiguity: for a causal signal, the onset of a compression from an explosive source is recorded as a negative number, a convention rooted in early refraction work where first arrivals broke downward. For the zero-phase wavelet that dominates modern processed data, SEG normal (positive standard) polarity represents a positive reflection coefficient, meaning an increase in acoustic impedance, as a central peak, conventionally shaded black on a variable-area or variable-density display. Reverse, or negative standard, polarity flips this so the same impedance increase appears as a trough. This is not a trivial bookkeeping matter. Whether a bright amplitude is read as a hard event (a tight carbonate or a cemented sand) or a soft event (gas-charged sand, coal, or an overpressured shale) depends entirely on which convention the data volume follows, and getting it wrong inverts the geologic story. The problem is compounded because different regions adopted opposite traditions: North Sea data historically used the reverse of the SEG zero-phase standard, so interpreters moving a workflow between basins must confirm polarity explicitly rather than assume it. In the Western Canadian Sedimentary Basin, where amplitude-versus-offset analysis is used to chase gas-charged Cretaceous sands and to map Devonian carbonate reefs such as Leduc and Nisku, the first step of any quantitative interpretation is to establish the polarity of the volume, usually by tying to a well with a measured sonic and density log so the synthetic seismogram and the field data agree. Polarity is verified at the seafloor or surface tie, carried through every processing step, and stated explicitly in the SEG-Y reel header and in the deliverable report, because a single sign error can turn a drillable gas anomaly into a phantom and send a CAD 5 million well to a dry hole.

Establishing Polarity with a Synthetic Seismogram

The definitive polarity check is a well tie. An interpreter computes acoustic impedance from a well's sonic and bulk-density logs, derives the reflection-coefficient series, convolves it with the extracted wavelet, and compares the resulting synthetic seismogram to the field trace at the well location. If the top of a known hard layer such as the Wabamun or a Nisku carbonate produces a peak in the synthetic and a peak in the data, the volume is SEG normal polarity. A mismatch means the data is reverse polarity, or the wavelet phase is wrong. This tie is repeated at several wells across a 3D survey because processing artifacts and phase rotation can vary spatially, and the confirmed polarity is then carried into every amplitude and inversion product.

Polarity in AVO and Bright-Spot Analysis

Amplitude-versus-offset work in the WCSB hunts for gas sands that lower acoustic impedance relative to encasing shale, producing a negative reflection coefficient at the sand top. On an SEG normal volume that top is a trough; the base of the gas sand, an impedance increase, is a peak. Interpreters look for this trough-over-peak doublet at the target. If polarity is mis-stated, the doublet reads inverted and a genuine Class III gas anomaly can be dismissed as noise, or a hard streak misread as pay. Every AVO deliverable in Alberta therefore states the polarity convention on the section and in the header so the next interpreter inherits an unambiguous volume.

Related Terms

Polarity only has meaning relative to a Wavelet, since it is the wavelet's central lobe whose sign the convention fixes; a zero-phase wavelet makes polarity readable directly off the section. The property being mapped is Amplitude, whose sign and magnitude drive bright-spot and AVO work. Polarity is verified within Seismic Data by tying to Acoustic Impedance computed from well logs, the rock property whose contrast at a boundary generates the reflection in the first place.

Real-World WCSB Scenario: A Polarity Error on a Viking Gas Prospect

A junior explorer in west-central Alberta licensed a reprocessed 3D volume to evaluate a Viking gas-sand prospect and mapped a strong peak at the target, interpreting it as a hard, tight sand and nearly dropping the lead. A review geophysicist tied the data to an offset well with sonic and density logs, built a synthetic, and found the volume was reverse SEG polarity: the processing house had flipped the sign and not flagged it in the report. The mapped peak was actually a trough in SEG normal terms, a soft event consistent with a gas-charged Viking sand.

With polarity corrected the prospect screened as a textbook Class III AVO anomaly. The operator drilled a CAD 4.8 million well that logged 11 m of gas-charged Viking sand and tied into a nearby gathering system within the year. A 30-minute well-tie check had reversed a decision to abandon a commercial gas pool.