Time Slice

A time slice is a horizontal display cut through a 3D seismic data volume at a constant two-way travel time. Instead of showing how seismic amplitude varies with depth along a vertical profile (as a conventional seismic section does), a time slice shows how amplitude varies across a horizontal area at the same instant in the seismic record. Geologists use time slices to visualize lateral changes in seismic character that reflect sedimentary features such as channels, fans, fault patterns, and carbonate buildups. Because channels and other elongated features are often easiest to recognize when viewed from above rather than from the side, time slices have become a standard interpretation tool in 3D seismic analysis.

Key Takeaways

A time slice is defined by its two-way travel time (TWT) in milliseconds, not by a depth. A time slice at 1,500 ms represents a snapshot of the seismic wavefield at the moment 1.5 seconds of two-way travel time has elapsed. This time corresponds to different depths in different parts of the survey area depending on the local velocity; a time slice therefore cuts across geological structure rather than following a constant depth horizon.
Time slices are most useful for identifying linear and curvilinear geological features that have a distinctive amplitude contrast with the surrounding rock. River channels, submarine fan channels, and incised valleys show as elongated high-amplitude strips. Faults show as lineaments where the seismic character changes abruptly. Carbonate reefs and salt diapirs show as circular or irregular high-amplitude patches.
A horizon slice is a different display: it extracts seismic amplitude along a specific interpreted geological surface (such as the top of a reservoir) rather than at a constant time. A horizon slice follows the shape of the interpreted reflector and is therefore geologically meaningful in a way that a time slice (which cuts through structure) is not. Both displays are used in interpretation, but for different purposes.
The spacing between time slices determines the vertical sampling of features visible in plan view. Standard 3D seismic datasets are sampled at 2 to 4 millisecond intervals in time. Time slices are often displayed every 4 to 8 ms, corresponding to roughly 10 to 20 metres of depth interval at typical shallow velocities. Thinner features may not be visible if the time slice spacing is too coarse.
Spectral decomposition of time slices decomposes the seismic amplitude at each point into its component frequencies. Displaying the response at a single frequency (for example, 25 Hz or 40 Hz) on a time slice can highlight features that are invisible on the broadband amplitude display, because thin beds resonate at specific frequencies related to their thickness (the tuning thickness effect).

What Is a Time Slice and How Is It Used?

Picture a loaf of bread sitting on a table. You can cut it vertically (across the slices as they come off the knife) or horizontally (parallel to the table). A conventional seismic section is a vertical slice through the earth. A time slice is the horizontal slice, cutting across the loaf parallel to the table top.

In a 3D seismic volume, the vertical slices show how reflections stack up in depth. The horizontal slices (time slices) show how those reflections vary laterally. A meander belt of ancient river channels, for example, might show as a faint squiggly pattern on vertical sections but as a clearly recognizable curving ribbon on a time slice. The human eye recognizes curvilinear patterns in plan view far more easily than in cross-section, which is why time slices dramatically improved geologists' ability to map sedimentary systems when 3D seismic became widely available in the 1990s.

In the deep Gulf of Mexico, time slices through turbidite sands have revealed the full geometry of channel-levee systems: meandering main channels flanked by overbank levee deposits, feeding into lobe-shaped terminal fans. These features are the primary targets for deepwater oil and gas exploration. A time slice from a well-defined turbidite channel system looks strikingly similar to a satellite photo of a modern river delta, which confirms the geological interpretation through visual analogy with modern environments.

Fast Facts

The first commercial 3D seismic survey was shot offshore Texas in 1975. Time slice interpretation became a standard workflow in the early 1980s as interactive seismic workstations allowed geologists to step through time slice volumes on screen for the first time. Before digital workstations, time slice interpretation required printing paper sections at every time level and physically laying them out in sequence, which was impractical for all but the most critical prospects. Today, seismic interpretation software (Petrel, Kingdom, OpendTect) displays time slices and animated time slice movies that geologists step through to trace channel systems across the survey area.

Reading a Time Slice: What Features Look Like

Amplitude on a time slice is displayed using a color scale, typically with one color for positive (peak) amplitudes and another for negative (trough) amplitudes, and white or transparent for near-zero amplitudes. The interpreter looks for patterns in this amplitude map.

Channels appear as elongated sinuous high-amplitude features when the channel sand has higher acoustic impedance than the surrounding shale, or as elongated low-amplitude features if the impedance contrast is reversed. Levee deposits appear as lower-amplitude wings flanking the main channel. Point bars inside meander loops show as concentric amplitude arcs.

Faults appear as discontinuities where seismic character changes abruptly across a linear or curvilinear trend. In salt basins (Gulf of Mexico, North Sea), the base of salt and the top of salt produce strong amplitude anomalies on time slices. Salt diapirs show as circular features where the seismic character inside the salt is chaotic (salt has no internal stratigraphy) surrounded by more organized stratified reflections.

Bright spots (high-amplitude anomalies associated with gas sands) often stand out on time slices as circular to lobate features if the gas sand has limited areal extent. Amplitude anomalies conformant to structure (following the four-way closure of a dome or anticline) are a strong indicator of hydrocarbons rather than lithology effects, and this conformance is much easier to verify on a time slice than on a vertical section.

Time Slices Versus Horizon Slices

Time slices cut at constant time and therefore cut through structural features at an angle, mixing reflections from different geological horizons in structurally deformed areas. In a steeply dipping anticline, a time slice at 1,800 ms might sample the top of the reservoir on the crest of the structure and a different, deeper horizon on the flanks. For structural mapping this is a problem; for looking at lateral amplitude variations without worrying about the exact stratigraphic level, it is acceptable.

Horizon slices solve this problem by following an interpreted reflector surface through the data volume. The seismic interpreter picks the top of the reservoir (or any other reflector) on every vertical line in the 3D grid and uses those picks to define the extraction surface. Amplitude is then extracted along this surface, which follows the true geological horizon rather than a constant-time plane.

In the Duvernay shale fairway of Alberta, operators use horizon slices extracted along the top of the Duvernay to map lateral changes in total organic carbon and maturity, which correlate to amplitude at specific frequencies. The horizon slice is geologically precise because it always samples the Duvernay top regardless of how the formation dips across the survey. A time slice in the same area would mix the Duvernay response with overlying Ireton shale response on the structural highs and with deeper Leduc carbonates on the lows.

A time slice is also called a horizontal section, a seismic plan view, or a constant-time section. Related terms include horizon slice (a seismic amplitude display extracted along an interpreted geological surface rather than at constant time; geologically accurate in deformed areas where a time slice would cross structural boundaries), seismic attribute (any quantity derived from the seismic data beyond raw amplitude; attributes displayed on time slices include instantaneous phase, frequency, and envelope, each highlighting different aspects of the seismic character), spectral decomposition (a seismic processing technique that decomposes the seismic amplitude into its frequency components; frequency-specific amplitude maps on time slices can reveal thin-bed features invisible on broadband amplitude displays), amplitude anomaly (a zone of unusually high or low seismic amplitude that may indicate hydrocarbons, gas hydrates, or lithology changes; anomalies conformant to structure on time slices are a key indicator of a direct hydrocarbon indicator), and 3D seismic (a seismic survey in which data is recorded over an areal grid rather than along a single line, producing a three-dimensional volume that can be sliced vertically as seismic sections or horizontally as time slices and horizon slices).

How a Time Slice Revealed a Missed Channel Pay Zone Worth 12 MMboe on a Norwegian Shelf Block

An exploration team had evaluated a 3D seismic dataset over a Cretaceous shelf block on the Norwegian Continental Shelf for two years, focusing their interpretation on conventional vertical seismic sections tied to two nearby wells. Their structural map showed a 4-way dip-closed anticline with two-way closure of 80 metres. A volumetric calculation indicated a prospective resource of 40 million barrels of oil equivalent (MMboe). The prospect was ranked moderate-confidence and was proposed for drilling but had not yet been approved.

A visiting geologist from the company's deepwater Gulf of Mexico team was shown the prospect data during a technical review. Accustomed to working with time slices to identify channel systems, she displayed a sequence of time slices through the Cretaceous interval. At 2,020 ms two-way time, a sinuous high-amplitude feature appeared in the northwestern corner of the data, about 6 kilometres from the anticlinal crest. The feature had the geometry of a submarine fan channel and had not been interpreted in the original work because the vertical sections crossing it did not show it prominently.

The team returned to the data and mapped the channel system in detail using both time slices and horizon slices extracted along the Cretaceous base. The channel connected to the anticlinal closure from the northwest, suggesting that turbidite sands could be filling both the structural trap and the stratigraphic trap of the channel itself. The combined prospect now included the original structural volume plus an estimated 12 MMboe of additional in-place resource in the channel fairway.

The revised prospect was approved for drilling. The well encountered oil in both the structural crest and in the channel sands, confirming the two-part trap. The channel sands alone would have justified the well. The key insight required only one hour of time slice navigation through existing data that had been available for two years. Changing the display type from vertical section to horizontal plan view changed what the interpreter could see.