Welcome: seeing a seismic cube

Part 1 — Foundations of Seismic

Learning objectives

Understand what a 3D seismic volume represents
Navigate inlines, crosslines, and time slices
Read amplitude color (red–white–blue polarity convention)

Welcome to 3D Seismic Interpretation. Before we dig into the physics, the geology, or the math, let's look at what we're actually going to be working with: a seismic volume.

A seismic survey sends sound waves into the earth and records their echoes. Each echo is a small reflection from a boundary deep below — the top of a sandstone, the edge of a fault, the roof of a salt dome. A 3D survey covers an area of the surface with a dense grid of recording positions, so each point in the subsurface gets imaged from many directions. The computer processes all those recordings and produces a cube of data: three axes of amplitude measurements, one value at every grid cell.

The three axes you will see everywhere

Inline — one of the two horizontal axes on the earth's surface. Historically, inlines follow the direction the boats or trucks moved during acquisition. Picking an inline gives you a vertical cross-section that looks a bit like a geological profile.
Crossline — the other horizontal axis, at right angles to the inlines. Another vertical cross-section, seen from the perpendicular direction.
Time (or depth) — downward. A time slice is a horizontal cut through the cube at one travel-time value. It looks like a map of the subsurface at a particular depth.

The interactive cube below is real seismic data — a teaching-sized window from the F3 Netherlands survey, acquired offshore in the Dutch sector of the North Sea. You are looking at a 200 × 200 grid of subsurface locations, with 300 time samples spanning the 400 to 1596 ms two-way-time window. That window covers a well-studied Tertiary-age section with mixed sand-and-shale stratigraphy, a dense fault network, and — if you look in the right place — a gas chimney and some classic bright-spot reflectors.

Try this

Drag the cube to rotate it. You are looking at three slices through the F3 volume: one inline (side face), one crossline (front face), one time slice (top face).
Move the Inline slider and watch the side face change. You are scrolling through the volume in one direction across real North Sea geology.
Move the Time slider. The top face changes — you are looking at a map-view at different two-way times. Around 600–900 ms you should see strong, continuous reflectors; deeper, the image becomes busier with faults.
Switch the palette from Red–White–Blue to Grayscale. The same data, different convention. Many interpreters prefer grayscale for structural work and RWB for stratigraphic work.
Slide the Gain up and down. Gain is a display setting only — it does not change the data, just how the amplitude range maps to color. Real F3 amplitudes reach tens of thousands of units, so you may need to turn gain down on high-frequency events.

References

Brown, A. R. (2011). Interpretation of Three-Dimensional Seismic Data (7th ed.). AAPG Memoir 42 / SEG IG13.
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.
Sheriff, R. E. (2002). Encyclopedic Dictionary of Applied Geophysics. Society of Exploration Geophysicists.

On amplitude color. In the Red–White–Blue palette above, red means a positive amplitude (at SEG-positive polarity, an increase in acoustic impedance — often the top of a harder layer). Blue means negative. White is zero. That visual convention — positive on one color, negative on the other, zero in the middle — is what makes polarity readable at a glance. Polarity conventions are not universal: some regions use the opposite sign. A recurring theme in this textbook will be check your conventions before you interpret.

In Part 1 proper we will build up the physics behind why those colors mean what they mean — acoustic impedance, reflection coefficients, the convolutional model of seismic, and how a squiggly trace becomes a recognizable image of subsurface geology. For now, play with the cube. Rotate it until you feel comfortable seeing three dimensions at once. That spatial fluency is the first skill an interpreter develops.

About the F3 dataset

F3 is a 3D seismic survey in the Dutch sector of the North Sea, originally acquired for hydrocarbon exploration. It has been released under an open license by dGB Earth Sciences and the Open Seismic Repository, and is the de facto teaching dataset in academic seismic interpretation. The subset you are exploring is a 48 MB window extracted from the full survey; the complete volume is hundreds of times larger. When you study a real interpretation project later in this textbook, it will almost certainly be F3 or a similarly-open volume.

Data courtesy of dGB Earth Sciences, distributed via the Open Seismic Repository under CC BY-SA 4.0.

Vocabulary you just learned

Volume — the full 3D grid of seismic amplitudes produced by a survey.
Inline / Crossline — the two horizontal axes of the volume. Together they tile the surface; each vertical cut along one is a cross-section.
Time slice — a horizontal cut through the volume at a fixed two-way travel time. Shows you how things look at "that depth" across the survey area.
Amplitude — the value at a single voxel. Positive or negative; its magnitude relates (through a chain of physics we will cover in §1.2) to the strength of a reflector.
Polarity convention — the agreement about whether a positive amplitude means "impedance increase" or "impedance decrease". Two conventions exist; always check which one a dataset uses.
Gain — display setting that scales the amplitude-to-color mapping. Does not alter underlying data.