Welcome: seeing a seismic cube
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 the physics, the geology or the mathematics, look at what you will be working with: a seismic volume.
A seismic survey sends sound into the earth and records its echoes. Each echo is a reflection from a boundary below, where the rock above differs from the rock beneath: the top of a sandstone, a fault plane, the roof of a salt dome. A 3D survey records over a whole area with a dense grid of sources and receivers, so each small patch of the subsurface, a bin, is recorded many times by different source and receiver pairs. Processing combines those recordings into one trace per bin, and the traces side by side make a cube: one amplitude value at every bin and every time sample.
The three axes you will see everywhere
- Inline, one of the two horizontal axes. Inlines usually run in the direction the vessel sailed at sea, or along the receiver lines on land. Fixing one inline gives a vertical section, a profile through the ground.
- Crossline, the other horizontal axis, at right angles to the inlines. Fixing one crossline gives the perpendicular vertical section.
- Time, downward: the two-way travel time of an echo, down to the reflector and back. A time slice fixes one time and gives a map. It is a map at one time, not at one depth, and it follows a single layer only where that layer is flat.
The figure below is real data: a 5 km square of the F3 Netherlands survey, offshore in the Dutch sector of the North Sea, with 200 inlines and 200 crosslines at 25 m and 300 samples per trace from 400 to 1596 ms two-way time. Move the three slices through it and read the same samples four ways: in 3D in (a), as a map at one time in (b), as a vertical section in (c) and as the single trace where the slices cross in (d). Start with the time slice. In this window a flat reflector sits near 470 ms, strong reflectors near 650 and 1050 ms, the layers dip between about 700 and 1000 ms, and between about 1200 and 1350 ms the reflectors break into short pieces.
The figure opens where the slices cross at inline 400, crossline 600 and 940 ms, on one sample of −1.20 times the cube’s RMS amplitude. It is negative, so the red-blue palette draws it pale blue: red is positive, blue negative and white zero. In the SEG normal polarity that the F3 header records, a positive amplitude on zero-phase data marks an increase in acoustic impedance downward, often the top of a harder layer. Polarity conventions are not universal, and some regions use the opposite sign, so a recurring rule in this book is check your conventions before you interpret. Gain is no different from the palette: it only moves the amplitude at which the colour saturates, and the table and the trace in (d) stay the same.
The same sentence says the time slice at 940 ms is 50 % positive: it cuts across layers that dip, so each layer crosses the map as a band. Move it up to 472 ms and 92 % of it is positive, because there it lies along one flat layer. A time slice is a map at one two-way time. A map of one layer, a horizon, has to be picked from reflector to reflector, which Part 2 does, and turning either into depth needs a velocity, for an average velocity above it, which Section 1.5 takes up.
The rest of Part 1 builds the physics behind those colours: acoustic impedance and reflection coefficients, the convolutional model, and how processing turns recorded echoes into an image of the subsurface. For now, rotate the cube until three dimensions at once feel natural. 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, acquired for hydrocarbon exploration and released under an open license by dGB Earth Sciences and the Open Seismic Repository. It is one of the most widely used open datasets for teaching seismic interpretation. The cube here is a 48 MB window of the full survey, small enough to load in a browser.
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, one trace per bin.
- Inline / Crossline, the two horizontal axes of the volume. Together they tile the survey area; fixing either gives a vertical section.
- Time slice, a horizontal cut through the volume at one two-way travel time: a map at one time, not at one depth, and not of one layer unless that layer is flat.
- Amplitude, the value of one sample. Positive or negative; its size relates, through the physics of Sections 1.1 and 1.2, to the strength of a reflection.
- 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, a display setting that moves the amplitude at which the colour saturates. It does not alter the data.
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.