2D vs 3D vs 4D vs 4C: survey dimensionality

The coarsest but most useful classification of a seismic survey is its dimensionality. It determines the image you can produce, the cost, and the class of problem it can solve.

2D

A single line of shots and receivers. The output is one 2D cross-section. Still useful for regional reconnaissance, frontier basin exploration, and over-land pipeline scouting — cheap and fast. Drawbacks: out-of-plane energy (reflections from sideways features) aliases into the image as “side-swipe” that misleads interpretation; any dip out of the line’s plane is mismigrated. 2D is rarely the final deliverable for modern exploration.

3D

A grid of shots and a grid of receivers. The data volume has three indexable axes: inline, crossline, time (or depth after imaging). Every reflection point is sampled from multiple azimuths and offsets, so out-of-plane energy is correctly placed, dip and azimuth are recoverable, and fold is uniform across the image area. 3D costs 5–20× more per km² than an equivalent 2D but is essentially mandatory for development drilling. Everything since the mid-1990s in mature basins is 3D.

4D

The same 3D geometry, re-shot over calendar time — usually 6 months to 3 years apart. The 4th dimension is time. Subtracting (or inverting) baseline from monitor reveals what changed between the two shoots: oil depletion, water-flood front, gas cap growth, CO₂ plume, compaction. The entire analysis depends on repeatability: how well the monitor replicates the baseline geometry. Streamer 4D achieves 30–50% NRMS; Permanent Reservoir Monitoring (§7.5) pushes it to 5–15%.

4C

Four-component recording: a vertical geophone (Z), two orthogonal horizontal geophones (H1, H2), and a hydrophone (P). The horizontal geophones capture shear (S) waves. Gas absorbs P waves but not S waves, so 4C images gas-obscured reservoirs where conventional P-wave sensors see only attenuation noise. S waves are also less attenuated by volcanic basalt — enabling sub-basalt imaging offshore Faroes, West Shetland, Atlantic margin. Fractured reservoirs show shear-wave splitting, giving azimuth-resolved fracture orientation. Standard equipment on most modern OBN and OBC surveys.

References

• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.
• Yilmaz, Ö. (2001). Seismic Data Analysis: Processing, Inversion, and Interpretation of Seismic Data (2 vols.). SEG Investigations in Geophysics 10.
• Vermeer, G. J. O. (2002). 3-D Seismic Survey Design. SEG Geophysical References 12.
• Lumley, D. (2001). Time-lapse seismic reservoir monitoring. Geophysics, 66(1), 50–53.