Land Seismic in the Desert: Vibroseis, Geophones and Fold
Key takeaways
- In the desert, vibroseis is usually used: heavy trucks that shake the ground with a frequency sweep lasting several seconds.
- The raw record is correlated with the sweep, so it looks like a short pulse.
- Geophones are planted along receiver lines; modern systems record wirelessly (nodes) or through cables.
- Fold is the number of times the same point is recorded, and it strengthens signal against noise. In our example, 240 channels at 25 m with a 50 m source interval give a fold of 60.
- The loose near-surface layer in dunes delays waves irregularly, which is fixed with static corrections.
Educational content only: procedures and operating values vary between companies and sites. Always follow your company’s approved procedures, your supervisor’s instructions and the permits in force at your site.
The crew: a mobile factory in the desert
A land seismic crew can include hundreds of people and dozens of vehicles. It works over a wide area for weeks or months and lives in a mobile camp that moves as the job advances. There is the survey team placing points by GPS, the layout team planting geophones, the source team with the vibrators, the recording room, and the maintenance, catering, medical and safety teams.
The source: vibroseis
Instead of explosives, open areas such as deserts usually use vibrator trucks. Each truck lowers a large steel baseplate onto the ground, presses it down with the truck's weight, and shakes it hydraulically. The vibration is not a single blow but a sweep whose frequency changes gradually, for example from 6 Hz to 80 Hz over 8 to 12 s. A fleet of 2 to 4 trucks usually works together, precisely synchronized, on the same point.
Because the sweep is long, the record picked up by the geophones is long and mixed. So a mathematical step called correlation compares the record with the original sweep, turning each reflection into a short pulse at its correct time, as if an explosive had been used.
Elsewhere, dynamite in shallow shot holes is used, especially where trucks cannot travel.
Receiving: geophones
A geophone is a small device with a coil and a magnet that turns ground motion into an electrical signal. It is planted with a spike, and several geophones are often wired together in a group to strengthen the signal and reduce some noise. Each group gives one channel in the recording.
Older systems send the signal through long cables to the recording truck. Modern systems use autonomous nodes with battery, memory and GPS that record on their own for days and are then collected and downloaded. This allows thousands of channels without cables.
2D and 3D
- 2D survey: source and geophones on one line, giving a vertical section beneath that line only. Cheaper, and used for early exploration of wide areas.
- 3D survey: source lines and receiver lines crossing on a grid, giving a cube of data that can be sliced in any direction. Much more expensive, but essential for field development and well placement.
Fold: why record the same point many times?
When source and geophone are some distance apart (the offset), the reflection comes roughly from a point halfway between them, the common midpoint or CMP. As the source moves, the same CMP is recorded by many pairs at different offsets. Later all these traces are added together (stacked), so true reflections are reinforced and random noise is weakened. The number of traces in each CMP is the fold.
A receiver line has 240 channels, the group interval is 25 m, and the source moves 50 m between shot points:
Fold = (Channels × Receiver interval) ÷ (2 × Source interval)
Fold = (240 × 25) ÷ (2 × 50) = 6,000 ÷ 100 = 60
And the 2D bin size is half the group interval:
Bin = 25 ÷ 2 = 12.5 m
The improvement in signal-to-random-noise ratio is roughly proportional to the square root of fold, so a fold of 60 gives about √60 ≈ 7.7 times better than a single trace.
The desert problem: near surface and noise
- Weathered layer: dry sand and dunes have very low velocity and variable thickness. Waves are delayed differently from point to point, so deep layers look wavy. The layer is measured with uphole surveys or refraction analysis, and times are corrected with static corrections.
- Ground roll: a strong, slow wave travelling along the surface that masks reflections. It is reduced by group design and by processing.
- Wind and vehicles: wind shakes the geophones and adds noise, so recording may stop in sandstorms.
Safety on a seismic crew
- Desert driving: the biggest risk to seismic crews is vehicle accidents, so journey management is applied and off-track night driving is usually banned.
- Vibrator trucks: very heavy; nobody approaches them while they work, and they keep a safe distance from wells, pipelines and buildings.
- Heat: long work in the sun requires enough water and rest breaks.
- Explosives: where dynamite is used, only licensed people handle it, with strict storage and transport procedures.
- Camp life: hygiene, food, water and medical cover are part of crew safety.
Takeaway for the technician
Land seismic is a huge logistics operation: a strong, synchronized source, thousands of accurately planted channels, a geometry designed for enough fold, and correction of the near surface. Data quality in the field sets the quality of everything that follows in processing and interpretation.
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