Dual-sensor / over-under / IsoMetrix
Learning objectives
- Explain dual-sensor deghosting: p + ρV·v_z removes the free-surface reflection
- Describe the over-under alternative: two streamers at different depths combined in processing
- Sketch a slant-cable geometry and its receiver-diverse ghost response
- Compare operational complexity of the three strategies
A single-depth hydrophone streamer has a hard ghost notch at f = Vₓ / (2h). Three strategies remove or avoid it.
Dual-sensor (the current industry standard)
Each receiver group on the cable co-locates a hydrophone (pressure) with a vertical geophone or accelerometer (vertical velocity). The combination
cancels the down-going (ghost) part of the wavefield, leaving the up-going field only. In practice the ρV_w amplitude ratio is estimated from the data itself for every segment. Production examples: PGS GeoStreamer, Schlumberger IsoMetrix. This has become the default for new streamer surveys since ~2015.
Over-under
Tow two parallel streamers at different depths (typically 8 m and 16 m). Their ghost notches sit at different frequencies (~94 Hz and ~47 Hz for those depths). A combined filter produces a broadband response with no deep shared notch. Operational cost: twice the streamer hardware, twice the cable wear. Largely superseded by dual-sensor for new acquisition but still in use for some older fleets.
Slant cable
One streamer tilted so the near-offset end is shallow (~6 m) and the far-offset end is deep (~40 m). Each receiver has its own h and its own notch, so a slant-stack across offsets produces a receiver-diverse average that has no single sharp notch. Cheaper than over-under and simpler than dual-sensor but the tilt changes binning geometry. Niche; used for low-frequency-focused surveys.
What they share
All three are about avoiding the single-depth-hydrophone ghost trap. Dual-sensor is the winner in the long run because it’s the only scheme that preserves full directional information — over-under and slant still lose information on steep arrivals that come in at angles where the envelope is weak.
References
- Tenghamn, R., Brown, J. (2000). A new dual-sensor towed-streamer technology. SEG Annual Meeting Expanded Abstracts, 1–4.
- Soubaras, R., Whiting, P. (2011). Variable depth streamer — the new generation. CSEG Recorder, 36(2), 27–31.
- Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.