Receiver arrays and group-forming

Classic land surveys hardwired N geophones (typically 6–36) in parallel within a single recording channel. Their outputs added in real time, so the field recording was the spatial average over the array. This is a beamformer — the same physics as §0.5’s antenna arrays — and its dominant field application is ground-roll attenuation.

Why it rejects ground roll

A deep reflection arriving near-vertically has a very high apparent velocity across the array — each geophone sees the signal nearly simultaneously, and they sum coherently. The array has unity gain for such events.

Ground roll (Rayleigh surface waves) creeps along the ground at 300–800 m/s with wavelengths of 20–100 m, comparable to typical array lengths. Different geophones see the wavefront at different phases, and the sum destructively interferes. An N=12 geophone group with L = 30 m applied to 500 m/s × 8 Hz = 62 m-wavelength roll has L/λ ≈ 0.5, giving substantial rejection; at smaller λ the rejection is near-total.

The f-k picture

In f-k (frequency-wavenumber) space, the array behaves as a low-pass filter in k at each frequency. The passband is |k| < 1/(2L) at low f and widens as f grows; events with high apparent velocity (low k for any given f) pass, events with low apparent velocity (high k) are suppressed. The same filter can be applied digitally after recording if every sensor is kept as its own channel — which is exactly what modern point-sensor acquisition does.

The shift to point sensors

Classic hardwired arrays have two drawbacks: they low-pass-filter the signal in space too (you lose high-k signal components along with the roll), and they mix statics effects from N different near-surface locations. Modern land 3D with digital recording (high-channel-count systems like Sercel 508XT, INOVA Hawk) treats every sensor as a point and forms digital arrays in processing — keeping full flexibility to change the array parameters after the fact, or to apply f-k filters without the physical mix-down. Ground roll is attenuated in processing (often with f-k mutes, Radon, or model-based schemes from §4 of the Processing textbook).

Hardwired arrays still appear in legacy recording systems and in noise-dominated environments where real-time noise-suppression is helpful. The physics is the same.

References

• Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.
• Vermeer, G. J. O. (2002). 3-D Seismic Survey Design. SEG Geophysical References 12.
• Cordsen, A., Galbraith, M., Peirce, J. (2000). Planning Land 3-D Seismic Surveys. SEG Geophysical Developments 9.
• Vermeer, G. J. O. (1990). Seismic Wavefield Sampling. SEG Geophysical Monograph 4.