Land noise sources and mitigation

Land seismic is noisy. A fresh shot gather looks cleanly signal-dominated only at the very deepest reflections; everything shallow is buried under a stack of coherent and incoherent noise. Four families dominate and each has a distinct signature in the time, frequency, and wavenumber domains.

Ground roll

Ground roll is dispersive Rayleigh waves travelling along the surface at ~300–800 m/s. On a shot gather it forms the characteristic “noise cone” that opens from the source. Amplitudes are typically 20–50× larger than the desired reflections. Bandwidth is low, 3–15 Hz — which is why a 5–8 Hz low-cut filter wipes out most of it without touching reflection signal above 15 Hz. Ground roll is the single most-studied noise in land seismic; entire processing flows (surface-wave adaptive subtraction, FK-filter variants) are dedicated to it.

Wind

Wind pushes on the geophone spike, the geophone case, and on grass or foliage immediately beside the geophone. The resulting trace jitter is broadband but energy is concentrated below 10 Hz (mechanical resonance of the spike + case). It is trace-independent: each phone sees different wind-induced motion, so stacking over many phones — receiver arrays or CDP stack — attenuates it strongly.

Power-line interference

A 50 Hz (Europe, Asia, Africa) or 60 Hz (Americas) overhead power line induces a clean sinusoid in every trace of nearby receivers. The tone is stable and coherent, so a narrow IIR notch filter (r ≈ 0.95 around the line frequency) removes it with essentially zero signal loss. Harmonics at 100 Hz (or 120) sometimes show up too and are removed with a second notch.

Cultural noise

Vehicle traffic, pumpjacks, rotary drill rigs, compressors, and construction produce broadband 5–40 Hz energy that is coherent over 50–200 m of nearby traces. It looks like a smeared-out “source” at a nearby location and cannot be easily filtered in frequency. Field mitigation: quiet the site (shut down the pump for the 2 s shot window), or shoot at night, or increase stacking fold to average it out.

Receiver arrays

A receiver “station” in modern land seismic is often a group of 6–12 geophones planted in a small pattern (typically 25–50 m long). The trace recorded by the station is the analog (or digital) sum of the group. A group acts as a spatial low-pass filter for horizontally-travelling waves — ground roll, wind-excited motion, cultural noise — while leaving near-vertical reflections largely intact. The group is the reason every land shot gather is tractable at all.

References

• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.
• Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.
• Yilmaz, Ö. (2001). Seismic Data Analysis: Processing, Inversion, and Interpretation of Seismic Data (2 vols.). SEG Investigations in Geophysics 10.