Coupling, planting, and QC

A geophone is a precision mechanical sensor. Its data quality is only as good as the coupling between its case and the ground. On a dry gravel road, a loosely-planted 10 Hz geophone can lose 12 dB of high-frequency signal and ring for 100 ms after every shot. Good planting is the cheapest data-quality investment a crew makes — and the one most often neglected.

Planting procedure

• Clear loose debris. Remove grass, small rocks, dried top-soil.
• Press the spike vertically. A geophone spike is usually 15–25 cm long; push it in at least 2/3 of its length. In hard ground, drill a hole with a hand auger and tamp the spike firmly.
• Check plumb. The case must be vertical for the Z axis to read pure vertical motion. A bubble level on top of each case is standard.
• Cover the case with dirt or a sand-bag to dampen wind noise on the housing.
• In dry soil: pour a litre of water around the spike before tamping. The water-saturated soil couples an order of magnitude better than dry.

Failure modes, their impulse-response signatures, and how to fix them

• Well-planted (A): clean, broadband impulse. Short post-arrival tail. This is the reference.
• Tilted (B): impulse has normal shape but horizontal motion leaks onto Z and vice versa. Polarisation wrong; PS processing breaks. Fix: re-plant vertically.
• Loose / dry-soil coupling (C): high-frequency roll-off; visible ringing at 10–30 Hz after the impulse. Fix: re-plant deeper, add water, tamp surrounding soil.
• Polarity reversed (D): perfect shape, wrong sign. Catch it before you leave the site: fix the wiring.

Field QC workflow

• Shake test at deployment: tap near each geophone, record the case-to-data delay and amplitude. Flag outliers.
• First-shot review: inspect the first production shot record. Stations with low amplitude, reversed polarity, or anomalous noise levels are re-visited same day.
• Cross-correlation between each station and its nearest neighbours during production. A negative correlation coefficient is almost certainly a polarity-reversed channel.
• Daily drift monitoring: station amplitude vs time. A channel that slowly loses amplitude is probably drying out (dry season) or getting un-tamped by foot traffic.

Why 3C coupling is harder

A 3C sensor has three orthogonal axes that must all couple. A station that is 10° from vertical — tolerable for a 1C Z-only record — mixes horizontal and vertical motion on a 3C record, invalidating the azimuth-preserving data processing that follows. Nodal 3C systems typically record their own tilt and record that value in the header for post-acquisition rotation; but the rotation cannot fix coupling-loss artefacts, only orientation.

References

• Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.
• Cordsen, A., Galbraith, M., Peirce, J. (2000). Planning Land 3-D Seismic Surveys. SEG Geophysical Developments 9.
• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.