3C / 4C sensors and mode-conversion pickup

A 3C receiver is three geophones (or three MEMS axes) mounted orthogonally. It captures the full particle-motion vector v = (v_x, v_y, v_z). Added to a hydrophone, it becomes a 4C station — the standard ocean-bottom-node configuration.

Polarisation of P and S

A P-wave (primary, compressional) has particle motion parallel to the propagation direction — think longitudinal spring wave. An S-wave has particle motion perpendicular to propagation, with two polarisation choices (SV in the sagittal plane, SH out of it).

For a wave arriving at angle θ from vertical onto a sensor at the origin, with velocity pointing back along the ray to the source:

At θ = 0 (vertical incidence), P lands entirely on Z and PS on X. As θ grows, P bleeds onto X and PS bleeds onto Z — a 1C vertical geophone under-counts P-energy by a factor of cosθ (a 40° incidence loses 23% of amplitude) and misses PS entirely.

PS waves and what they buy you

When a downgoing P wave hits an interface, it partitions into reflected P, transmitted P, and mode-converted SV (both reflected and transmitted). The PS reflection is sensitive to shear velocity contrasts that PP reflections are only indirectly sensitive to — giving an independent constraint on rock properties. For fractured reservoirs, anisotropic shales, or gas clouds, PS imaging is sometimes the only way to image through the overburden at all.

PS data lives on the horizontal components. Without a 3C sensor, you don’t get it. Full stop.

The 4C ocean-bottom node

An OBN (e.g., Sercel GPR300, Magseis Fairfield ZXPLR) sits on the sea floor for weeks at a time. Its 3C geophone captures vector particle motion; the hydrophone captures pressure. Together they let processing separate up-going from down-going wavefields (multiple attenuation via PZ-summation), separate PP from PS reflections, and rotate the horizontals into inline/crossline coordinates after deployment.

OBN survey design assumes 3C data. If the horizontals are contaminated by coupling problems (a common deployment issue on rough sea floors), the PS workflow falls over. Coupling QC (§2.7) is particularly important for 3C stations.

References

• Mougenot, D. (2013). MEMS-based 3C accelerometers for land seismic acquisition. The Leading Edge, 32(4), 388–396.
• Aki, K., Richards, P. G. (2002). Quantitative Seismology (2nd ed.). University Science Books.
• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.