1. n. [Geophysics]

Four-component (4C) borehole or marine seismic data are typically acquired using three orthogonally-oriented geophones and a hydrophone within an ocean-bottom sensor (deployed in node-type systems as well as cables). Provided the system is in contact with the seabed or the borehole wall, the addition of geophones allows measurement of shear (S) waves, whereas the hydrophone measures compressional (P) waves.

See: borehole seismic data,  compressional wave,  geophone,  ocean-bottom cable,  P-wave,  shear wave