During seismic exploration, P-waves (also known as primary or compressive waves) penetrate down into the earth.
Due to mode conversion, a P-wave can reflect upwards as an S-wave (also known as a secondary, shear or transverse wave) when it hits an interface (e.g., solid-liquid).
Unlike P-waves, converted shear waves are largely unaffected by fluids.
[1] By analyzing the original and converted waves, seismologists obtain additional subsurface information, especially due to (1) differential velocity (VP/VS), (2) asymmetry in the waves' angles of incidence and reflection and (3) amplitude variations.
However, c-wave analysis can provide additional information needed to create a three-dimensional depth image of rock type, structure, and saturant.