Seismic energy that has traveled partly as a P-wave and partly as an S-wave, being converted from one to the other upon reflection or refraction at oblique incidence on an interface. Since mode conversion is small for small incident angles, converted waves become more prominent as the angle of incidence (and usually offset) increases.
Converted wave analysis
During seismic exploration, P-waves (also known as primary or compressive waves) penetrate down into the earth. When a P-wave hits an interface (e.g., liquid-solid (as at the seafloor), or solid-solid (within the subsurface)), the boundary conditions (continuity of stress and displacement) in general require that the energy be partitioned, into both upgoing (reflected) and downgoing (transmitted) waves (both P and S). This occurs at every interface; the strength of conversion is included among the Knott-Zoeppritz_equations (of course there is no reflected shear wave at the seafloor). For plane waves incident upon a planar interface, the conversion is zero at normal incidence, and increases with increasing incidence angle. The same conversions happen for the upcoming waves. And similar conversions occur with incident S-waves. (In anisotropic media, there are two shear modes in each direction; this complicates the analysis.)
In exploration seismics, normally the most important of these various converted modes is the P-S conversion upon reflection; this is called the C-wave. Compared to P-waves, the (converted) S-waves are less affected by fluids. 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. 
As opposed to analysis of P-wave to P-wave (P-P) reflection, C-wave (P-S) analysis is more complex. C-wave analysis requires at least three times as many measurement channels per station. Variations in reflection depths can cause significant analytic problems. Gathering, mapping, and binning c-wave data is also more difficult than P-P data. However, c-wave analysis can provide additional information needed to create a three-dimensional depth image of rock type, structure, and saturant. For example, changes in VS with respect to VP suggest changing lithology and pore geometry.
See also main page: C-wave.
- Probert, T.; Robinson, J.P.; S. Ronen, R. Hoare, D. Pope, J. Kommedal, H. Crook, A. Law (2000), "Imaging Through Gas Using 4-Component, 3D Seismic Data: A Case Study From The Lomond Field", Offshore Technology Conference (Houston, TX), doi:10.4043/11982-MS
- Stewart, Robert R.; Gaiser, James E.; Brown, R. James; Lawton, Don C. (28 Feb 2002). "Tutorial, Converted-wave seismic exploration: Methods". Geophysics (Tulsa, Oklahoma: Society of Exploration Geophysicists) 67 (5): 1348–1363. doi:10.1190/1.1512781.