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., solid-liquid), it can reflect upwards as an S-wave (also known as a secondary, shear or transverse wave). Other P-wave to S-wave (P-S) conversions can occur, but the down-up conversion is the primary focus. Unlike P-waves, converted shear waves are largely unaffected 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.
- 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.