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.^[1] By analyzing the original and converted waves, seismologists obtain additional subsurface information, especially due to (1) differential velocity (V_P/V_S), (2) asymmetry in the waves' angles of incidence and reflection and (3) amplitude variations. ^[2]

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 V_S with respect to V_P suggest changing lithology and pore geometry.^[2]

References

1. ^ 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 
2. ^ ^{a b} Stewart, Robert R.; Gaiser, James E.; Brown, R. James; Lawton, Don C. (28 Feb 2002). "Tutorial, Converted-wave seismic exploration: Methods" (PDF). Geophysics (Tulsa, Oklahoma: Society of Exploration Geophysicists) 67 (5): 1348–1363. doi:10.1190/1.1512781. http://www.geosc.uh.edu/docs/geos/faculty-files/pdf/stewart/ps-methods.pdf.