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). 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 (VP/VS), (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 VS with respect to VP suggest changing lithology and pore geometry.[2]
References edit
- ^ Probert, T.; Robinson, J.P.; Ronen, S.; Hoare, R.; Pope, D.; Kommedal, J.; Crook, H.; Law, A. (2013). "Imaging Through Gas Using 4-Component, 3D Seismic Data: A Case Study From The Lomond Field". All Days. doi:10.4043/11982-MS.
- ^ 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. 67 (5). Tulsa, Oklahoma: Society of Exploration Geophysicists: 1348–1363. doi:10.1190/1.1512781. Archived from the original (PDF) on 3 March 2016. Retrieved 13 October 2010.