| This thesis discusses the imaging of a marine oil reservoir underneath a gaseous zone using multicomponent data. Oil reservoirs often are not totally permeable to gas and may let some of it leak out. The presence of gas in the pores of a rock slows down compressional waves traveling through it. Structure imaging with compressional waves is thus impossible. But shear waves are not affected by the presence of gas and so can be used to image structures.; A marine multicomponent survey is performed using geophones on the sea bottom as receivers and an air gun as a source. The data set can be split in two components corresponding to either the purely compressional waves or the mode converted waves. Mode converted waves are waves which start as compressional waves and convert to shear waves at the reflection point. These waves are affected by the gas only on their compressional leg. So, by selecting the rays which go through the gas zone only as shear, it is possible to have traces unaffected by the gas and which can be used to image the reservoir.; Imaging a reservoir with mode converted waves requires the knowledge of the vertical compressional velocity, the vertical shear velocity and the two anisotropy coefficients (δ and ε). Compressional velocity is determined by global search optimization and Normal Move Out velocity analysis. The vertical shear velocity is estimated by correlating the depth of reflectors on the compressional image with the time of arrival of corresponding mode converted reflectors. One of the anisotropy coefficients, δ, is taken to be zero under the assumption that anisotropy at small angles on compressional waves is small. The second coefficient, ε, is estimated by using a velocity scan and a hyperbolic approximation of the time of arrival of mode converted waves in an anisotropic medium. |
