A Study On The Application Of AVO Theory In Seafloor Elastic Parameters Estimation

Seafloor elastic parameters play an important role in many fields such as gas hydrate exploration and cold seepage detection. The goal of marine seismic exploration is always focused on the strata under the seafloor sediments. AVO technology is an effective method to extract the physical parameters of the underground media from pre-stack seismic data. However, conventional research on AVO technology is also focus on the strata under the seafloor sediments. The AVO response of the interface between seawater and seafloor sediments is always ignored. The goal of this thesis is to estimate seafloor sediments elastic parameters with the AVO response of the interface between seawater and seafloor sediments.There are three interface conditions for the seafloor reflector, the continuity of normal strain, the continuity of normal stress, and that the tangential stress vanishes at the interface. Based on the interface conditions, this paper derivates the Zoeppritz equations to describe the reflection and transmission behavior of the seismic waves on the seafloor. The seafloor interface is a strong elastic contrast interface. Conventional approximations based on weak contrast assumption are not applicable for the seafloor case. Therefore, this paper derives an approximation by series truncation procedure without any weak contrast assumption. Both the exact equation and approximation form the basis of the research in this thesis. This paper presents a two-step linear method to invert for the seafloor sediments. In the first step, the gradient and intercept are estimated based on the linear relationship between the reflection coefficients the sine squared incidence angle in the near-angle domain. Then, the ratio of S-wave velocity and P-wave velocity is estimated in the far-angle domain. The seafloor sediments parameters can be solved with the gradient, the intercept and the ratio. Based on the two-step linear method, this paper further investigates the influence of the seismic wavelet on the inversion result and present a strategy of inversion with amplitude of reflected waves directly through rock physics constrains. This research also investigates the influence of the near-angle range accuracy on the inversion procedure. Aiming at the deficiency of two step linear inversion method, this paper tries to conduct the inversion with the exact equation through unconstrained optimization methods. This paper studies the performances of constant-step gradient method,the steepest descent method, the Newton method and the conjugate gradient method. With kind of methods,inversions can be performed directly in the whole angle domain. Therefore, the stability is much higher than that of the two-step linear method.At last, this paper performs inversion with field data from the South China Sea.