Land seismic data acquisition and preprocessing

Abstract

Complexities of the near surface and the varying acquisition-surface topography have degrading effects on land seismic images. This is because of trapped Rayleigh waves in the near-surface, and the distortions occurring to the body waves while passing through the complex near-surface. Removal of these two effects depends on the acquisition design and the preprocessing method. The acquisition design proposed in this thesis includes two phases. The first phase involves selecting initial parameters based on wave sampling theory. In the second phase, the subsurface model is incorporated. The main objective of this phase is to evaluate and update the initial design parameters. In this thesis the evaluation is based on the quality of pre-stack imaging results. These results are obtained using the concept of focal beams which allows assessing the effects of an acquisition geometry on the image quality without going into explicit modelling of seismic data. The results obtained in this thesis demonstrate that the complex near-surface causes the seismic wavefronts to be distorted without causing major shadow zones. This is principally different for the situation where the complexities of the underlying macro-model are relatively close to the target. The proposed method of removing the signal distortions is based on wavefield redatuming. This is carried out using one-way focusing operators that are obtained without knowledge of the underlying macro-model. This thesis introduces the estimation of the 3D one-way focusing operators using cross-spreads without a need for trace interpolation. In addition, a method for estimating 2D focusing operators in one-step is introduced and applied on a complex 2D synthetic and field data set. This method is based on global non-parametric inversion of tracked two-way reflection traveltimes in the common source gathers, using Fermat's principle as a constraint.