Seismic inversjon

Abstract

Seismic signals contain information about the subsurface. To extract this information we use our knowledge about how waves propagates to generate seismic images. As a wave propagates through the earth, it responds to changes in elastic properties. Relative changes in elastic impedance cause reection of seismic energy according to Snell's law, and these reflections allow us to record signals from the earth filtered waves. If the earth was perfectly elastic, the wave energy would not be absorbed, but in reality only energy in a frequency range band from 10 Hzto about 90 Hz is measured, making the reconstruction of the elastic earth model a non-unique inverse problem. In particular, the lack of high frequencies puts a limit on the resolution, while the lack of low frequencies only allow us to inferrelative changes. In this thesis seismic inversion techniques are introduced, with an emphasis on how the seismic wavelet aects the inversion, and how the different inversion methods handle the inherent non-uniqueness. The methods are divided into post-stack and pre-stack inversion. Pre-stack data contains information about the shear properties of the rocks, in addition to the compressional properties also. This information comes at the cost of a lower signal to noise ratio, and a more costly inversion procedure.

I was also fortunate to work with real prestack data from the North Sea. I usedthis data for post-stack and pre-stack inversion using software from Hampson & Russel . The purpose of this case study was mainly to learn the workflow of a typical seismic inversion, and to get familiar with real seismic data.