Search

The aim of walkaway VSP experiments is to image the region beneath the receiver. The information in the data is obscured by propagation effects in the region above the receiver such as free surface multiples, internal reflections and mode conversions. This thesis presents a method of extracting the reflectivity response of the region beneath the receiver from walkaway VSP data, assuming the earth to be horizontally stratified. For marine experiments the source is an acoustic source in the water. Measurements of source signatures clearly show shot-to-shot variations. VSP processing, such as the separation of upgoing and downgoing waves, is based on the assumption of shot-to-shot repeatability. However, shot-to-shot variations are usually ignored during processing. I present a straightforward method for correcting for these variations. The source signature must be measured and the geometry of the measurement must be known. The recorded source wavelets are all shaped to a standard wavelet using filters in the frequency domain. The same filters are applied to the geophone data, thus removing the effect of the source variations. The method is demonstrated on real data. As a plane horizontally-layered earth is laterally invariant, a walkaway VSP can be viewed as an experiment with a single source and a horizontal array of geophones at depth. The data are processed in the horizontal wavenumber-frequency domain, in which plane wave components are separated. I present a method for recovering the reflectivity of the region beneath the receiver in this domain. The full wavelength at the receiver level can be computed for a plane-horizontally layered earth as a superposition of plane wave responses.