Use of Multiples in VSP Depth Imaging

Abstract

Vertical seismic profiling is a borehole seismic method that provides valuable and reliable information about the local geological structure; with superior resolution compared to surface seismic. The recorded multicomponent wavefield can easily be decomposed into upgoing and downgoing wavefields and consequently primaries and multiples can be easier identified. This high resolution data is used to link well logs and surface seismic data.

Historically only primary reflections are being used to establish subsurface velocities as well as imaging the local subsurface geological structure. The advantage of VSP imaging compared to conventional surface seismic is that receivers are sitting close to the image zone, hence improving lateral and vertical resolution. The drawback in VSP imaging of primaries is due to the limited aperture around the well. One way to overcome such limited aperture in VSP imaging is to include multiples in imaging of the subsurface, particularly downgoing first order free surface multiples.

The main objective of this thesis is to use the mirror symmetric principle for the purpose of subsurface imaging of downgoing first order free surface multiples. By proper preparation of the input data as well as the velocity field, a Kirchhoff migration approach is used in order to image the first order downgoing free surface multiples.