Vector Kirchhoff migration of virtual‐source VSP data

Abstract

Seismic interferometry, also referred to as the virtual source method, is becoming a popular technology in VSP data processing for imaging subsurface structures that lie under complex and poorly understood formations. Using the virtual source method, seismic sources on or close to the free surface can be redatumed to the receiver locations in a borehole, creating a series of virtual common shot gathers at each receiver, in order to avoid the determination of a complex overburden velocity model necessary for proper image migration. The second is the removal of distortion in the seismic signal that travels through a complex overburden by positioning a virtual source close to the target zone(s). Most virtual source imaging methods currently in use process and migrate only a single scalar component. VSP data is recorded using three-component (3C) borehole receivers which allow the recording of the 3D vector characteristics of the waveform. 3C VSP receivers likewise record complex signals as both compressional and shear waves are generally captured. Due to both the 3C vector nature and the complexity of the different wave modes in VSP data, it is necessary to develop new virtual source imaging methods to migrate the full 3C vector wavefield simultaneously. In this paper, we present a methodology to perform three-component vector migration for virtual source VSP data. First, we redatum each of three components of the VSP data to their virtual receiver positions in a borehole using seismic interferometry. Next we perform a vector summation of the Kirchhoff prestack depth migration for all three components of the virtual source VSP data. The numerical test results from elastic finite difference modeling suggest that this new vector migration of virtual source VSP data can effectively image steeply dipping structures, such as a salt flank, while the knowledge of the overburden structure and velocity is not required. Migration amplitude image strength and quality is significantly enhanced with the migration of all three vector components instead of one scalar component as is usually done in conventional virtual source migration methods. In addition, a simultaneous 3C migration of the virtual source VSP data has the added benefit of simplifying the VSP pre-processing as it is not necessary to try and maximize the reflection onto a single component prior to migration. .