Applications for Vector Coordinate Systems of Converted Waves Obtained by Multicomponent 3-D Data
Authors: Gaiser J.E. 
Published: October 2008

Recently, 3-D four-component marine ocean-bottom cable (OBC) data has come into broad use for a variety of applications. However, understanding how these new components and their associated acquisition and processing coordinate systems are related to the vector wavefields has become a challenging task for the exploration industry. The compressional wavefield (P-wave) is obtained from the pressure response of the hydrophone plus the particle-motion response from the vertical geophone. A converted P-wave to shear wavefield (PS-wave) is obtained from two horizontal geophone components oriented in an acquisition coordinate system of in-line and cross-line particle motion. The coordinate system of interest for multicomponent vector wavefields describes the polarization of particle displacement.

There are many uses of PS-wave data that have been transformed from acquisition coordinates to source coordinates of radial and transverse shear-wave (S-wave) motion. In this case only the radial component oriented in the source-receiver azimuth is retained as an estimate of S-wave reflectivity. One of the main uses is for discrimination of the matrix and fluid types. S-waves yield insights into the nature of subsurface lithologies and the nature of the pore-saturating fluids because they are not affected appreciably by different pore fluids such as gas. The types of information available from converted waves, useful for lithology discrimination, include layer-based Vp/Vs velocity analysis based on correlations or isochron measurements. Another indicator of lithology is horizon reflectivity differences. Since P-waves and S-waves respond to different rock properties, the strengths of a P- or S-wave reflection from the same interface may differ greatly. One of the most dramatic applications of converted waves is in structural imaging within gas chimneys and beneath shallow-gas overburdens. The use of converted waves for depth imaging is also becoming common. Independent prestack depth images of the base of salt, using the P-wave and PS-wave data, can yield important confirmations as to the validity of a structural interpretation.

A very important use of PS-waves is for vertical fracture detection, accomplished by further transforming common azimuth prestack data to radial and transverse receiver coordinates. This provides the necessary data for four-component rotations and layer-stripping algorithms to analyze S-waves polarized into slow and fast arrivals (a form of anisotropy known as birefringence). One application is to enhance the fast and slow S-wave for lithology discrimination, i.e. calculation of Vp/Vs and inverting PS-waves for impedance. Fracture intensity and fracture direction (related to the degree and direction of anisotropy) may be quantified between specified horizons of interest to define preferred zones of fracturing that can have a major impact on reservoir models. Models that incorporate fracture density and orientation can lead to optimal recovery of hydrocarbons and more refined use of 4-D time-lapse measurements.

Introduction

Marine 3-D four-component ocean-bottom cable (OBC) data has recently come into broad use for a variety of applications. However, understanding how these new components and their associated acquisition and processing coordinate systems are related to the vector wavefields has become a challenging task for the exploration industry as a whole. The compressional wavefield (P-wave) is obtained from the pressure response of the hydrophone plus the particle-motion response from the vertical geophone. A converted P-wave to shear wavefield (PS-wave) is obtained from two horizontal geophone components oriented in an acquisition coordinate system of in-line and cr