Pre-Stack Depth Migration

Pre-stack depth migration technology at PGS includes anisotropic ray tracing to compute travel times for input to Kirchhoff prestack depth migration and full wave equation depth migration using an efficient explicit finite difference operator.

Our anisotropic ray tracing is a hybrid ray tracing - wavefront construction algorithm.  It is a dynamic ray tracer, able to shoot more rays when required where the complexity of the velocity model increases, and can perform ray infill to minimise holes in travel times.  Other features include ray tracing capability for P-waves and converted waves where the model is VTI anisotropic.

Travel times computed by ray tracing are input to our Kirchhoff prestack depth migration.  Key features of our implementation include:

• Flexible aperture definition based on maximum imagable dip
• Automated anti-alias filtering
• Amplitude compensation options
• Turning ray handling
• OBC (seafloor) and converted-wave handling; different travel time tables for shot and receiver

Our flagship wave equation depth migration application is based on an explicit finite difference operator.  The design of the constrained operator allows for efficiency in performing the wavefield extrapolation for 3D wave equation depth migration, without compromising the quality of the results.  For further details please refer to: J. Ren et al, An efficient 3D depth migration with explicit finite-difference operators, EAGE 2004.

Recognising the value of cost efficiency to our clients, PGS has developed a hybrid wave equation migration approach based an a combination of methods:  The shot and receiver wavefields are extrapolated using a combination of Gazdag phase shift, split-step Fourier, and implicit finite difference techniques.  The methodology incorporates an automated selection criterion dictated by the complexity of the model.

PGS has been an industry leader in depth imaging since 1992, when we were the first to implement the vertical update procedure for Kirchhoff prestack depth migration commercially.  Our depth imaging professionals at PGS centers worldwide have processed large volumes of pre-stack depth migration from a diversity of geological settings and covering a full spectrum of imaging challenges and processing objectives.  PGS continues its leadership position in depth imaging with new technologies including an efficient wave equation depth migration, and velocity model building and tomography in the context of our 3D visualization technology, holoSeis.  Our philosophy is to work closely with our clients to better understand the characteristics of their data and the velocity regime as well as their expectations in depth domain processing; we can then recommend a velocity model building and depth imaging strategy best suited to the specific requirements of their project.

Input 3D velocity model slice (left), standard Kirchhoff pre-stack depth migration (PSDM) result (middle), and PGS wave equation pre-stack depth migration result (right).

Wave equation pre-stack depth migration is a natural complement to the high shot gather density, tight 3D spatial sampling and high trace densities provided by PGS' HD3D acquisition technology, ensuring artifact-free imaging, high resolution, and optimum data quality.

Comparison of Kirchhoff PSDM (left) to Wave Equation PSDM (right) from Gulf of Mexico. Note the improved structural definition (blue circle).