Structural Depth Imaging

WesternGeco Structural Depth Imaging (SDI) specifically addresses the problems of velocity model building and imaging in the depth domain for complex geologic settings. Thrust-belts, sub-basalt imaging and complex overburdens are typical targets for SDI.

A suite of workflows and algorithms have been developed for this purpose, including:

• Processing and imaging of long offset seismic arrivals (wide-offset workflow)
• Simultaneous Joint Inversion of seismic - gravity - magnetotellurics for improved velocity model building in very complex geology
• Shot-domain Beam PSDM in the angle domain with anisotropic (VTI/TTI) suppor

Wide-offset imaging workflow

The wide-offset imaging workflow is most efficient when seismic data are acquired with an extended offset geometry. The definition of long-offsets is relative to the depth of the target and can be defined as seismic data recorded at distances as great as three times the predicted depth of the target. By doing this, a wide range of seismic phases are collected, all carrying information about structures and velocities at depth. In conventional processing, most of the seismic phases other than near-vertical reflections are disregarded, whilst in the wide-offset workflow they are used to derive robust 3D velocity models and enhanced seismic images in the depth domain.

Simultaneous Joint Inversion

Electromagnetic and gravity fields are not affected by the propagation problems that affect seismic waves in complex velocity fields. For this reason the integration of multiple geophysical measurements with seismic data can add huge value to exploration programs.

The simultaneous Joint Inversion (JI) of multiple geophysical domains (i.e. seismic-gravity-electromagnetics) in the framework of a depth imaging workflow provides extended capabilities for resolving complex velocity fields in conditions of poor signal-to-noise ratio. Our patent-pending Joint Inversion methodology is designed to work in the pre-migration domain (i.e. JI of first breaks, gravity, electromagnetics), to target the reconstruction of the shallow velocity section (steeply-dipping outcropping structures, velocity inversions, hidden layers), and in the post-migration domain where the JI is performed between post-migration seismic residuals (i.e. common-image point (CIP) residuals) and the residuals of the gravity and electromagnetic fields.

Shot-domain Beam PSDM

Shot-domain Beam PSDM in the angular domain (with anisotropic VTI/TTI support) is an important facet of SDI in improving the imaging algorithms for complex geological settings. The complex velocity fields characterizing thrust-belt environments and other geologic scenarios generate illumination problems of deep-seated sub-thrust targets that can be handled by multi-arrival imaging algorithms such as shot-domain Beam prestack depth migration (B-PSDM). The angle-domain implementation of B-PSDM together with hit-count trace equalization enable a number of amplitude-friendly processes of the image gathers. B-PSDM can also handle complex anisotropy situations such as TTI.

Related references

• 2008 Virgilio M., D. Colombo and A. Dyke, Seismic imaging strategies for thrust-belt exploration: extended offsets, Seismic/Gravity/EM simultaneous Joint-Inversion and Anisotropic Gaussian Beam Pre-Stack Depth Migration, 2008 CSPG CSEG CWLS Convention
• 2007 Colombo D., M. Mantovani, M. De Stefano, D. Garrad and Hussain Al Lawati, Simultaneous Joint Inversion of Seismic and Gravity data for long offset Pre-Stack Depth Migration in Northern Oman, CSEG
• Colombo D., 2005, Benefits of Wide Offset seismic for commercial exploration targets and implications for data analysis, The Leading Edge, 352-363 April edition.