The quality of seismic images in thrust belt is often disappointing or worse. Several factors reduce the effectiveness of seismic methods in complex thrust belts and these can be divided into:
  · Physical propagation effects of the wave field;
  · Insufficient modeling capability of the recorded data

The presence of high impedance bodies close to surface or outcropping (e.g. carbonate sheets) represents a major obstacle to the propagation of seismic energy. The energy is reflected from the top of the body, absorbed by it and scattered. Extending the offsets during acquisition while maintaining the usual source and group intervals widens the possibility of undershooting localized complexities as well as gaining the advantage of improved S/N ratio for seismic reflected phases close to the critical reflection angle (i.e. wide angle reflections).

The second class of problems, which causes unsatisfactory imaging in thrust belts, is mainly related to the inadequate strategies adopted for resolving the complex velocity field in poor S/N conditions.

Geosystem uses a new approach to seismic exploration in thrust belts by considering the problems described above from a new perspective, which involves extended offset acquisition geometries and an innovative approach for velocity determination in depth domain. Longer offsets involve the use of imaging algorithms able to handle the non-hyperbolic move out and the severe ray-path distortions occurring as a consequence of a complex and laterally varying velocity field. For this reason imaging has to be carried out with Pre-Stack Depth Migration methods which take advantage of the information contained in the long offset energy and deliver improved results.

Tomographic velocity analysis using turning rays in the Canadian Foothills (the dashed line indicates the resolved model section).

Thrust belts are in general characterized by velocity patterns consistent with geologic structures. The velocity field expected is characterized by weak gradients within the rock formations, sharp discontinuities and strong control from the geology. All these characteristics suggest layer-based velocity model-building approaches (see Seismic Tomography).

Since the conventional offset reflected phases alone are insufficient to reliably constrain the velocity field, we extend the range of seismic phases by extending the offsets and collecting diving waves, head waves, near-vertical and wide-angle reflections. All these phases carry information of the subsurface and are used for (data-driven) 3D velocity model building. We base our work on data-driven velocity model building coupled with the search for the simplest model supported by the data. However, when consistent geologic information is present, this is integrated in the velocity model building procedure to generate enhanced velocity models for depth imaging.

Wide Offset Pre-Stack Depth Migration is used for imaging the complex thrust-belt structures and enabling undershooting of absorbing/scattering layers. This task is achieved through robust velocity model building.

Wide offset imaging results for a complex thrust-belt structure in the Canadian Foothills (from Colombo et al., CSEG 2004). Notice the improvement of the image in the center of the section due to the large offsets.

The figure above shows the improvement obtained in a complex thrust belt in the Canadian Foothills by the use of the wide offset workflow. The offsets used for imaging were in excess of two times the depth of the target (offset to depth ratio of 2:1).

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