Wide Offset Seismic Imaging is a new seismic analysis/processing/imaging workflow developed at Geosystem primarily to enhance Thrust Belt Imaging but with extensions to Sub-Basalt and Sub-Salt imaging cases. Proprietary algorithms on 3D tomographic velocity determination and migration were developed in the framework of extensive research projects financed by oil companies which recognized Geosystem’s outstanding expertise and know-how in the fields of inverse methods and imaging in depth.

The wide offset imaging workflow obtains its maximum efficiency when seismic data are acquired with Extended Offsets Recording Geometry, which is designed to record seismic data at distances as far as three times the predicted depth of the target. By doing this, a wide range of seismic phases are collected, all carrying information of the structures and velocities at depth. In conventional processing, most of the seismic phases different from 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 depth domain.

Examples of Common Shot Gathers with wide angle reflected phases.

APPLICATION RANGE

Wide Offset Imaging can be applied to any seismic dataset including conventional offset seismic and it is especially suited for problematic prospects characterized by low S/N ratio. These include the cases of shallow high-impedance absorbing layers or complex structural tectonic regimes characterized by sharp lateral velocity changes which cause difficult velocity determination and severe ray-path distortions. Typical geologic environments where these conditions occur are:

- Thrust Belts / Foothills
- Outcropping Carbonates
- Shallow Volcanic covered areas (e.g. basalts)
- Sub-Salt targets

TIME/DEPTH WORKFLOW

Specific treatment of the seismic data is performed in time domain before depth imaging. The Time Processing procedures to be applied vary in relation to the geologic setting and environment (e.g. land/marine). For the specific case of land seismic data particular emphasis is placed on the evaluation of:

- Field statics (e.g. tomography)
- Residual statics
- Velocities

The “character” of the seismic data is preserved, favoring as much as possible the conservation of signal bandwidth.

Depth Processing is then performed focusing on velocity model building and pre-stack depth migration. Tomography can be performed from all offset diving waves, reflected event travel-times and Common Image Gather residuals. Imaging is performed using PSDM algorithms. Forward modeling (Ray-Tracing or FD acoustic/elastic modeling) is used interactively in the model building process.

INTEGRATION WITH NON-SEISMIC METHODS

Geosystem has developed the ability to integrate and use Gravity and MT data for improving the Vint/Depth model accuracy. Currently this is achieved through iterative modeling or inversion iterations with two-way transformation of parameters. Schemes for the Simultaneous Joint Inversion of multiple parameters (i.e. velocity-resistivity or velocity-density) are under development. Geosystem’s achievements in wide offset imaging are to be ascribed mainly to the unique procedure adopted to resolve the Vint field in the depth domain, which remains the central problem for a correct and reliable seismic imaging.

Wide Offset Imaging has been applied for thrust-belt and sub-basalt exploration in various regions covering Bolivian foothills, Colombian thrust-belt, Canadian foothills, Columbia Basin (Washington), Utah foothills, Indian basalt plateau (Deccan), Egypt (onshore/offshore), Poland and Romanian Carpathians, Zagros (Iran), Oman thrust-belts, Papua New Guinea thrust belts, Apennines and Pyrenees.

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