The highly significant technical developments taking place in the seismic industry, combined with the ever more demanding challenges of oil and gas production, are opening up a completely new range of potential applications for seismic.
Gradually, seismic acquisition and processing, which are essentially technologies restricted to exploration and appraisal, will become methods of reservoir characterization and monitoring at the disposal of asset managers.
The offshore environment, with increasingly deeper water developments, and the attached risks and costs, will constitute a particularly important domain of application for these new technologies.
For years, the CGG group has been developing and promoting a number of highly significant new techniques for reservoir characterization and monitoring. One of these, the acquisition of shear waves (S-waves), is particularly relevant in the context of reservoir characterization.
The combination of conventional pressure waves and the X, Y, Z components of shear wave propagation provides invaluable reservoir information in terms of lithology, anisotropy, better illumination of tricky horizons (gas chimneys, salt domes or basalt capped pay zones for instance) and better monitoring of fluid fronts.
To acquire shear waves in a marine environment, multi-component geophones must be deployed on the sea bed.
Through its subsidiary, Subseaco, the CGG group has developed a unique, state-of-the-art capability for 4-component seabed acquisition, which is qualified for 2D and 3D surveys in water depths down to 1500 m.
The system consists of multiple arrays of individual nodes linked by combined telemetry/power cables. Each node is a self-contained 4C digital sensor, housing a total of three-component geophones, one hydro-phone, one compass for azimuth measurement and an inclinometer for tilt acquisition.
All sensors are interfaced to the surface monitoring system through the highly reliable and field-proven Sercel SU6 telemetry digital electronics.
In addition, a very versatile power management system, designed by the Norwegian company, DataRespons, in co-operation with Subseaco, allows each node to be individually activated or deactivated from the surface without affecting the rest of the array.
All equipment is fixed inside the canister. No gimbals are used for the geophones, as proper calibration is achieved through the compass and inclinometer readings. This very specific feature contributes greatly to the excellent quality of the acquisition.
Another very important aspect which also contributes to the performance of the system lies in the perfect coupling of the nodes to the seabed. Each node is planted into the seabed by means of a ROV (remotely operated vehicle) and its manipulator arm.
According to each specific seabed strength, a particular shape is designed for the lower part of the node and fitted onto the bottom of the pressure canister housing the sensors. This lower part, referred to as the "skirt", guarantees optimum coupling.
The overall system, cables and nodes, is deployed from the deck of a standard, dynamically positioned survey-class vessel.
Intrinsically, this technology addresses in a very elegant and effective manner the three major challenges facing the concept of multi-component seabed acquisition, namely: positioning (performed in real time by the ROV), orientation of geophones (compass and inclinometer) and finally coupling.
The resulting data quality is outstanding, as established by two surveys, one 2D, one 3D, performed in the North Sea in early 1997.
by Christophe Pettenati-Auzière, V.P. Seismic Acquisition
