Onshore 3D surveys have generally been limited to sparse wide-azimuth geometries for both technical and economic reasons. The result is that typical land seismic data have been poorly-sampled and noisy. As a result, image quality has been poor compared to marine seismic, despite the wide-azimuth geometry.
With the advent of high channel count recording systems, high-productivity vibroseis techniques and advances in wide-azimuth processing, onshore 3D seismic is undergoing a revolution. This revolution is the move from sparse to high-density WAZ acquisition and it is being made possible by high-specification SuperCrews.
Using techniques such as CGG’ V1 single vibrator acquisition, a step-change in source productivity has been achieved.
V1 allows a large number of vibrators to act independently as separate siesmic sources. This is done within a slip-sweep (overlapping sweep) acquisition scheme where long sweeps are used and wait times (slip times) between sweeps are as short as possible.
For high-density surveys, this creates a huge increase in productivity and allows CGG crews to routinely record over 600 source points per hour.
CGG is at the forefront of high-density land WAZ acquisition, with two high-channel crews currently in operation in the Middle East.
These CGG SuperCrews are recording data at up to 100 times the density of conventional land surveys, and at rates that are breaking records regularly. Remarkably, this makes the new-generation land acquisition denser than current marine WAZ surveys by a factor of 10.
This is important in the onshore environment to ensure that the seismic wavefield (including signal and coherent noise) is adequately sampled, so that the full benefits of wide azimuth technique can be realized:
- Improved noise and multiple attenuation with true 3D WAZ algorithms
- High-resolution velocities and statics for improved imaging
- Azimuthal velocity analysis for improved imaging
- Wide-azimuth illumination of targets for improved imaging
|(a) Narrow-azimuth data reprocessed in 2008||(b) Wide-azimuth acquisition and processing|
|(from Wombell et al., EAGE Subsalt Imaging Workshop, Cairo 2009)|