In the case of onshore 4D projects, the challenges are somewhat different from marine 4D acquisition. Although repeatability of source and receiver locations are generally easier to achieve, the 4D variations caused by changes in near-surface conditions can be orders of magnitude higher.
Careful planning of the initial baseline survey and a consideration of the expected changes in the response of the near-surface, as well as the reservoir, are essential to assure that a usable 4D signature is obtained.
Positioning accuracy, and therefore repeatability, has been revolutionised by the use of “stakeless” GPS positioning systems. Not only are actual coordinates recorded more accurately, but integrated navigation systems ensure that receivers and sources are deployed as close to the pre-defined targets as possible. Stakeless surveying can eliminate many of the navigation errors which previously affected 4D repeatibility.
The move to high-density and wide-azimuth land acquisition geometries, particularly with single source-single receiver configurations, has great benefits for land 4D. The use of point source and point receivers results in more consistent and repeatable responses and removes unwanted array effects which create azimuthally varying attenuation. 4D processing in particular benefits from:
- More accurate, higher-resolution statics solutions
- More accurate imaging with higher-resolution velocities
- More effective noise attenuation utilising 3D algorithms
SeisMovie
SeisMovie is a permanent seismic monitoring solution developed by CGGVeritas in conjunction with Gaz de France and IFP. The system was initially designed for the monitoring of underground gas storage whereby gas is stored in natural subsurface reservoirs.
Using a permanent buried installation ensures excellent 4D repeatability and coupling. By placing the source and sensors below the weathering layer, SeisMovie eliminates near-surface variations to further improve 4D repeatability over extended periods.
