Subsalt Imaging: An Optimized Design for the WATS Acquisition Method
Total conducted an extensive seismic acquisition campaign using the Wide-Azimuth Towed Streamer (WATS) method to obtain images of subsalt targets in deep offshore West Africa. Upstream expertise, cutting-edge equipment and powerful computing resources yielded an optimized design with the best cost-benefit ratio.
Improving subsalt imaging
The Keys for High-Potential Exploration
Mapping the most complex geological structures, particularly when salt bodies are present, constitutes one of the greatest challenges for geophysics. The conventional offshore seismic data acquisition method known as Narrow-Azimuth Towed Streamer (NATS) is not suited to these environments.
The data collected this way, from just one direction (or azimuth), do not yield reliable images of the subsurface. For this reason, we transitioned to the next generation of seismic acquisitions: Wide-Azimuth Towed Streamer (WATS). Depicting the same subsurface reflecting point from multiple azimuths, this method significantly improves subsalt imaging and yields a more reliable image of the geological realities.
We decided to implement this innovative approach for Block 32 in deep offshore Angola, one of the most difficult areas in the world to map due to a geological environment comprising considerable salt bodies. Our aim was twofold: to assess the subsalt turbidite reservoirs that were revealed but not sufficiently mapped by our two previous NATS surveys (in 2000 and 2007) and to continue our exploration of the block. This required covering a 1,500 km2 area to perform one of the two most extensive proprietary WATS acquisitions in the world.
Due to the marine assets involved, a WATS survey requires substantial investments, five to ten times greater than those of a NATS survey. Given this financial and budgetary reality, the design to be performed had to be optimized by envisioning the approach that would minimize acquisition costs while ensuring that we achieve our imaging goals. That was the aim of the 2012-2013 feasibility study.
A comprehensive simulation, from data acquisition to imaging
A feasibility study consists of simulating the entire geophysical chain, from synthetic data acquisition to seismic image compilation, for each design undergoing testing. This means integrating advanced geophysical and geological expertise, cutting-edge imaging tools and powerful computing resources such as Pangea, Total’s supercomputer.
We began by building an acoustic model that was as realistic as possible, comprising a 3D cube for velocity (of the seismic waves) and a 3D cube for density (of the subsurface). The velocity field included all the information about the geological complexity of the area being mapped. Meanwhile, the density cube modeled the stratigraphic levels for the region. Three-dimensional geological structures, such as channels, were inserted into the target acquisition levels to assess the designs’ capacity to brighten them.
This model served as the basis for the two following steps, which were repeated for each simulated acquisition approach:
- modeling: the acquisition is simulated by modeling the propagation of seismic waves within the model between each shot point and receiver
- migration: the synthetic data obtained are migrated to carry out seismic images, which are then compared to the already completed model to assess the performance of the design being tested
By implementing our High-Performance Computing (HPC) resources, we could use modeling tools based directly on the Reverse Time Migration (RTM) wave equation, which came out of our depth imaging R&D. Costlier in terms of computing power but more reliable than tools based on ray-tracing, these resources make it possible to better understand structural heterogeneity and produce higher quality images in areas of great geological complexity.
$25 million IN savings
The first proposed design was the same as the one for the single azimuth (unidirectional) WATS acquisition performed by Total in 2009 in another region of Block 32.
However, the great complexity of the target area would have required us to increase the density of the approach in order to achieve our imaging goals. This would have affected the duration of the acquisition as well as its cost (estimated at $188 million).
We therefore optimized the initial design by simulating around fifteen scenarios with different acquisition parameters: recording instruments, minimum and maximum offsets (distance between source and receiver), the direction of the acquisition, the density of shot points, and so forth.
Ultimately, we chose an acquisition approach from two perpendicular azimuths (bi-azimuth), requiring the use of two source ships and a streamer ship towing twelve 8,000-meter-long streamers spaced at 100-meter intervals! The deciding factor? An acquisition “grid” that is less dense but uses two azimuths, saving around $25 million and significantly improving target imaging.
In conclusion, the images of Block 32 compiled using data acquired by the 2013-2014 B-Wats campaign proved similar to the synthetic images from our feasibility study, demonstrating the reliability of this cutting-edge method and our ability to make the necessary advances for improving subsalt imaging.