Wide-Azimuth Field Data Application
The effectiveness of imaging with separated wavefields depends strongly on the overall streamer coverage, receiver density, source-receiver geometry and subsurface acoustic impedance. The more sea-surface reflection information that can be collected by the chosen source-receiver geometry, the better the potential of improving the final image. For this reason, SWIM® technology is particularly advantageous in improving subsurface illumination and imaging resolution when used with Wide-Azimuth (WAZ) survey geometries.
To illustrate this, we have applied the SWIM technique to a WAZ data set acquired in the deepwater Gulf of Mexico. Results demonstrate the improved imaging resolution, especially for complex structures of salt boundaries.
The images above show a comparison of depth slices (3000 m below the sea-surface) from the imaging of primaries only (left) and imaging with SWIM (right) using a full-salt TTI (Tilted Transversely Isotropic) velocity model. The results illustrate much improved resolution using SWIM where a better lateral definition of the rugose top-salt geometry is resolved (highlighted using arrows inside the red boxes).
The close-up images above confirm that using SWIM produces more continuous and clearer salt boundary images than primaries alone.
The significantly improved illumination and resolution provided by SWIM can be accessed best in pre-stack domain by looking at angle gathers. Subsurface illumination using primaries is limited by the shot density, which is controlled by the shot and sail line spacing (typically several hundred meters). Sampling of the sea-surface reflections used in SWIM is determined by receiver and cable spacing, which is typically much denser than shot and sail line spacing. Therefore, imaging with sea-surface reflections generates more finely sampled angle gathers, especially in the crossline direction.
The angle gathers containing the water bottom, sediment, and top salt can be seen above (A-D). In the plots, the angle gathers are displayed in inline (zero-degree azimuth) and crossline (90° azimuth) directions, with maximum angles ranging from –70° to +70°.
B shows zero-degree azimuth angle gathers from SWIM, which are sampled much more finely than the gathers from imaging with primaries only shown in A. Moreover, in the 90° azimuth direction, only a few reflection angles are imaged using primaries only, C. The image in D shows the densely populated angle gathers from SWIM that are higher resolution and easier to interpret. The densely populated angle-domain images greatly improve imaging resolution in the stack domain.