Narrow-Azimuth Field Data Application
Imaging using sea-surface reflections improves the illumination and resolution of shallow subsurface targets when applied to Narrow-Azimuth (NAZ) data. The acquisition footprint and poor near-surface image resolution present ongoing challenges, particularly in shallow-water environments when only primaries are used for imaging.
When imaging using primaries only, the acquisition footprint is caused by large shot spacing in the crossline direction. By using each receiver array as a virtual-source, SWIM® increases shot density in both inline and crossline directions and can improve the illumination area and mitigate the sail line footprint.
The large and sparse angular illumination from the imaging of primaries creates lower resolution images, especially for shallow structures. By imaging denser and relatively smaller reflection angles, SWIM generates continuous higher resolution images.
To improve the image resolution, high-density 3D (HD3D®) acquisition survey geometries can be designed, comprising denser sail line spacing and richer near-offset information. However, this increases the cost of both acquisition and data processing.
Sea-surface reflections provide a significantly wider and denser sampling of shallow reflectors than is possible with primary arrivals. Therefore, SWIM provides a possible low cost alternative to high-density acquisition geometries in shallow-water environments.
We illustrate the near-surface image improvements by imaging sea-surface reflections from a NAZ data set from offshore Malaysia. In a 25 km × 23.4 km testing area, 49 consecutive sail lines of dual-sensor streamer data are imaged using separated wavefield data.
The image above shows depth sections at 105 m below the sea surface, in an area with 70 m water depth, for the primary-only image (left) and the SWIM image (right). Significant gaps in the shallow overburden are visible in the primary-only image, caused by the required mutes to eliminate refracted energy arrivals. In contrast, the SWIM images have a complete and high resolution near-surface image, which provides a shallow channel image of excellent clarity and structural detail. Data: Lundin.