A modeling study of a subsalt exploration problem was used to help understand imaging issues and to help plan solutions to those issues. By comparing images from various acquisition geometries ranging from wide (WATS) to narrow (NATS) one can say WATS and certain XWATS geometries should resolve most of the subsalt imaging problems for the area of interest. Despite these improvements subsalt illumination remains an issue for all types of acquisition geometries as demonstrated by interpreting images and creating amplitude maps. WATS geometries seem to be more robust when faced with velocity model inaccuracies.
Will the imaging improvements seen elsewhere translate to the specific area of interest?
Which of the proposed acquisition geometries is more likely to produce more improvement?
Which imaging technology is best with the exploration WATS (XWATS) data?
How do errors in the model affect the image quality of XWATS data?
Are subsalt reflection amplitudes a useful interpretation tool?
Over the last several years Wide Azimuth Towed Streamer (WATS) acquisition has been shown to provide significant improvement in imaging and multiple attenuation for complex geology (Regone, 2006; Sava, 2006; Barley, et.al., 2007, Beaudoin, et.al., 2007; Corcoran, et.al, 2007; Howard, 2007; Michell, 2007). Several service providers are now offering multi-client Exploration WATS 3D surveys. We wanted to better understand potential imaging improvement possible through the use of WATS acquisition in the Gulf of Mexico. Specific questions to be addressed are:
A modeling approach similar to that described by Regone (2006) was used in this study. The model for a specific exploration project was constructed using well data, seismic depth-imaging velocities, and salt surfaces provided by the interpreters of the area. As a first step interpreted surfaces represented the shapes of the water bottom, top of salt, bottom of salt and major stratigraphic boundaries were imported into the interpretation system. Each surface required editing to create closed volumes. Figure 1 shows the final top and bottom salt surfaces. Depth migration derived sediment velocities were used as the background into which the salt was inserted between the surfaces in Figure 1. A constant salt velocity of 14, 800 ft/s was used.
The model was used to compute seismic shots using a constant-density acoustic two-way wave-equation algorithm. Approximately 7,000 shots were computed each using a 50 m by 50 m gridded receiver array centered on the source covering a square 18.6 km on a side. Thus, each shot was recorded by approximately 140,000 receivers. The shot line spacing was 150 m which was chosen so that subsets of the full data could closely approximate proposed multi-client surveys. Subsets of the full shots were created for a typical Narrow Azimuth Towed Streamer (NATS) geometry and several XWATS geometries (Figure 2). The critical acquisition geometry features for each set of shots are shown in Table 1. Typical XWATS and NATS surveys use a sail line spacing of 450 m. All datasets used a shot spacing of 100 m and receiver spacing within each cable was 50 m. The number of cables and the placement of the source were varied between the datasets.
