ABSTRACT

The determination of the base of a salt body in some areas of the Gulf of Mexico is extremely difficult utilizing seismic data alone. This is due primarily to subsalt imaging problems. Full Tensor Gradient data was combined with the seismic data in an effort to determine the edges and base of salt in the Green Canyon area. A 2-D TGS regional line that runs from Green Canyon southeast to Walker Ridge was used in this study. The line is approximately 86 miles long with water depths ranging from approximately 1,000' in the northwest to 8,000'+ in the southeast. A section (approximately 30 miles) of this line runs diagonally across a grid of 3-D gradient data acquired by Bell Geospace, Inc in 1995. The seismic line was interpreted utilizing a workstation interpretation system. The top of salt was identified and a reasonable interpretation made as to the bases and edges of the salt bodies. These horizons were then imported into a gravity gradient-modeling program. The response of the data measured by Bell Geospace, Inc., versus that of the original model indicated significant differences relating to the size, shape, and thickness of the salt bodies within the area covered by the gradient data. The bases and edges of the salt bodies in the model were then altered to match the various gradients and gravity data as closely as possible. The gradient data indicates that various horizons continue through the seismic data "wipe out" zones under the salt bodies. The salt at one point is less than 2,500' thick creating the potential for subsalt objectives.

A comparison of the original salt model, the gradient salt model and a model using only conventional gravity data was made. The gravity data was unable to determine the thickness of the salt as it is composed primarily of long wavelengths which inhibit its ability to image shallow mass density changes. There are very significant differences in the three models, which show that the gradient data is much more sensitive and affected by the salt masses, while the conventional gravity data is more affected by the deeper layers and the basement. A determination of the base of salt can be accurately made by incorporating Full Tensor Gradient data in the interpretation process.