SG - 4D is a repeat 3-D survey. Ideally it identifies changes in the reservoir due to production to date. i.e. it maps fluid movement. So, the 4th dimension is actually time.
Normally you would want the repeat survey to be the same in all aspects, but Woodside did state a high res survey was possibly planned. It may be that the original survey was not an ideal design for a subsequent 4D (the changes in seismic response due to fluid movement are subtle, and to image such subtle changes in 4D you normally need fairly high resolution), so the new survey would be higher resolution for potential additional 4D in future years.
Hope that helps
HDR
hardman resources limited
