Split spread acquisition
This article is shorter than usual because a day before the publishing deadline I had nothing to write about. But, with a day to spare, I stumbled across an item posted on LinkedIn by Andrew Long (PGS) with a comment by John Cant (a Perth based geophysical consultant).
Andrew commented on a photograph of an offshore seismic survey with an unusual acquisition geometry - a split spread with the source arrays positioned in the “middle” of the receiver array so that there are receiver locations in front (negative offset) and behind (positive offsets) the sources. This is a common geometry for onshore surveys, but it has been difficult to implement offshore. As Andrew says, this geometry with wide sources provides “the best platform for high resolution imaging of the shallow sediments and overburden”. The photo published by Aker BP ASA https://www.linkedin.com/company/akerbp/ and OMV https://www.linkedin.com/company/omv/ with annotations by Andrew Long () shows the Ramform Atlas towing ‘negative source offsets’ as part of their Poseidon CCS survey, but this is only the most recent example as a similar set up was used to acquire the Keraudren Seismic survey (Santos 2019).
Figure 1. View of split spread acquisition with source arrays (bottom right) positioned some distance behind the front of the streamers (centre). Photo by Aker BP ASA and OMV.
This idea is not new. Twenty years ago, BHP recognised a split spread geometry would be better for SRME – Surface Related Multiple Extermination/Elimination/Estimation. They recorded a small 44 km2 experimental survey in an area where water bottom multiples were rife and primary reflections were very difficult to identify. At the time it was difficult to implement a negative offset because a separate source boat would be required. It was also feared that having the streamers in close proximity to the source could damage the towed equipment. So, the survey acquired data in two passes by sailing in opposite directions. The two parts of the split spread were then merged (). As John points out, this experiment was unsuccessful. It seems obvious now, but tidal differences in water depth of each pass led to errors in matching the two halves, and this mismatch was more pronounced at depth.
Figure 2. Split spread gather created by merging two separate passes in opposite directions. The inset shows the mismatch at zero offset between the left and right sides of the gather.
The use of a single boat with extremely long source umbilicals and independently steerable source sub-arrays means this solution is now common for surveys with a particular near-surface focus such as CCS. Of course, this geometry could end up in a tangled mess if it weren’t for the expertise of the onboard crew on the back deck.
Acknowledgments
This article is based on a LinkedIn discussion posted in mid-October 2023 by Andrew Long with comments by John Cant.