PGS First Break Article on Towing Strategies for Improved Efficiency and Better Coverage
In the November edition of First Break, PGS Chief Scientist Andrew Long outlines how improvements in efficiency, spatial sampling and near offset coverage can be gained by different source and streamer towing strategies.
Several towing concepts break the convention of towing two source arrays between the two innermost streamers in a multi-streamer 3D configuration (‘dual source shooting’). More than 20 years ago, three arrays (‘triple source shooting’) of one or more sub-arrays were towed to improve cross-line spatial sampling, but inline spatial sampling and fold was compromised by inefficient recycling times on air gun compressors and limited recording lengths.
Modern acquisition systems enable continuous recording, very short physical shot intervals, and up to six source arrays being deployed between the innermost two streamers; always with the ambition of improving cross-line spatial sampling.
Survey efficiency and spatial sampling for the ‘conventional’ scenario where all source arrays are towed between the innermost two streamers are illustrated and compared with scenarios of increasingly large source separations and sources being towed outside the innermost two streamers. Sail line efficiency increases with increasing source separation if a predictable pattern of zero fold sublines centered around the sail line boundaries can be accommodated in signal processing and imaging. The number of zero fold sublines increases with increasing source separation.
Alternatively, if the sail line separation is not adjusted, being based upon the nominal sail line separation for ‘conventional’ source towing, the zero fold sublines are mitigated by the finite fold contributions from the sublines of the adjacent sail lines in an interleaved manner. Sail line efficiency is therefore not changed, but the near offset distribution will be changed for each subline.
Acquisition Strategy for Shallow Targets
Resolving shallow exploration targets in shallow water areas such as the Barents Sea is a challenge for conventional marine seismic acquisition and imaging techniques. New novel acquisition concepts are needed. Widmaier et al. SEG 2017, outline their recipe for improved near offset acquisition without the need for an additional source vessel. It involves dual-sensor streamers, high density acquisition, wide towing of the sources (dual, triple or more) and variable length streamers. This can improve near offset/near angle distribution in marine streamer acquisition in a cost effective way and using existing technology and equipment.
The images below show the near offset distribution for three acquisition geometries. Firstly, for a conventional streamer spread of 12*75 m with a dual source separation of 37.5 m. Secondly, for a streamer separation of 18*50 m, with staggered streamer fronts to reduce source-receiver offsets, and dual source with 175 m source separation. Thirdly, the case is for 16*50 m streamer setup, staggered streamer fronts but triple source with 116.67 m source separation.
The sail line separation is shown as a black dashed line and is 450 m for the first two images and 400 m for the third. Three adjacent sail lines are shown for each geometry.
CMP positions are along the x-axis and source-receiver offsets are shown on the y-axis. The color of the dots represents incidence angle at a target reflector and it can be seen that for the wide tow source and staggered front geometries significant near angle information will be recorded. This near angle information is essential for accurate AVO analysis.
Deploying more sources and reducing the spread width can improve the near offset distribution but cost and quality implications have to be taken into account.
|Acquisition Parameters||Conventional Dual Source||Wide Tow Dual Source||Wide Tow Triple Source|
|Streamer Spread||12 * 75 m||18 * 50 m||16 * 50 m|
|Cross Line Bin Size||18.75 m||12.5 m||8.33 m|
|Sail Line Separation||450 m||450 m||400 m|