Wider Source Separation
Wide tow sources can increase efficiency. Alternatively, a combination of high-density spreads and wider towing of sources can offer a cost-effective method to achieve denser spatial sampling and improve near-offset distribution. By tuning the configuration, both shallow and deeper targets can be optimally imaged.
Read about the latest tailored acquisition solutions, highlighting the benefits of wide-tow sources, in our First Break article, ‘Redefining marine towed-streamer acquisition’.
Marine seismic source arrays are traditionally towed between the innermost two streamers. The resulting CMP-surface-coverage-per-sail-line is sometimes referred to as the 'brush'. The width of this coverage brush corresponds to half the width of the streamer spread, and the brush grows wider with increasing source separation. Consequently, the sail-line separation can be increased to gain efficiency. However, wider source separation introduces a pattern of zero-fold sublines centered around the sail-line boundaries, this needs to be accommodated for by data reconstruction in signal processing and imaging.
Recently, PGS completed the Viking Graben GeoStreamer X survey, using a single vessel operation in the North Sea, successfully towing a 225 m wide source with triple arrays, with sources distributed very close to the streamer front ends.
Improved Near Offsets
If the sail-line separation is not adjusted but is based upon the nominal sail-line separation for ‘conventional’ source towing, then the zero-fold sublines are mitigated by the fold contributions from the sublines of the adjacent sail lines in an interleaved manner.
With this solution, sail-line efficiency is not changed. Instead, the near offset distribution will be improved for each subline. The improvement in the near-offset coverage for shallow AVO analysis is clear in the slider below, showing a comparison of a standard source set-up with a wide tow source configuration. The wide-tow source approach, combined with a high-density acquisition, can help to resolve imaging challenges with shallow exploration targets in shallow water areas such as the Barents Sea.
Wide source towing is not restricted to dual or triple source configurations. The concept can be applied to configurations with higher source counts. The ultimate vision is to distribute many smaller sources, e.g. across the front-end of a streamer spread, or throughout the spread. Distributed source solutions are however not yet feasible with towed marine seismic technology and inventory. However, in light of recent developments in marine vibrators and alternative source technologies such as eSeismic, distributed sources may become reality in the not-too-distant future.
Slider showing the near-offset distribution for a quad-source configuration with 12.5 m standard source separation (left) and 62.5 m wide source separation (right). The streamer separation is 50 m in both examples. The red dashed lines indicate the center of each sail line. CMP-X positions are along the x-axis, and source-receiver offsets are along the y-axis. The wide-tow source configuration provides an improved near offset coverage for shallow AVO analysis.