The deviation of the towed receiver cables from a simple line-astern configuration is known as “feathering”. Feathering is generally caused by currents, and impacts the homogeneous fold coverage across different offsets before infilling, particularly at far offsets. This impact can be more serious if currents change direction in the survey area, resulting in feathering variations both in amplitude and direction across contiguous sail lines.
Feathering of each cable during MGL0812 was monitored in real time from Spectra, both graphically and numerically. This capability was particularly important during acquisition of infill lines so they could be navigated in real time to fill in the missing offsets according to the feathering of the moment.
Average feathering during the cruise was 0°±5° (one standard deviation), with maximum values of up to 11°.
Figure shows feathering map of primary lines for the primary 3D box (racetracks 1 and 2). Feathering is measured as angle with respect to the bearing of the sail line, averaged for the 4 cables. Positive and negative values correspond to cables displaced to the north and south of the sail line, respectively.
To account for uneven data trace distribution over the target 3D survey region due to streamer feathering as well as other survey shutdowns mid-line, 24 infill and repeat lines were collected. Of this total, 10 were repeated lines for system failures and marine wildlife and 14 were infill lines to obtain uniform fold for the near to mid source-receiver offsets.
Figure above shows bin fold as percentage of the nominal full fold before (a) and after (b) infill and reshoot. Source-receiver offsets from 200 m to 1662 m are taken into account. Both axes indicate bin numbers. Bin size horizontally is 37.5 m and vertically 6.25 m. Bin flexing is 50% at near offsets and 150% at far offset with a linear taper in between.