of the largest waves observed precisely was reported by USS Ramapo in a storm in the Pacific Ocean in 1933. An officer on the deck used the crow’s nest to estimate the wave height and corresponding wavelength, knowing that the length of the ship was 146m. Te estimated wave height was 34m and the corresponding wavelength was 342m, yielding a wave steepness of 1/10.
Figure 5.17: Turbulent flow around a seismic streamer, visualised by a single hole dye release.
5.3.3 Turbulent Flow Around Streamers
In addition to noise created by ocean waves, there are currents in the water layer, and close to the streamer the flow is not necessarily laminar, as shown in Figure 5.17. Elboth et al. (2010, 2012) studied this flow in detail, and demonstrated that the surface material of the streamer is crucial. By using a superhydrophobic surface material they revealed that the towing noise could be lowered by approximately 10 dB in the frequency range between 1 and 10 Hz. Tis observation is important for broadband seismic, where the lower frequencies are essential. Te broadband paradigm shift that we witness today is multi- causal: the streamer depth has increased, the manufacturing of various types of streamers has improved significantly, especially with respect to noise attenuation, and finally the algorithms for data processing have also improved. Te result is high quality data, enabling an improved image of the subsurface.
5.3.4 Noisy Birds and Tugs
In order to control its depth, a streamer is equipped with devices that enable adjustment of the vertical (and in some cases also the horizontal) position. Tese devices are called ‘birds’ and are attached to the streamer at certain intervals, typically a couple of hundred metres. A noise record obtained in calm weather is shown in Figure 5.19, where we can notice a significant increase in noise level close to the birds and also close to the front and tail ends of the streamer. Trace 120 is closest to the seismic vessel, and we observe a slight decrease in noise level (red dashed line) as we get further away from the ship. Tis indicates that some of the noise observed is actually vessel noise caused by the ship propellers and engine. Te slightly increased level at the head and tail (6–8
microbars) is probably caused by tugging from the vessel, tail buoys and lead-in sections. As the sea condition gets more marginal, this tugging noise will increase and create unwanted vibrations along the streamer. As the streamer depth increases, the angle between the head and tail buoys rises, leading to extra tug noise. Tis additional problem is normally solved by introducing passive sections prior to the active receiver sections in the streamer. However, there
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Figure 5.18: Noise spectra with and without superhydrophobic (SHS) surface on the streamer. We observe a significant noise reduction for frequencies between 1 and 10 Hz.
Figure 5.19: Measured noise in calm seas (practically no wind). The noise level is lower than 0.5 microbar for the mid streamer, and shows significant increase close to the birds and the front and tail end of the streamer. Notice the slight decrease in noise level away from the ship (trace 120 is closest to the ship). This is a normal, fluid-filled streamer.
Elboth et al., Geophysics, 2010
Martin Landrø
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