Variable depth streamer — The new broadband acquisition system (2011)

The importance of recording the full range of frequencies (low as well as high) is widely accepted. High‐fidelity, low‐frequency data provides better penetration for the clear imaging of deep targets, as well as providing greater stability in seismic inversion. Broader bandwidths produce sharper wavelets and both low and high frequencies are required for high‐resolution imaging of important features such as thin beds and stratigraphic traps. The industry has been facing many issues that have limited the performance of marine seismic surveys with respect to bandwidth. Among them, we find mechanical and acoustic noise, source and receiver ghosts and attenuation with depth. Until recently, conventional de‐ghosting was found to be sub‐optimal. Thanks to recent advances in technology and also in operational capabilities, we have seen several improvements, in particular with the use of solid streamers, deep towing and notch diversity. We describe a different technique to achieve broadband marine streamer data. The proposed solution is a new combination of streamer equipment, novel streamer towing techniques, and a new de‐ghosting and imaging technology. The technique takes full advantage of the low noise and low‐frequency response of the new generation of solid streamers (see Figure 1). It then uses receiver notch diversity to yield a broadband spectrum. Conventional acquisition (see Figure 2) with its receiver ghost represents a tuned receiving array which enhances some frequencies and completely cancels others (at the ghost notches). In variable depth streamer acquisition (see Figure 3) a variable depth streamer is used so that the receiving array is detuned and receives all frequencies. As a result, the method creates an exceptionally sharp and clean wavelet for interpretation. It can be optimized for different water depths, target depths and desired output spectra. Figure 4 shows how the variable depth configuration improves low frequency response (it is the average streamer depth that is key parameter here), while at the same time using notch diversity to avoid higher frequency notch problems. This approach to towed streamer broadband seismic is particularly efficient, flexible and customizable for a range of environments and applications. The acquisition parameters such as variable depth streamer profile, maximum streamer depth and source depth can be tuned to provide the maximum possible bandwidth for a given geological setting and water depth. In particular this technique can take full advantage of towing solid streamers at what are currently considered as extreme depths to benefit from the improved low‐frequency response of the hydrophones and reduced sea‐state noise. To date a variety of test lines have been acquired in different settings with streamer maximum depths as large as 60m. The novel approach to de‐ghosting is fully 3D. It makes no 2D assumptions and has no limitations in the cross‐line direction making it suitable for wide‐azimuth as well as 3D surveys. This flexibility means that the technique can be used for a range of applications. The increase in penetration from the extension of the bandwidth at the low end will benefit the imaging of deep targets and those below complex overburdens. Shallow targets (such as shallow hazards) will benefit from the fully from the total bandwidth available and recordable. Recent trials have achieved usable bandwidths between 2.5 and 150 Hz. Datasets using this technique have been acquired in a variety of geologic settings around the world ranging from deep water salt environments such as West Africa and the Gulf of Mexico to shallow water settings such as the North Sea. Examples will be shown displaying the advantages of the increase in resolution, the improved texture in the image which is useful for interpreting lithology and increased low frequency penetration in sub‐salt environments. These examples include 3D and AVO applications. Figure 5 shows an example of how the variable depth streamer acquisition increases the low end of the bandwidth and reduces the noise levels in the final image below salt in the Gulf of Mexico.