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ABSTRACT Variable-depth streamer acquisition is becoming a key technique for providing wide bandwidth seismic data. Varying the receiver depth creates wide receiver ghost diversity and produces a spectacular increase in the frequency bandwidth. However, compared to conventional data, this variable-depth streamer data implies a major challenge in processing: how to deal with various receiver ghosts. The ghosts have to be preserved up to the deghosting step. Here we present the implication for the following de-multiple methods: Shallow-Water De-multiple, Tau-P deconvolution and Surface-related multiples elimination in deep and shallow water environments.
ABSTRACT This paper starts by revisiting the receiver deghosting problem, showing that in order to be optimal in terms of signal-to-noise ratio, it should not performed as a preprocessing stage as it is done usually. A new deghosting algorithm is described, based on computing a migration together with a mirror migration, and performing a joint deconvolution of these two images. It is true amplitude, being able to extract the true deghosted reflectivity, that is the reflectivity that would have been obtained should the water surface be not reflecting. The paper then describes how this new method allows to deghost acquisitions based on pre-stack notch diversity, such as the slant streamer technique. We revisit this acquisition technique in two ways: firstly, we optimize the depth profile of the streamer to ensure diversity for all reflectors depths, leading to variable-depth streamer rather than slant streamer. Secondly, recognizing that the stack performs an imperfect deghosting, leaving a residual ghost, we perform the residual deghosting with our new deghosting technique. This variable-depth streamer acquisition and processing has been tested on several locations. Here, we show results from a West Africa acquisition, where a 2.5–150 Hz bandwidth was achieved. This broad bandwidth translates into improved results for the acoustic impedance inversion. Variable-depth streamer data seems to have the potential to fill the usual gap between the high frequencies of the seismic velocities and the low frequencies of the reflectivity, the 2.5–5 Hz octave being the overlapping zone.
- Geophysics > Seismic Surveying > Seismic Processing (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (0.69)
Challenges In Processing Variable-depth Streamer Data
Lin, Dechun (CGGVeritas) | Sablon, Ronan (CGGVeritas) | Gao, Yan (CGGVeritas) | Russier, Damien (CGGVeritas) | Durussel, Vincent (CGGVeritas) | Romano, Vera (CGGVeritas) | Soubaras, Robert (CGGVeritas) | Whiting, Peter (CGGVeritas) | Gratacos, Bruno (CGGVeritas)
ABSTRACT Variable-depth streamer acquisition is emerging as a key technique for providing wide bandwidth seismic data. With several data sets acquired across the world, it has consistently produced high quality images in terms of seismic resolution, layer stratigraphy and low-frequency penetration. By varying receiver depth, variable-depth streamer acquisition introduces receiver ghost diversity over different offsets. Such diversity enables a joint deconvolution method to fully remove the receiver ghost. Variable-depth streamer data also tends to be less noisy due to the deep tow of cables. These two factors allow variable-depth streamer data to have a spectrum from 2.5 Hz up to the source notch. Challenges in processing include: how to maintain the full bandwidth in the data, how to effectively remove multiples, and how to robustly build a velocity model. This paper will discuss each of these challenges and their solutions.
- Europe > North Sea (0.16)
- Oceania > Australia (0.15)
- North America > United States (0.15)
Variable Depth Streamer – The New Broadband Acquisition System
Soubaras, Robert (CGGVeritas) | Whiting, Peter (CGGVeritas)
ABSTRACT 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.
- North America > United States > Illinois > Madison County (0.25)
- Europe > United Kingdom > North Sea (0.25)
- Europe > Norway > North Sea (0.25)
- (2 more...)
- Geology > Rock Type (0.36)
- Geology > Geological Subdiscipline (0.35)