A superior broadband marine seismic solution offers a new combination of leading equipment, unique acquisition techniques and patented de-ghosting and imaging technology to provide better subsurface images. Jo Firth reports.
Whenever seismic data is acquired, the reflections recorded have different frequencies, and the difference between the highest and lowest frequencies is the bandwidth. The broader the bandwidth, the sharper the wavelet and the higher the resolution of the resulting seismic image.
However, complications arise when data is acquired with a large bandwidth. Different sources produce different bandwidth signals, the Earth quickly attenuates higher frequencies, there are issues with just how much of any signal is preserved by the receivers, and the data needs to be appropriately processed once recorded.
The solution lies in answering the challenges that are under our control, ie source and receiver bandwidth and high-fidelity processing as unfortunately we cannot change the Earth's behaviour.
Seeking the best solution
The seismic industry now widely accepts the importance of recording the full range of frequencies (low as well as high) to achieve high-resolution imaging. At one end of the scale, high-fidelity, low-frequency data provides clear imaging of deep targets, as well as greater stability for undertaking inversion. At the other end, higher frequencies are important for high-resolution imaging of important shallow features such as thin beds and small sedimentary traps.
When the streamers are towed at greater depths, this reduces sea-state noise and improves low-frequency signal-to-noise ratios. The drawback is that the streamers record a 'ghost' signal, the reflection of data back down to the streamer from the sea surface, causing a notch in the amplitude spectrum which comes in at lower frequencies as the cable gets deeper - 100Hz at 7.5m streamer depth and 50Hz at 15m. Up to now this has resulted in a choice between recording shallow for high frequencies or deep for noise-free low frequencies.
The BroadSeis solution developed by CGGVeritas acquires both the low and high frequencies with each Sercel Sentinel solid streamer in a specific geometry configuration. The cable ghost notch varies along the cable and this notch diversity is used by our patented new de-ghosting and imaging techniques to produce an ideal wavelet, with the best signal-to-noise ratio and best low-frequency content. It is this unique combination which provides the clearest seismic images, especially at depth, around key target locations.
Low noise characteristics
The BroadSeis solution capitalises on the extremely low-noise characteristics and precise low-frequency response of the Sentinel solid streamer. Solid streamers are quieter and can be towed deeper than other streamers (over 40m), meaning that BroadSeis provides better low frequencies. This is also supported by the new generation of Sentinel streamers that have lower instrument low-cut filters, which allow recording down to 2Hz, providing a full extra octave of data at low frequencies over previous systems.
Conventional wisdom believed that seismic sources did not produce sufficient low frequency to warrant recording below 5Hz, but recent data from the TIDES Tsunami Research survey project conducted by CGGVeritas in Indonesia clearly demonstrate that frequencies below 5Hz are being both emitted and recorded.
As well as offering a quieter solution, with better signal-to-noise ratio and better low frequencies, BroadSeis uses fully-proven, readily-available equipment and is immediately operational.
The combination of Sercel Sentinel streamers and Nautilus depth controllers is ideal for BroadSeis acquisition and leads to efficient operations. The control of solid streamers in BroadSeis mode is very robust and stable, especially when Nautilus is used, even at 45m. The streamers retain their exceptional low-noise characteristics at all depths.
As BroadSeis requires no new equipment where solid streamers are already deployed, it is immediately available on most of the CGGVeritas fleet. There are fewer operational difficulties and, in addition, there is no loss in productivity due to having to tow additional streamers.
One of the unique features of BroadSeis is a new, fully 3D de-ghosting algorithm that makes no 2D assumptions and has no complications for de-ghosting ray paths in the crossline direction. This makes it uniquely ideal for 3D and all wide-azimuth acquisition configurations, as well as being possible for 2D data.
Although the BroadSeis technique includes de-ghosting, it is possible to deliver raw field data to clients, giving them the option of applying any processing techniques desired, and offering them unrivalled flexibility.
With its industry-first combination of an optimised and uniquely shaped solid streamer configuration with patented new de-ghosting and imaging algorithms, BroadSeis delivers a signal spectrum of unprecedented bandwidth.
The result is an exceptionally sharp and clean wavelet for interpretation. The notch diversity is designed and optimized for exploration and production targets, delivering the best possible resolution to interpreters. The variability of the cable depth can be tuned for different water depths, target depths and desired output spectra.
BroadSeis is suitable for all reservoirs, outperforming other industry solutions for high-definition broadband imaging.
As targets go deeper, BroadSeis can rely upon a larger useful bandwidth, particularly in the low-frequency range, for imaging beneath complex overburdens, etc. Conversely, for shallow targets, BroadSeis benefits fully from the total bandwidth available and recordable. This delivers the sharpest wavelet and the best resolving power in the industry as well as the added penetration and imaging power of boosted low frequencies.
The combination of Sentinel streamers, new towing geometries and breakthrough processing technology creates stunning final images of the subsurface. BroadSeis provides a simple, available and robust solution for broadband marine acquisition.
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Jo Firth is with CGGVeritas, Crawley, UK. www.cggveritas.com