Broadband acquisition a step-change for deepwater imaging

Broadband acquisition techniques have provided a step-change in seismic image quality of similar magnitude to the earlier changes from 2-D to 3-D and from narrow-to wide-azimuth (WAZ) acquisition. The value of broadband data is recognized for almost all seismic applications, and especially for deepwater environments, where the targets are frequently below complex overburdens. Imaging these deep targets is challenging as only a limited range of frequencies can penetrate to them, making recording and preservation of the low frequencies essential. However, high frequencies also are required to provide detailed images of the overburden and near-surface to obtain good velocity models for accurate imaging and so that drilling hazards can be avoided.

In a 2-D data comparison from the GoM, the BroadSeis data show improved resolution and clearer layer differentiation as well as greater subsalt illumination. (Data courtesy of CGGVeritas Data Library)

Broadband acquisition

Trilobit nodes recorded high-resolution data directly under a drilling platform where towed streamers could not provide coverage. The Trilobit timeslice data is inserted into the conventional data (top) and shows much higher resolution. Bright spots, indicating gas accumulations, are clearly visible on this and the inline section (bottom). (Data courtesy of Apache)

Deep-towed hydrophones or ocean-bottom nodes provide the optimum low frequencies. BroadSeis uses variation in streamer depth to provide receiver ghost notch diversity, allowing the streamer to be towed deeper to improve the low-frequency signal-to-noise ratio without compromising the high frequencies. This ghost notch diversity is exploited by proprietary deghosting and imaging techniques to produce a wavelet with excellent low-frequency signal (down to 2.5 Hz) and maximum bandwidth (six octaves) for optimum imaging of deep targets.

Using a proprietary curved streamer shape rather than a straight slant provides a steeper slope and therefore greater notch diversity for the near offsets as well as allowing a greater average streamer depth. The exact shape of the streamer can be tuned according to the water and target depths to maximize bandwidth at reservoir levels.

Due to the deep tow of the majority of the receivers, only the nearest offsets are significantly affected by weather-related noise, which can be attenuated using standard techniques. Using purpose-designed X-Bow seismic vessels, which perform better in marginal weather, high-quality data can be recorded earlier and later in the season. The Oceanic Sirius seismic vessel acquired three surveys in the North Sea in late 2011 in adverse weather conditions where conventional acquisition would fail to deliver satisfactory results.

Since BroadSeis uses only hydrophones, there is reduced susceptibility to noise from streamer steering. This makes it fully compatible with fanned streamer acquisition for reduced infill in areas of strong currents; a recent 3-D survey offshore the Bahamas was recorded with zero infill. Steering the streamers also increases 4-D repeatability. BroadSeis data have been successfully matched to conventional baseline survey data using 4-D prestack co-processing, demonstrating its compatibility with legacy surveys. However, the full benefits of broadband 4-D reservoir monitoring will not be realized until BroadSeis monitor surveys have been performed over a BroadSeis baseline.

The exceptionally sharp wavelets with minimal sidelobes provided by variable-depth acquisition enhance the fine stratigraphic detail and reveal the genuine seismic reflection response of geologic formation boundaries. This clarifies impedance contrasts and creates sharp images of small features. The extra-low frequencies give an envelope to the seismic signal, shaping the larger-scale impedance variations or major lithology variations to provide clear differentiation between sedimentary packages and increase confidence in correlating seismic interpretation across faults and other major structural features.

Deghosting is both true-amplitude and fully 3-D, so it preserves AVO anomalies. The improved low frequencies provide greater stability and more quantitative inversion results. In conventional seismic data, the lack of low frequencies requires a low-frequency model to be incorporated in the inversion process, adding a bias to the final results which may not be correct. With broad-bandwidth data, high-resolution seismic velocities are used to define the low-frequency model in the 0-5 Hz range while the reflectivity provides information from 2.5 Hz, giving access to more meaningful quantitative results.

Seabed nodes

In mature areas with considerable infrastructure, long-endurance Trilobit nodes provide the ideal partner to towed-streamer broadband acquisition since they can be placed beneath rigs to infill holes in the towed-streamer coverage. These nodes record down to 0.1 Hz on the hydrophones and 1 Hz on the geophones. The node data can be coprocessed with the towed-streamer broadband data to produce seamless coverage of modern, high-quality broadband data in areas where there may currently only be data that predates the rig. The resolution of the near-surface broadband data allows drilling hazards such as shallow gas pockets and channels to be imaged over the entire survey area, even directly under platforms.

A deghosted broadband source can provide more high frequencies and removes the visible bubble and source ghost. (Data courtesy of CGGVeritas)

As an added bonus, Trilobits record 4-C multicomponent data so the shear waves can be analyzed for a more accurate model of the subsurface. Shear waves can be used for imaging under gas clouds since they are largely insensitive to pore fluid content. They also can be used for lithology characterization and add information to distinguish between lithology, fluid, and pore pressure effects.

Both Trilobits and BroadSeis can benefit from a broadband deghosted source, which fills the source notch to provide frequencies to 200 Hz (at 2 msec) and provides full source designature to remove the bubble and ghost, further enhancing interpretation. Due to the exceptionally low frequencies recorded by BroadSeis using a conventional source, the improvement in low frequency is incremental, although a boost in the 7-20 Hz range is expected from a deep-deployed broadband source, which will benefit subsalt and sub-basalt imaging.

Broadband acquisition is revolutionizing deepwater seismic by providing low frequencies for imaging deep targets while providing high frequencies for detailed resolution of the overburden and near surface. With more than 30,000 sq km (11,583 sq miles) of data acquired to date, in a range of water depths and geological settings, variable-depth streamer acquisition has proved itself to be a robust, efficient, and effective solution. In all cases, increases in bandwidth at both low and high frequencies provided significant improvements in imaging and data quality.