DISCover deep interpolated streamer coverage, is designed to efficiently deliver 3D seismic data with an enhanced bandwidth, providing increases in both low frequencies for deeper penetration and high frequencies for improved resolution. The method, enabled by the Q-Marine seismic system, simultaneously acquires data from several shallow-towed streamers, designed to optimize mid- to high frequencies, with data from a smaller number of deeper towed streamers, providing low frequencies. These data are combined in processing to optimize signal-to-noise (S/N) ratio over the entire bandwidth. Potential applications include improved imaging beneath highly absorptive overburdens such as basalt and salt, or acquiring data suitable for inversion to absolute rock properties. Data acquired in this way are also more robust to poor weather conditions than conventionally acquired data.
Conventional towed-streamer systems deploy sources and streamers at shallow depths, typically between 6 and 10 m. This configuration enables recording of the high frequencies needed for resolution, but attenuates the low frequencies needed for stratigraphic and structural inversion. Shallow towing also make the data more susceptible to environmental noise such as waves, swell and wind. Towing sources and streamers at deeper depths enhances the low-frequency content of the data and can increase the signal-to-ambient-noise ratio; however, it attenuates the high frequencies.
Designing tow-depths for conventional towed-streamer surveys therefore requires a compromise to optimize bandwidth and S/N for a specific target depth or two-way traveltime, often at the expense of other shallower or deeper objectives. Data recorded by streamers towed at different depths can avoid this compromise, delivering the high-frequency characteristics of conventional shallow-towed configurations and at the same time the low-frequency characteristics of deep-towed data. Further enhancements to data quality can be derived from deploying a multi-layered source array.
WesternGeco Q-Marine technology has delivered the accuracy needed to enable the benefits of DISCover to be realized. Q-Marine streamers can be steered with the required horizontal alignment and accurate depth control.
The importance of low frequencies
As seismic data processing, imaging, and inversion techniques evolve, the presence of low frequencies in the recorded data has become increasingly important. Low frequencies have always been crucial for the imaging of complex structures in areas of highly absorptive media. However, new methods for developing velocity models, such as full waveform inversion and other inversion techniques for deriving rock properties from seismic data, require the presence of low frequencies in the data below that achievable from shallow towed streamers alone.
Example
Data for a 3D case study using the DISCover method were acquired off the NW Shelf of Australia during December 2008. Six shallow streamers were towed at a depth of 6 m with two additional deep streamers deployed at a depth of 20 m. A novel over/under source design was also used to strengthen the source output in the low frequencies and further enhance the low-frequency S/N ratio. Initial results from the combined data show considerable enhancement in the low frequencies compared to data from the shallow streamers alone (see the images in the right-hand column).
DISCover is offered today from any of the Q-Marine enabled vessels in the WesternGeco fleet.