- Offset ranges of up to ~20 km are recorded by two vessels using streamer lengths of 10 km
- Increases safety and efficiency and reduces noise
- Beneficial to deep velocity model updates using Full Waveform Inversion
- Ultra-long offsets can be achieved using sparse OBN with towed streamer acquisition
A dual vessel configuration consists of a vessel with a streamer spread and sources that is complemented by a second vessel towing either sources or streamers. Alternatively, ocean bottom nodes can be deployed in typically sparse configurations to complement the offset coverage from streamer acquisition.
The maximum long offset requirements and the corresponding spatial sampling depend on the geological targets and can be derived during survey planning. Velocity model building based on refraction FWI has increased the demand for longer offsets.
Dual vessel operations frequently use a greater number of sources compared to conventional towed streamer operations. Blended acquisition with shot dithering can be used to optimize shot sampling, fold, and acquisition turnaround. Low-frequency sources can form part of a multi-source solution to improve the signal-to-noise ratio for the long offset recordings.
The frequencies and spatial sampling recorded by the longest offsets should be tailored to the subsurface requirements. Proprietary shot dithering schemes are used to enable blended acquisition with dense inline shot sampling, and the final imaged data benefits from the high fold and broadband frequency content characteristic.
Flexible Source and Streamer Configurations
Signals from the primary vessel provide the near half of the full offset range, whereas the secondary source or streamer vessel provides the far half of the offset range (see figure below). Shorter cables are less susceptible to feathering and line turns are more efficient, reducing the risk of tangles and simplifying streamer maintenance in areas affected by barnacles.
Simultaneous Long Offset (SLO) acquisition uses blended-shooting to yield equivalent shot sampling to traditional acquisition with one vessel.
It is also possible to record the extra offsets with a second streamer vessel deployed either ahead or behind the primary acquisition vessel. The second streamer vessel can be configured with a sparse spread to use the record data for FWI model building, or with a full spread to record data suitable for both model building and imaging (see figure below). This configuration could be used in areas where environmental regulations limit the use of additional sources, or on surveys where the separation of near and far source contributions might is expected to be problematic. Wide-tow multi-source shooting may also be deployed by the primary vessel to optimize the crossline spatial sampling and near offset distribution beneficial to shallow imaging.
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