Dual vessel operations enable efficient acquisition of very long offsets beyond the length of a seismic streamer.

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Schematic illustration of the vessel and source arrangement for ELO acquisition with both vessels towing sources.

Acquisition Solutions for Complex Subsurface Challenges

Extended long offsets (ELO) are increasingly used in seismic acquisition to improve imaging of deep and structurally complex targets. By extending the offset range, ELO enables better recording of low-frequency refracted energy, which is critical for accurate velocity model building using Full Waveform Inversion (FWI).

These extended offsets are typically achieved using dual-vessel configurations. One vessel tows streamers and sources, while the second tows either additional sources or streamers to expand the offset range. Hybrid acquisition designs may also combine sparse ocean-bottom node (OBN) deployments with streamer acquisition to enhance both offset and azimuth sampling. Offset length and spatial sampling are defined during survey planning based on the geological objectives.

Advantages of Extended Long Offsets

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Offset ranges of up to approximately 20 km can be recorded using two vessels, each towing streamers up to 10 km in length.

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Improves acquisition safety and efficiency, and helps reduce coherent and ambient noise levels.

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Supports deeper and more accurate velocity model updates when used with Full Waveform Inversion (FWI).

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Can be achieved using hybrid acquisition, combining sparse ocean-bottom node (OBN) deployment with towed streamer data.

Tailored Solutions

Dual-vessel operations often employ more sources than conventional towed streamer acquisition. Blended acquisition with shot dithering can optimize shot sampling, fold, and turnaround time. Low-frequency sources may be included in multi-source configurations to improve signal-to-noise ratios at long offsets. Frequencies and spatial sampling captured by the longest offsets should be matched to the imaging requirements of the subsurface. Proprietary shot dithering techniques enable blended acquisition with dense inline sampling, resulting in high fold and broadband content in the final imaged data.

Flexible Source and Streamer Configurations

Signals from the primary vessel typically provide the near half of the full offset range, while the secondary vessel contributes the far offsets (see figure). Shorter streamers are less susceptible to feathering, and tighter line turns reduce the risk of tangles and simplify maintenance in barnacle-prone waters. Simultaneous Long Offset (SLO) acquisition applies blended shooting to deliver shot sampling comparable to conventional single-vessel acquisition.

Extended Long Offsets - Fig2
An alternative dual vessel ELO configuration with a second streamer vessel without sources behind the primary streamer vessel.

Enhancing Offset Recording with Dual Streamer Vessels

Extra offset coverage can also be achieved using a second streamer vessel positioned ahead of or behind the primary vessel. This secondary vessel may be equipped with a sparse spread to collect data for FWI velocity model building, or with a full spread to support both model building and imaging (see figure below). This setup is useful in areas where environmental regulations restrict additional sources or where separating near and far source contributions is expected to be difficult. Wide-tow multi-source shooting from the primary vessel can also improve crossline sampling and near-offset distribution, enhancing shallow imaging.

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