ACQUISITION - MARINE

Safer, Quieter, Better

Dovetail™Effi cient streamer acquisition of high-quality data

Far offset spread increased by 50% using 2º “fanning”. VBe applied to real-time bin coverage intended to duplicate the effect of trace interpolation in processing and facilitate steering.

Despite a dramatic decrease in infill from more than 40% to less than 10% on this Gom survey, the binfold remained within the contract specifications.

Using Nautilus, streamer fanning is straightforward to implement.

Dovetail™ is an effi cient streamer acquisition solution, especially where there is signifi cant and unpredictable feathering. It is a combination of fanned streamer acquisition using Nautilus®, offset-dependent bin size expansion for steering and intelligent interpolation during processing. Dovetail signifi cantly reduces the need for infi ll while remaining within bin fold specifi cations, leading to reduced survey time and HSE exposure and faster data receipt, with no degradation of data quality.

WHY USE DOVETAIL?

Dovetail combats poor far-offset fold coverage due to extreme feathering, caused by such things as strong local currents etc, by using three interlinked components:

• Acquisition:

- Fan-Mode streamer acquisition by Nautilus, which can be programmed to produce a fanned spread automatically

- Variable Bin Expansion (VBE) applied to onboard binning to facilitate vessel steering for maximum coverage.

• Processing:

- Trace interpolation using REVIVE5D, an intelligent 5D anti-leakage Fourier algorithm

BENEFITS

• Meets binfold specifications for survey with a drastic reduction in infill/prime ratio (current pattern-dependent)

• Reduced survey time

- Faster data receipt

- Reduced HSE exposure

• No reduction in data quality

• Reduced infill means less “wasted” duplicate acquisition and therefore increased processing efficiency

• Smaller holes (in terms of numbers of CMP lines) in the data for safer interpolation

• Better sampling at mid/far offsets

• Large reduction in real-time steering/overlap

• Sail for pre-plot/near offset to achieve higher resolution

8100 m

fan=20

delta=100 m

delta=150 m

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Dovetail data

Contract specs

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DOVETAIL ACQUISITION

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ACQUISITION - MARINE

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Fold of coverage before interpolation, over a test area covering adjacent swathes recorded with and without Dovetail acquisition.

It is impossible to tell where the data was acquired using “Dovetail” and interpolation, or where using conventional acquisition with over 30% infill.

It is impossible to tell where the data was acquired using “Dovetail” and interpolation, or where using conventional acquisition with over 30% infill.

Conventional

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Fold

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Dovetail

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BURIED ACQUISITION

The requirement for infill implies large amounts of duplicate (buried) data which will effectively be discarded in processing. Therefore a survey with 40% infill means that at least 28.5% of the acquisition is “wasted”. In practice, probably only 90% of the overall coverage is recorded, so this is more likely to be 35% wasted data.

Dovetail acquisition reduces the amount of “wasted” data, and reduces the maximum distances that are interpolated, although there may be more short-distance, safe interpolation to fill small gaps. With Dovetail empty bins are more likely to be evenly spread, rather than being concentrated in fewer places.

Using Dovetail acquisition there is no reduction in the fold specifications for the survey, these are just fulfilled more efficiently.

INTERPOLATION

To estimate the effect of interpolation in processing, variable bin expansion is applied to the data for binning. Generally a larger expansion distance is allowed at longer offsets, to mimic the larger interpolation distances allowed in processing, since higher frequencies are attenuated at longer offsets and greater depths. Using Dovetail to increase the streamer separation towards the tail of the streamers applies this principle to the acquisition.

Using a de-aliased anti-leakage, irregular Fourier transform and an intelligent 5D algorithm for the interpolation means that there is no degradation of the data. REVIVE5D uses the true source and receiver locations of the data, preserves amplitudes across offsets and azimuths accurately, and thus honours the true 3D character of the data (important in areas of significant feather).

Depth Migration

Dovetail DovetailConventional Conventional

Pre-Migration

Fold before interpolation

Fold after interpolation

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I-15

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