Solving Imaging Challenges in a Deepwater Complex Ooze Regime

Derisk by resolving complexities and creating accurate velocity models and reliable images.

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The Location

Location

Outer Vøring, Norwegian Sea

Survey Year

2016

Survey Type

3D GeoStreamer

Configuration

16 streamers, 75 m separation, 8100 m length

Survey Size

5 500 sq. km

Water depth

800 - 1 600 m

Water depth

25 m

This 2016 project (orange outline), acquired by Ramform Tethys deploying 129 km of in-sea recording cable, remains the biggest spread ever towed by a seismic vessel in northern Europe. Globally, this achievement has only been matched by the sister vessels in the Ramform Titan-class fleet. Visit our data library for more information on data availability.

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The Challenge

The Challenge

Reservoir targets in the area sit deeper than complex ooze bodies. In order to accurately image the targets the ooze bodies must be effectively resolved. Deep water and lack of refractions from strong negative velocity contrasts at the top of the ooze bodies mean that traditional FWI is not optimal. The ooze also causes strong scattering of the wavefield and results in distortions and amplitude diming effects which must also be addressed.

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The Solution

Data

Good S/N due to multisensor and deep tow

Processing

4 ms

Signal Processing

Full 3D demultiple

Velocity Model Building

FWI using reflections and refractions

Frequency Range for FWI

2-12 Hz

Q modeling

Variable Q model to capture high-absorption ooze bodies

TGS FWI solution includes reflections and refractions to provide deeper and higher resolution velocity models. Small-scale velocity variations and sharp velocity contrasts between the ooze bodies and surrounding lithology are captured. Q modeling is included to compensate for amplitude distortions and to ensure absorption is incorporated during the migration.

Ooze Bodies Better Resolved with FWI

Before Image After Image
Initial
12 Hz FWI
PSDM stack and 1 700 m depth slice for the initial (left) and final FWI velocity model overlays (right). Ooze bodies are much better resolved with reflections and refractions in TGS FWI.
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The Results

Better imaging of Targets Beneath the Ooze

High-resolution velocity models down to depths of 4 km have been generated due to the availability of FWI frequencies down to 2 Hz and the inclusion of reflections in the calculations. Targets beneath the ooze are better imaged as the high-resolution velocity models have been combined with detailed attenuation (Q) models in a Q-Kirchhoff prestack depth migration.

Increasing FWI Frequency to 27 Hz For Superior Delineation of Shallow and Deep Ooze Bodies

Before Image After Image
12 Hz FWI
27 Hz FWI
12 and 27 Hz velocity model overlays on a 1530 m Q-KPSDM section. Note the resolution of localized ooze features in the 27 Hz FWI velocity model.

27 Hz FWI + Q-WEM for an Accurate Image
Corrected for Velocity Variations Due to Ooze

Before Image After Image
Q-KPSDM
Q-WEM
Q-KPSDM run with a 12 Hz FWI velocity model versus Q-WEM run with a 27 Hz FWI velocity model and masked shallow Q model. Note the Q-WEM image is significantly less distorted below the thick ooze body. The resolution of faults and geological contacts and facies boundaries is improved.