Sea bed diffraction and impact on 4d seimsic data eage2010-baard osdal_notes

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Classification: Internal

Sea bed diffraction and impact on 4D seismic data –observations from synthetic

modeling and field dataB.Osdal, Statoil,

S.Grude and M.Landrø, NTNU


Objective

• Better understand the influence of sea bed diffractions on water layer correction in 4D processing

– Corrections for water layer changes very important in 4D processing

– Diffracted multiples degrade 4D quality


Motivation

a)

4D differenceNo water layer correction

1 km

4D differenceWith water layer correctionMigrated stack


Sea bed diffractions and impact on time shift correction

• Data examples

– 2D FD modelling of seabed diffractions

– Field data from Norne 2006 and 2008


2D modelling of point diffractions

Model seabed

Ice scour: 5-15m wide and 0-15m deep

Modelling

NMO corrected CMPs


2D modelling of point diffractions

Real

Modelling

Observations:• Diffractions can not be flattened using standard primary reflection NMO correction• Asymmetrical amplitude strength asymmetrical ice scour shape is likely

NMO corrected CMPs


Acquisition mis-position and seabed time shift

Acquisition mis-position- 0.2

Distance from apex (m)- 100 0- 200 200100

Tim

eshi

ft (m

s) - 0.4

- 0.6

- 0.8

- 1.0

5m0m 10m

- 100 0- 200 200100 - 100 0- 200 200100

Model seabed

Ice scour: 5-15m wide and 0-15m deep


Acquisition mis-position and seabed time shift

Acquisition mis-position

Mis-positioning creates non-negligible time shift error

Distance from apex (m)- 100 0- 200 200100

Tim

eshi

ft (m

s) - 0.4

- 0.6

- 0.8

- 1.0

5m0m 10m

- 100 0- 200 200100 - 100 0- 200 200100


Approximations for time shift error caused by tuning

V1, V2 - base and monitor sea velocity

,

- Amplitude ratio between seabed and diffraction

h – water depth

x – distance from diffraction apexa= mis-positioning between base and monitor

Valid for small offsets, where the two signal are inside same lobe

Harmonic input signal


Approximations for time shift error caused by tuning

Approximation ok up to ca. 40m from apex point

Distance from apex (m)0 80

Tim

eshi

ft (m

s)

-0.2

- 0.6

- 1.0

- 1.4

5m0m 10m

-40 40-80

approximation

0 80-40 40-80 0 80-40 40-80


2D FD modelling and field data0 1km

Seabed Norne

385

380(m)

390Field data Norne


Seabed time and time shifts

ms

Time Timeshift after tidal correction 1 swath

ms

1km


Seabed time shifts

ms

Mean = -1ms

DVp = ~ 2.5 m/s

Timeshift after tidal correction

Time shift related to: Variation of sea velocity (lateral and with calendar time) Geometrical mis-positioning Error in tidal correction Structural dip and complexity of seabed

1 swath


Seabed time shifts

ms



1 swath


Seabed time shifts

ms

Mean = -1ms

DVp = ~ 2.5 m/s

Timeshift after tidal correction 1 swath


Velocity profiles 2006 and 2008

2008

Calculated means 2006280706: 1488 m/s310706: 1487 m/s030806: 1486 m/s090806. 1487 m/s140806: 1487 m/s180806: 1487 m/s

Calculated means 2008080608. 1484 m/s170608: 1486 m/s

-”Observed” velocity change ~2-3 m/s- variation within each survey

Based on measured temperatures and salinities

2006


Seabed time shifts

ms



1 swath


Time shift related to complex sea bottom and geometrical mis-position

DCMP (m)Blue = Best lineRed = Worst line

Seafloor(m)100m

mean

Seabed time shift (ms)



Seafloor(m)100m

mean


2D FD modelling(DVp=3m/s)



mean


2D FD modelling(DVp=3m/s)

Observation Seabed time shift strongly dependent on geometrical repeatability and complexity of the seabed


Complex sea bottom and geometrical mis-positionSmoothing of time shift

Modelled data –Seabed time shift (ms) Too weak smoothing

Seabed (m)

Smoothing line by line remove much of the disturbance from complex sea bottom

- assume that the seabed irregularities do not create systematic time shift error and strong smoothing is needed



Systematic time shift related to geometrical mis-positioning occurs- Should not correct for this since this is handled in the 3D migration. Strong smoothing or one average time shift value for each swath is therefore preferred.

Field Data - Seabed time shift (ms)

100mBlue = Best lineRed = worst line

mean



ms

Timeshift after tidal correctionTimeshift correction after tidal correction Mean for each line 1 swath

MotivationDiffractions and diffracted multiples

Unmigrated

4D diff

Diffracted multiples a problem in 4D difficult to repeat in 4D difficult to remove in processing

Final 3D migration

1km

ms4D diff

4D diff


Diffractions and diffracted multiples FD modelling

• 2D model

seafloor

1km

1km

Reservoir

Point diffractions below reservoir

Classification: Internal Base

Water corr. applied Water corr. applied

Diffractions and diffracted multiples 4D data quality – Unmigrated data

No mis-positionDv=0m/s 3m/s

Mis-position 5mDv=0m/s 3m/s

Classification: Internal Base No mis-position

Dv=0m/s 3m/sMis-position 5m

Dv=0m/s 3m/s


Diffractions and diffracted multiples 4D data quality – Migrated data

Classification: Internal Base No mis-position

Dv=0m/s 3m/sMis-position 5m

Dv=0m/s 3m/s


Diffractions and diffracted multiples 4D data quality – Migrated data

For 4D data diffraction and diffracted multiples become non- repeatable and degrade 4D data quality due to

- acquisition mis-positioning- changes in sea velocity


Conclusions

Seabed time shifts strongly depend on changes in water velocity, geometrical repeatability and complexity of the seabed

Applying constant or strong seabed time shift smoothing line by line will correct for changes in water velocity, error in tidal correction and complexity of sea bottom.

For 4D data diffraction and diffracted multiples are non-repeatable due to

- acquisition mis-positioning- changes in sea velocity


Acknowledgments

• Statoil and their partners ENI Norge AS and Petorofor permission to use their data

• NFR for financial support to the ROSE project at NTNU

• Børge Arntsen and Jon A. Haugen and for help and support with the FD modelling and Hossein Mehdi Zadeh for Promax help

Classification: Internal Classification: Internal 2010-06-11Classification: Internal 2010-05-03Classification: Internal 2010-05-03

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Sea bed diffraction and impact on 4d seimsic data eage2010-baard osdal_notes