Estimation of layer thickness and velocity changes using 4D prestack seismic data

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Estimation of layer thickness and velocity
changes using 4D prestack seismic data
by THOMAS RØSTE Trondheim, 24 November 2005
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Objective

• Introduce a new method which estimates changes in
  thickness and velocity for a sequence of layers
• Test the method on data from a synthetic model

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The synthetic model
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The compacted reservoir section
(Layer 4)
Res
(Layer 5)
Max compaction 10 m
Top Reservoir section after compaction
(red) Bottom Reservoir thickness changes
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Example of event picking in a prestack gather
OFFSET (m)
Top Reservoir horizon
TWT (ms)
Base Reservoir horizon
Top and bottom mutes (above and below the green
lines) are applied prior to traveltime picking
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Interpolated traveltimes for max amplitude
versus offset
OFFSET (m)
TWT (ms)
picked
Top Prestack gather after mute Bottom Picked
traveltimes versus offset
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• Finding traveltime for zero offset (T0)
• The picked TWT as a function of offset, T(X),
• can be utilized to find the traveltime for zero
  offset
• Assuming

where
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Estimated ?T0/T0 for Top Reservoir horizon
Red dots Initial estimation. Blue crosses
Smoothed. Dashed line Modeled values.
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Methodology

• Assume - Prestack-gather at position x0
• - Laterally varying velocity field,
  v(x),
• but non-varying with depth within the layer

- TWT for a ray-path with offset 2h can then be
expressed by
where S Total ray-path length
v(s) Velocity as a function of ray position
s(x0,h)
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- a is determined by minimizing the least square
error in relative time shifts (eq ())
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• When best fitted a-value is found
• The changes in layer thickness and velocity are
  given by

where I have assumed that ?T0/T0 are only
influenced by velocity and thickness changes
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Example of finding the best fitted a (for one
position) Sequence SeaSurf TopRes
Best fit a - 0.6
Bad picks (red dots) automaticly excluded prior
to best a-fit
Red x The picked relative time shifts, ?T/T.
Dashed Initial and last guess of a. Blue solid
Best fitted a (from eq ()).
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Results for all positions for sequence SeaSurf
TopRes
Estimated changes in thickness (top) and velocity
(bottom) Dashed line Modeled values
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Discussions

• Uncertainties
• The method depends on undisturbed events
• Assume straight ray-path lines
• Anisotropy
• Time lapse anisotropy is not incorporated in the
  methodology (Must be included if larger then 20
  of maximum possible)

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Conclusions

• Prestack time shift analysis offers a method to
  discriminate between layer thickness and velocity
  changes
• The method shows good results when tested on data
  from synthetic models

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Subsidence map of Water Bottom between -92 -02
(bathymetri)
Subsidence close to well A30B approx 0.5m (max)