Survey Planning

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Survey Planning Illumination with NORSAR-3D
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Overview

• This short presentation gives some applications
  of NORSAR-3D ray tracing.
• Survey aperture
• Fold and Amplitude, Planned v Modelled
• Survey offset and azimuth
• Migration aperture
• Effect of Overburden
• S.R.M.E. Aperture

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Simple Slope Model
A simple model was used to demonstrate the
technique. The model contains a variable velocity
field overlying a reflector with a cosine dip
region in the center.
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Aperture Study, All Azimuth Survey
The survey used here is a Wide Azimuth survey.
In the illustration, the closely spaced shot
lines are shown, together with the receiver
boundary square for the shot location marked with
the cross. The receiver pattern moves as the shot
location moves.
Shot Pattern
Receiver boundary for indicated shot
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Aperture Study Area of Interest
Having ray traced the survey, the Hit Count
attribute is plotted in the reflection point
domain. This shows where the reflection points
are and the density. The display shows the
attribute after masking it to a 4km x 2km area of
interest.
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Aperture Study Shots Contributing
Down dip extension
An alternative domain for display is the shot
domain. This display shows the shots that
contributed only to the area of interest on the
target. Clearly, the survey needs to be extended
in the down-dip direction.
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Overview

• This short presentation gives some applications
  of NORSAR-3D ray tracing.
• Survey aperture
• Fold and Amplitude, Planned v Modelled
• Survey offset and azimuth
• Migration aperture
• Effect of Overburden
• S.R.M.E. Aperture

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Reflection Points, 0 deg Dip
This diagram relates the reflection point
positions to the streamer locations assuming a
flat, horizontally layered Earth. The reflection
points and shot locations for two shots on
adjacent shot lines are shown, but only the
streamers for the left shot are drawn for
clarity. The reflection point positions for the
outer streamers are identical to the CMP
positions.
Outer Reflection points
Streamers associated with left shot.
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Reflection points, 10 deg dip south
This diagram is equivalent to the previous
diagram except that a 10 dip to the south has
been included in the reflector. This asymmetry
skews the reflection points away from the CMP
points as depicted.
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Hit Map, E to W Direction
Hit map for E-W The central part of the map, the
attribute is striped. This is caused by the dip
of the reflector.
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Hit Map, Alternating Directions
This map was made using the same model and survey
parameters as the previous map except that the
sail line directions alternate. This alters the
hit count pattern
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SMA 20Hz, E to W Directions
Would this appear in migrated seismic data? Plot
the migration amplitude using SMA The stripes
persist.
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SMA 20Hz, Alternating Directions
This shows the Simulated Migration Amplitude
llumination for the survey shot in alternating
directions.
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Overview

• This short presentation gives some applications
  of NORSAR-3D ray tracing.
• Survey aperture
• Fold and Amplitude, Planned v Modelled
• Survey offset and azimuth
• Migration aperture
• Effect of Overburden
• S.R.M.E. Aperture

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Illumination in SEG Salt Model
The SEG/EAGE salt model was used to illustrate
this topic. The salt model consists of a salt
body in a sedimentary velocity field. A plane,
horizontal target has been added.
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Hit Map, E-W Survey, 6km Streamer
This is the hit count attribute plotted for an
east-west survey using a 6km streamer array.
Away from the salt, the reflection count is
continuous. Underneath the salt, the reflection
count varies because of the focusing effects of
the salt. Under the southwest corner of the salt
there is an illumination hole.
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Hit Map, N-S Survey, 6km Streamer
This is the hit count attribute plotted for a
north-south survey using a 6km streamer array.
Under the southwest corner of the salt, the
illumination hole persists but the details are
different. If one had a particular prospect
location in mind, a choice might be made between
the east-west and north-south surveys.
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Hit Map, N-S Survey, 10km Streamer
This is the hit count attribute plotted for a
north-south survey using a 10km streamer array.
There is very little difference between this map
and the one made with the 6km streamer array.
The 10km streamer is probably not worthwhile.
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Maximum Offset, 10km Streamer
For each image point, this attribute shows the
maximum offset that contributed to that image
point. It confirms that for most of the target,
7km streamers are enough. Only in a few places
below the salt would a longer streamer contribute
to the image.
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Shooting from target

• In the previous SEG/EAGE salt model examples,
  non of the acquisition geometries filled the
  illumination hole.
• An efficient alternative analysis for these
  local trouble spots is the flower plot display.
• A shot is placed in the illumination hole at the
  target. One-way rays are propagated from the shot
  up to an array of receivers on the surface.

Dense set of receivers.
Shot point placed in the shadow zone
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Shooting from target Flower Plot

• Rays with equal departure inclination and
  opposite azimuth are paired and plotted
• The figure shows that the east-west and
  north-south azimuths would not produce any ray
  pairs. However, the northwest-southeast direction
  produces ray pairs at long offsets.

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Hit map for the new survey

• Repeating the streamer survey using a northwest
  southeast sailing direction does indeed fill the
  illumination hole.

N125
Complete illumination
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Overview

• This short presentation gives some applications
  of NORSAR-3D ray tracing.
• Survey aperture
• Fold and Amplitude, Planned v Modelled
• Survey offset and azimuth
• Migration aperture
• Effect of Overburden
• S.R.M.E. Aperture

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Max. CMP-CRP, 10km Streamer
Again using the SEG/EAGE salt model, the CMP-CRP
distance attribute is plotted in the reflection
point domain. Away from the salt the CMP-CRP
distance is effectively zero, indicating that
small apertures are required for imaging
reflections. However, significant apertures are
needed under the salt.
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Overview

• This short presentation gives some applications
  of NORSAR-3D ray tracing.
• Survey aperture
• Fold and Amplitude, Planned v Modelled
• Survey offset and azimuth
• Migration aperture
• Effect of Overburden
• S.R.M.E. Aperture

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Total Illumination Amplitude
The illumination map here was made using the
SEG/EAGE salt model and a streamer survey. All
rays that reflected from the target were used, no
matter what part of the model they had traveled
through.
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Illumination Amplitude, No Salt
This illumination map was made in the same way as
the previous one, except that any ray that had
passed through the salt was rejected. It shows
the image that requires only sedimentary ray
paths, and can be called the high confidence
image, because it is not subject to uncertainties
in the salt model.
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Overview

• This short presentation gives some applications
  of NORSAR-3D ray tracing.
• Survey aperture
• Fold and Amplitude, Planned v Modelled
• Survey offset and azimuth
• Migration aperture
• Effect of Overburden
• S.R.M.E. Aperture

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Surface Bounce Points
Bounce
Bounce
The SRME method estimates the multiple by using
the surface bounce. The 2D SRME method can be
used when the surface bounce occurs within the
streamer array. Ray tracing can be used to see if
this is a valid assumption.
Surface Bounce Points
Streamer Array
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Salt Top Multiple
If the surfaces generating the multiple are
approximately horizontal, then the bounce points
will occur within the streamer array. This
extreme example illustrates ray paths for the top
salt multiple from a single shot.
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Salt Multiple, Surface Bounce Points
The surface bounce points for the top salt
multiple from the single shot are depicted in
red. Clearly the bounce points are not contained
within the streamer array, indicating that a 3D
SRME technique would be necessary.
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NORSAR-3D Work Flow
Surfaces Properties
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Model Components

• Generated internally
• Imported surfaces
• Depth/time grids
• GoCAD trimeshes
• GoCAD model
• Imported properties
• SEGY property field

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NORSAR-3D Work Flow
Surfaces Properties
Survey
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Surveys

• Simple Surveys Internally
• Marine streamer
• Ocean Bottom Sensor
• Complex surveys import
• P1/90
• SPS
• ASCII (for VSP)

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NORSAR-3D Work Flow
Surfaces Properties
Survey
Ray Code
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3D Wavefront Construction
Direct wavefront
Shot point and streamer array.
Salt
Reflected wavefront returning to surface.
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3D Wavefront Construction
This shows the result of the ray traced shot
represented as the more conventional ray diagram.
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NORSAR-3D Work Flow
Surfaces Properties
Survey
Ray Code
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Summary

• A variety of import formats for NORSAR-3D models
  and surveys.
• Wavefront Construction is fast and robust
• Variety of survey analysis tools in NORSAR-3D

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Ask for a demonstration to see how NORSAR-3D can
benefit your project
Insight through modelling