Tutorial 3 Refractor assignment, Analysis, Modeling and Statics

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Tutorial 3Refractor assignment,Analysis,
Modeling and Statics
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Refractor assignment, analysis and modeling are
all accessed under the Model menu.
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Refractors are assigned under Branch Assignment
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If you want to show sources and detectors on the
map, check these then Reload plot.
This is a base map of your bin fold coverage
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Click on base map to see picks centered on the
closest bin
Here are the picks centered on the base map
position plotted by offset vs time
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Currently we are assigning picks to the first
refractor

• By dragging the line you have done 3 things
• You have set a first refractor offset range
• You have estimated a refractor velocity
• You have estimated a delay time

To set refractor branch information drag a
best-fit line over the range of picks you want to
assign to the first refractor
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These values are provided here and can be edited
The zero offset intercept estimates 2x the delay
time
The slope of the line estimates the refractor
velocity
Refractor offset distance is shown by the red zone
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As before the offsets, velocity and delay time
appear here and are editable
Click on Apply LMO correction to picks
Now move the slider
Drag the line again to specify the offset range
for First refractor
Another way to use the Branch Assignment window
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The base map provides a map of the branch
parameters
To accept all these branch assignment parameters,
you must push OK apply changes button
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After accepting your new branch assignment field,
you will be asked if you want to interpolate the
delay times and velocities to the source and
detectors
If you push Yes, this will initialize the
source and detector databases with these delay
times and refractor velocities. If you already
have a refractor solution from previous session,
these new values will replace the old solution.
You will have one more chance to change your mind.
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If you said Yes to interpolating the delay
times and velocities to the source and detector
tables, you will next see this process window.
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Applying analyses to traces

• Next, you will see how to apply a delay-time and
  refractor-velocity solution to the traces
• The following two slides show the program options
  that you can use to help QC delay time and
  refractor velocities

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From a conventional pick window
Push the M toggle button
Under Options/Display under Background color
options, Click on Use the branch numbers (if
assigned)
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Eliminate the traces that dont belong to the
refractor Push X toggle button
Push T toggle button to limit the time window
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Applying the branch assignment derived delay
times and refractor velocities

• Recall that above we assigned branch offsets in
  the Branch Assignment window
• Recall that by assigning branch offsets, we also
  determined a crude delay time field and a
  refractor velocity field
• Now we will apply those fields to our traces,
  using the technique we just described
• Note that we have also turned off the refractor
  background color to simplify the display

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A perfect refraction solution (refractor
velocities and delay times) would flatten the
refractor to zero time.
This shot is pretty good, meaning the refractor
velocity and delay times for this source and its
detectors are probably close to correct
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This source did not respond as well. The simple
delay times interpolated from the branch
assignment are not correct in detail. This does
not mean that this source has a problem. It just
means that the delay time and refractor velocity
field are not accurate for this location. The
flatness should improve when we actually compute
refractor velocities and delay times from the
picks themselves in the next step.
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Lets see how the delay-time and velocity
solution we picked in branch assignment looks in
another window.
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Inline-crossline azimuth-limited common-offset
pick window

• We will look at the solution applied to the
  traces that fall within a narrow offset range and
  a narrow azimuth range
• We will look at these limited traces across an
  entire prospect

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Here is the common-offset window with the
branch-derived velocity and delay times applied.
As with the source record display, flattened
traces imply a good solution.
In general, refracted arrivals along this inline
and crossline line up pretty well on zero.
This cross line shows significant residual shape.
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Click on RVC for a conventional least-squares
solution
Compute conventional refractor velocities and
delay times by going to Model/RVC delay
time/velocity computation sequence
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This runs your data through a standard sequence
of steps shown here
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Analysis QC

• At this point
• You have picked refracted arrivals
• You have assigned your picks to refractors
• You have computed refractor velocities and delay
  times
• You have also estimated source and detector
  geometry errors
• This is automatically performed as part of the
  standard sequence
• It estimates source and detector mispositions

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At this point in the tutorial you will examine
your velocity and delay time fields
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Click on Model/3D (and 2D) model building window
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In this window, the surface elevations,
weathering velocity and weathering thickness are
accessed through weathering layer
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Refractor delay times, refractor velocities and
elevation of the refractors are accessed via
First refractor, Second refractor, etc.
Note Some versions of Seismic Studio require
you to click on Weathering layer definition
before you can examine refractor parameters
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This window can be used to construct simple
refractor-based earth models.
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In this case, we will use the default constant
weathering velocity of 2000.
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The result is this First refractor elevation
surface.
To smooth the refractor elevations (and cause the
weathering velocity to be modified) click on
Modify attribute Note Modify attribute will
modify the attribute that is currently being
displayed.
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Specify the smoothing radius here.
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To compute statics, click here
The weathering velocity is no longer constant
2000.
This now displays the smoothed first refractor
elevation.
If you change your mind, you can undo the
modification here.
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Statics in Seismic Studio

• Seismic Studio computes an individual static
  value at each source and detector location.
• Statics are calculated as the sum of vertical
  times through each model layer, then to an
  intermediate datum, then to a final datum.
• Both the intermediate datum and final datum are
  optional.

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Statics in Seismic Studio
Surface
Weathering velocity set in model building,
typically varies spatially
Refractor
Refractor velocity varies spatially
Intermediate Datum
Replacement velocity constant, user-specified
Final Datum
For this model, at any station location, the
static will be the sum of 3 times.
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Statics in Seismic Studio
Surface
T1
T1 layer-thickness / weathering-velocity
T2 refractor-to-intermediate datum thickness /
refractor-velocity
T2
T3 intermediate-to-final datum thickness /
replacement-velocity
T3
For this model, at any station location, the
static will be the sum of 3 times.
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Statics in Seismic Studio
Surface
As mentioned above, both the intermediate datum
and final datum are optional
T1
T1 layer-thickness / weathering-velocity
T2 refractor-to-final datum thickness /
replacement-velocity
T2
If no intermediate datum is requested, for
example, then the static would be the sum of two
times
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Accessing the Statics Wizard
Each of Seismic Studios model building windows
has a Compute statics button.
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This is the first page of the Statics Wizard
If you want either an intermediate datum or a
final datum, check them here.
Click Next gtgt
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If you requested an intermediate datum, you
design it here
The wizard shows you some model statistics to
help you
For this model, we choose an flat intermediate
datum of -100 to be just beneath the refractor
Click Next gtgt
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If you requested an final datum, you design it
here
Again, the wizard shows you some model statistics
to help you
Final datum elevation and replacement velocity
are often specified by the project client
Click Next gtgt
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If your data have uphole information associated,
then this page provides several
options. Otherwise, you can ignore this page.
Click Finish
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In the 3D (and 2D) model building window, click
Plot statics to see the statics you just
computed.
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What to do with the statics

• You can see some stacks of the traces with
  statics applied
• You can export the statics for use by other
  processing systems

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To Stack traces in Seismic Studio click on
Stacks
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Slice Stacking in Seimic Studio

• Will be presented in a special tutorial

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Exporting statics

• Statics are computed for each source and detector
  in the survey
• There are several options for exporting the
  statics for use by processing systems
• This tutorial will show you one option
• Export source/detector tables

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Click on Export/Export source/detector tables
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We will create a format called demo statics
This window is actually a general purpose
database exporting facility
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First, we will define which source parameters we
want to output with the statics
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Add whatever identifiers you want
Dont forget the statics!
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If the source parameters are completed, do
similar for the detector parameters
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Once we have defined the formats, we must name
the output files for each table
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Type in a file name that makes sense
Push Save
Dont forget to check here
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Do the same for the detector statics file
Push OK to create the files
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Sample source statics page
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Conclusions

• This tutorial shows you a standard
  analysis/modeling path through Seismic Studio
• On simple data, this may be an adequate template
• For more difficult data, more advanced procedures
  may be required
• Advanced procedures can be learned via a Renegade
  training class