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Tutorial 3Refractor assignment,Analysis, Modeling and Statics
Refractor assignment, analysis
and modeling are all accessed under the Model
menu.
Refractors are assigned under Branch Assignment
This is a base map of your bin fold coverage
If you want to show sources and detectors on the map, check these then “Reload plot.”
Click on base map to see picks centered on the closest bin
Here are the picks centered on the base map positionplotted by offset vs time
Currently we are assigning picks to the first refractor
To set refractor branch information drag a best-fit line over the range of picks
you want to assign to the first refractor
By dragging the line you have done 3 things:1. You have set a first refractor offset range2. You have estimated a refractor velocity3. You have estimated a delay time
Refractor offset distance is shown by the red zone
The slope of the line estimates the
refractor velocity
The zero offset intercept estimates 2x
the delay time
These values are provided here and
can be edited
Another way to use the Branch Assignment window
Click on “Apply LMO correction to picks
Now move the slider
Drag the line again to specify the offset range for
First refractor
As before the offsets, velocity and delay time
appear here and are editable
The base map provides a map of the
branch parameters
To accept all these branch assignment parameters, you
must push “OK – apply changes” button
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.
If you said “Yes” to interpolating the delay times and velocities to the source and detector tables, you will next
see this “process” window.
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
From a conventional pick window …
Under Options/Display under “Background color options,”Click on “Use the branch numbers (if assigned)”
Push the “M” toggle button
Push “T” toggle button to limit the time windowEliminate the traces that don’t belong to the refractor – Push
‘X’ toggle button
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
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
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.
Let’s see how the delay-time and velocity solution we picked in branch assignment looks in another window.
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
As with the source record display, flattened traces imply a good solution.
Here is the common-offset window with the branch-derived velocity and delay times applied.
In general, refracted arrivals along this inline
and crossline line up pretty well on zero.
This cross line shows significant residual shape.
Compute conventional refractor velocities and delay times by going to Model/RVC delay time/velocity
computation sequence
Click on RVC for a conventional least-squares solution
This runs your data through a standard sequence of steps shown here
Analysis QC
At this point 1. You have picked refracted arrivals2. You have assigned your picks to refractors3. You have computed refractor velocities and
delay times4. 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
At this point in the tutorial you will examine your velocity and delay
time fields
Click on Model/3D (and 2D) model building window
In this window, the surface elevations, weathering velocity and weathering thickness
are accessed through “weathering layer”
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
This window can be used to construct simple refractor-based earth models.
In this case, we will use the default constant weathering
velocity of 2000.
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.
Specify the smoothing
radius here.
This now displays the smoothed first refractor elevation.
The weathering velocity is no longer constant 2000.
To compute statics, click here
If you change your mind, you can undo the modification here.
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.
Statics in Seismic Studio
Surface
Refractor
Intermediate Datum
Final Datum
Weathering velocity … set in model building, typically varies spatially
Refractor velocity …varies spatially
Replacement velocity … constant, user-specified
For this model, at any station location,the static will be the sum of 3 times.
Statics in Seismic Studio
Surface
For this model, at any station location,the static will be the sum of 3 times.
T1
T2
T3
T1 = layer-thickness / weathering-velocity
T2 = refractor-to-intermediate datum thickness / refractor-velocity
T3 = intermediate-to-final datum thickness / replacement-velocity
Statics in Seismic Studio
Surface
If no intermediate datum is requested, for example, then the static would be the sum of two times
T1
T2
T1 = layer-thickness / weathering-velocity
T2 = refractor-to-final datum thickness / replacement-velocity
As mentioned above, both the intermediate datum and final datum
are optional
Accessing the Statics Wizard
Each of Seismic Studio’s model building windows has a “Compute statics” button.
If you want either an intermediate datum or a final
datum, check them here.
Click “Next >>”
This is the first page of the Statics Wizard
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 >>”
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 >>”
If your data have uphole information associated, then this page provides several options.
Otherwise, you can ignore this page.
Click “Finish”
In the “3D (and 2D) model building window,” click “Plot statics” to see the statics you just
computed.
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
To Stack traces in Seismic Studio click on “Stacks”
Slice Stacking in Seimic Studio Will be presented in a special tutorial
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
Click on Export/Export source/detector tables
This window is actually a general purpose database exporting facility
We will create a format called “demo statics”
First, we will define which source parameters we want to output with the statics
Add whatever identifiers you want
Don’t forget the statics!
If the source parameters are completed, do similar for the detector
parameters
Once we have defined the formats, we must name the output files for each table
Type in a file name that makes sense
Push “Save”Don’t forget to check here
Do the same for the detector statics file
Push “OK” to create the files
Sample source statics page
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