Tutorial 3 Refractor 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.” This is a base map of your bin fold coverage

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

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

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

Another way to use the Branch Assignment window 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

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 … Push the “M” toggle button Under Options/Display under “Background color options,” Click on “Use the branch numbers (if assigned)”

Eliminate the traces that don’t belong to the refractor – Push ‘X’ toggle button Push “T” toggle button to limit the time window

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 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

This cross line shows significant residual shape. 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.

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

This runs your data through a standard sequence of steps shown here

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

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.

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.

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 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.

Statics in Seismic Studio Surface T1 T1 = layer-thickness / weathering-velocity T2 T2 = refractor-to-intermediate datum thickness / refractor-velocity T3 T3 = intermediate-to-final datum thickness / replacement-velocity For this model, at any station location, the static will be the sum of 3 times.

Statics in Seismic Studio Surface As mentioned above, both the intermediate datum and final datum are optional T1 T1 = layer-thickness / weathering-velocity T2 T2 = refractor-to-final datum thickness / replacement-velocity If no intermediate datum is requested, for example, then the static would be the sum of two times

Accessing the Statics Wizard Each of Seismic Studio’s model building windows has a “Compute statics” button.

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 >>”

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 >>”

Otherwise, you can ignore this page. 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

We will create a format called “demo statics” This window is actually a general purpose database exporting facility

First, we will define which source parameters we want to output with the statics

Don’t forget 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 Don’t forget to check here Push “Save”

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