tutorial 3 refractor assignment, analysis, modeling and statics

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