Seismic data recorded with a different frequency ranges depending on the objective. For shallow hazard seismic surveys, we usually requires a high resolution seismic to delineate the subtle features of the near surface. For that reason, we acquire higher frequencies. Whereas in case of the exploration seismic, we require both low and high frequencies. We as processing geophysicists, try to preserve the amplitudes and frequencies through the processing sequence. At the beginning of the parameter testing, we should determine the low frequency filter by testing.

Create a workflow 0040_Low_frequency_filter:

1. Read seismic traces

2. Sort traces

3. Seismic loop

4. Bandpass filter

5. Save seismic by gather

6. Read seismic traces

7. Stack Imaging before and after band pass filter

1) Read seismic traces loads seismic trace from the previous step, input file name is 0001_Geometry. Execute the module by double click on it or press on run button from the upper menu.

2) Sort traces Here we need to sort seismic traces for Seismic loop. Add Sort traces module and set FFID header for Trace Grouping and CHANNEL as Trace Sorting as it is shown below

3) Seismic loop Connect trace headers vector (Input sorted headers) from the Sort traces module output and seismic (Input SEG-Y data handle) from Read seismic traces. We should pay attention to settings tab of Seismic loop. Any module placed/inserted inside the Seismic loop module automatically connects to previous module output. In case we want to connect to a particular input/output then we should go to the settings tab of Seismic loop and uncheck Auto connection. By default it is checked:

4) Band pass filter We need to add several band pass filters to test the optimum low frequency filter. Also, we should go to the settings tab and uncheck Auto connection so that the we make the reference/connection as we want (not auto connection, but user-defined one). In this example, Seismic loop - Gather as input to the Band pass filter as reference/connection. The same goes to other band pass filter modules within Seismic loop module.

Provide the band pass filter parameters and display the results. In the below image, we display the low frequency results of 0-3, 0-4 and 0-5 Hz for a single shot gather along with input shot gather:

INPUT GATHER:

BAND-PASS 0-3 Hz:

BAND-PASS 0-4 Hz:

BAND-PASS 0-5 Hz:

5) Save seismic by gather is used to save the seismic data in g-Platform’s internal format with .gsd extension. This module should be used within (inside) the Seismic Loop. Select the optimum low pass filter from the test results (for example, 0-3 Hz). Disable the remaining band pass filter modules or delete them. Write a name for an output seismic data set 0020_BandPass and execute the Seismic loop on the entire data set by using launch all modules button.  Make sure to  Turn off all difference calculations prior to launch  low frequency filter workflow for the entire seismic data. 

QC step (stack comparison):

Create a stack before and after low pass filter, to compare the stack response with and without low pass filter:

6) Read seismic traces Loads seismic trace from the previous step, input file name is 0020_BandPass to create the stack. Also we can load 0001_Geometry data set and add Band pass filter module inside the sub-sequence -before NMO of Stack Imaging module. Execute the module by double click on it or press on run button from the upper menu.

7) Stack imaging - before/after low frequency filter. This module is complex interactive application for velocity analysis, creating mute function, stacking CMP gathers, but we don't need to use all these option in this chapter. This step is only for QC, so we just need to build two stacks and compare them: before and after spherical divergence correction. Add Stack imaging module and write a comment: before low pass filter and add the second Stack imaging with comment: after low pass filter:

Pay attention on what is inside Stack imaging module, it is sub-sequence. Sub-sequence helps to avoid creating a separate workflow to apply an AGC or Deconvolution or other processing modules, we just simply insert those modules in the sub-sequence inside the main module. Open sub-sequence flow in Stack imaging module:

There are four sub-sequences inside Stack imaging:

•Sub-sequence - For VA: processing for velocity analysis, i.e. apply some procedures like band-pass or AGC, and then velocity spectrum is calculated;

•Sub-sequence - Before NMO: processing before applying NMO corrections to gathers, for example we can apply static corrections;

•Sub-sequence - After NMO: processing before applying NMO corrections to gathers, for example we can apply denoise procedures;

•Sub-sequence - After Stack: processing after stacking CMP gathers, for example we can apply denoise procedures or spectrum balancing.

We can either Import stacking velocity (we don't have it yet) for creating a stack or pick the velocities (make a simple single velocity law). Use action menu of Stack imaging module to import the already picked velocities by selecting the option Load velocity picking:

Define all necessary input data items:

Right click on Stack Imaging, select Vista Groups followed by All Groups. Select 2D Groups. Go to the View Manager and remove unnecessary windows.

Change windows location, refresh them by clicking on Adjust to size button. Select some CMP point randomly on the map by clicking on points. Make a single simple velocity law:

Do not focus on velocity spectrum and picking in this chapter, because we will go deeply into velocity analysis later. For now we just need to build a stack. First, create a mute function using interactive Current NMO seismogram window. To pick mute on Current NMO seismogram, we should click on the gather and a white dot with a brown color will appear. If you click on the View Properties, you can change the properties of the mute curve. Similarly, we can save these mute picks by simply selecting the Save mute picking option in the action menu as shown in the below image:

Pay attention that stack won't be on a constant datum. Try to compare two results: datum is on topography and datum is on constant/final. For shifting seismic data from topography on constant datum you should change the following parameters:

Execute Stack imaging and check result. Switch off unnecessary layers (Vista Items), this is a stack before low pass filter:

Next, we need to create the second stack after low pass filter. Define input data items, load the same velocity and mute files, add low pass filter module into sub-sequence and execute Stack imaging. Open two stack windows: before and after low pass filter:

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