Thursday, 11 August 2016

Combining different Noise Cancellation/Spectral Enhancement volumes using Time Variant

The aim of this blog post is to describe how to combine different Noise Cancellation/Spectral Enhancement volumes using a time variant method.
This workflows can be used for:
  1. Creating a combined Noise Cancellation volume in which different noise filter were applied to target different noise contents that vary vertically and can be bounded by a horizon.
  2. Creating a Spectral Enhancement volume with different enhancement parameters that can be divided by a horizon.
The following workflows is for two Noise Cancellation volumes where there is a need to apply a more aggressive noise filter below the horizon and only a gentle filter above the horizon, but the same workflow applies for the Spectral Enhancement volumes too.

Input needed for this workflows are horizons which separate the areas with different noise content vertically, Noise Cancellation volumes, which target the different intervals and a time volume, that is used for the combination. Example in this post will show the workflows to combine 2 areas which is separated by a horizon.
Figure 1: Original Seismic Volume

The first step is to create two Noise cancellation volumes, a gentle noise filter above the horizon and an aggressive filter below the horizon. The next step is to combine these two volumes into one. To combine these two volumes, a time volume is needed, which is unflatten using the horizon and formatted for the combination purpose.

The following is the workflow to create the a formatted Unflatten Time Volume.
  1. Go to Workflows>Processes and Workflows>Processes>Utilities>Time Volume.Input your seismic volume to create the time volume.
    Figure 2: Time volume displayed using the spectrum colour bar, where green is the higher value.  
  2. Use the Horizon Tools to unflatten the Time Volume. Go to Tools> Horizon Tools> Flatten/Unflatten. Toggle Seismic as volume type. Then specify the input horizon, the input volume will be the newly created Time Volume. The output volume will need to be named as Unflatten Time Volume. Change the mode to Un-flatten Horizon and Click Apply. Once you input the horizon, a Set Flattened value, which is the mean of the input horizon, will be displayed at the bottom of the Horizon Tool menu. Write down the value as it will be used to format the Unflatten Time Volume later.
    Figure 3: Unflatten Time Volume   
  3. The next step is to format the Unflatten Time Volume so that one of the Noise Cancellation volumes can be assigned to the data above the horizon and the other Noise Cancellation can be assigned to below the horizon. Go to Workflows>Processes and Workflows>Processes>Volume Math>Parser. Input the Unflatten Time Volume, and use the following parser expression: ((im1>0)*(im1-A)) where A is the “set flattened value to” number that was given when unflattening the time volume. Now you have the formatted Unflatten Time Volume.

    Figure 4: Formatted Unflatten Time Volume displayed as Redwhiteblue colourbar. Red are the negatives values, whites are 0 values and blues are the positive values. It will be observed that along the horizon, values are all 0, this will be used as a transition zone between the 2 volumes.
The final steps now is to combine the two noise cancellation volume using the formatted Unflatten Time Volume. To combine the volumes, go to Workflows>Processes and Workflows>Processes>Volume Math>Parser. The first volume should be the formatted Unflatten Time Volume as im1, Gentle Noise Cancellation as im2 and Aggressive Noise Cancellation as im3. The Parser expression used will be:

((im1<0)*im2) + ((im1>100)*im3) + (((im1>=0) & (im1<=100))*(im1*im3/100)) + (((im1>=0) & (im1<=100))*((100-im1)*im2/100))
As a part of QC, you can compare the Combined Noise Cancellation result with the Aggressive and Gentle Noise Cancellation volume, or look at the Difference volume of each of the Noise Cancellation volumes (Original-Noise Cancellation Volume) to see the related noise in the Combined Noise Cancellation volume.
Combining these two volumes in this way, will avoid abrupt changes along the horizon. Instead there will be a transition zone of 100ms taken from both volumes. This is not limited to only two areas of Noise Cancellation, more volumes can be combined, in a sequence, using the same workflow.This workflows can also be used to combine different level of Spectral Enhancement as long as there are horizons that separate the areas with different Spectral Enhancement.

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