Thursday, 4 June 2015

AVO Screening (Part 4): AVO Product and Reflection coefficient difference

Gradient and Intercept volumes can be generated from the angle stacks as discussed in Part 3. The Gradient and Intercept volumes can be combined in a number of ways to generate different AVO attributes, such as AVO Product or Reflection coefficient difference.

AVO Product

The AVO Product is simply Gradient * Intercept. It has been found to nicely highlight AVO class III responses (Rutherford & Williams, 1989). Soft sands with hydrocarbons will have a strong negative intercept and a strong negative gradient, so they will show a strong positive response in the AVO product. Non-hydrocarbon bearing areas will be weak or have negative products (Avseth et al., 2010)

The AVO product can be calculated in GeoTeric simply using the Parser expression: 

where im1=gradient and im2=intercept. 

The scale factor will need to be adjusted depending on the data type and dynamic range of the input volumes.

AVO Product showing a class III anomaly in the Rosewall-Calliance dataset

Reflection coefficient difference

Castagna and Smith (1994) compared different AVO attributes, and found that the reflection coefficient difference, Rp - Rs, might be a better gas-sand discriminator than the AVO product, because it will work for any type of sand, whether these are AVO class I, II or III. They verified that Rp - Rs can be expressed in terms of intercept and gradient as follows:

Rp - Rs ≈ (R(0) + G)/2

Or in GeoTeric’s Parser language:

where im1=gradient and im2=intercept.

Previous posts in this series can be found here: 
Part 1 AVO Screening :Difference and ratio volumes
Part 2 AVO Screening: AVO colour blends
Part 3 AVO Screening: Gradient and Intercept volumes

Coming soon- part 5 where we will look at AVO Screening: scaled refelctivity

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