Thursday, 8 September 2016

Effect of Post-Stack Noise Attenuation in Acoustic Inversion

It's well known that there are many methods of data conditioning that improve the signal to noise ratio. In this case study the post stack noise attenuation process applied to the data was carefully performed to preserve the signal associated with geological features within the Pre-Salt context of the Santos Basin. The objective of the noise attenuation is to improve the input volume to the inversion process. 
Figure 1 - common noise in the pre-salt context

Mothodology - Noise Attenuation
The workflow to reduce the noise is out lined below and illustrated in Figure 2:

  1. Determine the grid oriented filter size, a large filter in the X&Y directions and small in Z, will reduce the continuity of the vertical noise associated with the salt/migration smiles. In order to create an input for the Steering Volumes, the Smoothing Attribute is calculated. Recommended filter size for this case: 9x9x5.
  2. The Steering Volumes, of Dip and Azimuth are calculated from the Smoothing Attribute, this becomes the reference volume.
  3. Finally the SO Noise filter which is an structurally oriented mean noise filter, is applied to the original seismic volume, which is guided by the reference volume.
Figure 2 – Noise Attenuation Workflow for Pre-Salt

Methodology – Acoustic Inversion
In geosciences, through reflection seismic data, the seismic inversion process estimates the quantitative rock-petrophysical properties such as porosity, lithology and fluid saturation of a reservoir. This process, in a simply way, is based on a straightforward convolution of a reflectivity model and an estimated wavelet to produce synthetic traces, which are compared to the seismic input. The reflectivity model is updated to ensure there is a high correlation between the synthetic and seismic data (figure 3).
Figure 3 – Deterministic Inversion workflow
Noise Attenuation

As can be seen in the figure below (figure 4), the SO Noise filter has removed a lot of the migration smiles associated with pre-salt seismic data. The input data is on the left and the noise attenuated data is on the right. The reflectors are more continuous and the faults are still visible.
Figure 4 – Original Seismic (Left) x Noise attenuated seismic (right)

Acoustic Inversion
In this case study, both the input data and the noise attenuated data were inverted.  On the left is the inversion results using the original seismic data and on the right the inversion results using the noise attenuated data.  The inversion results using the noise attenuated as an input contains more continuous layers, which is more consistent with what well data would contain.  This result can be correlated to petrophysical properties and the reservoir mapper more easily as compared to the result on the left.
Figure 5 – Acoustic inversion over raw seismic (left) x Acoustic inversion over noise attenuated seismic (Right). the colors nearer to yellow have low impedance and nearer to blue are the high ones.
By applying a noise attenuation filter to noisy pre-salt data, it allows for better interpretation of events on the output data, but also provides a better input into other attributes, such as an inversion. 

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