Simultaneous inversion
In simultaneous prestack inversion, multiple partial-offset or angle sub-stacks are inverted simultaneously. For each angle stack, a unique wavelet is estimated. Subsurface low-frequency models for P-impedance, S-impedance and density constrained with appropriate horizons in the broad zone of interest, are constructed using the approach explained above. The models, wavelets and partial stacks were used as input in the inversion, and the output was P-impedance, S-impedance and density. The general workflow of simultaneous inversion is shown below.
General workflow for simultaneous inversion.
(a) Wavelets extracted the near-, mid-, and far-angle stacks; (b) frequency spectra for the wavelets extracted in (a). Notice the gradual reduction in the frequency content in going from the near- to the far-angle stack wavelets, as well as the roll-offs on the higher frequency side. (Adapted from Chopra et al., 2017)
The variation of frequency content with offset is expected and can be noticed above on angle dependent wavelets extracted from partial angle stacks. This lateral deficiency in frequency deteriorates the results in terms of resolution of seismic data and the attributes derived therefrom. While shale formations may be thick, some high TOC shale units may be thin. It is therefore desirable to recover the attenuated frequencies of seismic data before putting them through the impedance inversion process for a better delineation of a reservoir. At SamiGeo, we prefer a workflow in which we first balance the frequency content of near-angle stack and then balance the mid- and far- angle stacks with respect to it in an amplitude-friendly way as shown below.
Preferred workflow for enhancing the resolution of prestack impedance inversion attributes.
QC of spectral balancing
Six partial angle stacks in the form of gathers before and after spectral balancing are shown on the first two tracks, respectively, for all the figures. Their AVA responses before and after the spectral balancing are also shown next to them. Notice that while there is small change in the amplitude of the events after the spectral balancing, the relative amplitude variation with angle is very similar which suggests that the spectral balancing has been done in an AVO friendly way.
Improvement in inversion
Angle dependent wavelets (a) before and (b) after spectrally balancing angle stacks. An enhancement in the frequency content of the data is noticed after spectral balancing.
Segments of S-impedance sections generated using angle gathers (a) before, and (b) after spectral balancing analysis. The impedance logs at the locations of the wells are shown overlaid as a curve, as a color strip. While comparison is self-explanatory, the improvement in the inversion after following the balanced gathers in the inversion is highlighted with ellipses and rectangle.
References
Chopra, S., R. K. Sharma, G. K. Grech, and B. E. Kjølhamar, 2017, Characterization of shallow high-amplitude seismic anomalies in the Hoop Fault Complex, Barents Sea, Interpretation, Vol. 5, No. 4 (November 2017); p. T607–T622.