Constrained inversion to absolute acoustic impedance:

• Synthetic zero offset traces are generated wherever sonic and density logs are available.
• The seismic wavelet near wells is estimated from multiple cross correlations between local seismic and synthetic traces.
• Seismic data can be inverted to absolute acoustic impedance by constraining the latter using low frequency impedance trends extrapolated along structure from the nearest well.
• Well data is not used directly in the inversion.

The work is done under contract via a specialist contractor such as ØDEGAARD (UK) Ltd. using the simulated annealing inversion algorithm within their ISIS package.

 

Inversion to relative acoustic impedance (no well constraint):

• The seismic wavelet is estimated from the seismic data on the assumption that the data is minimum phase and the earth’s reflectivity amplitude spectrum is white.
• The phase spectrum is modified to be approximately zero phase such that the resulting data approximates the band-limited reflectivity of the earth. The relative (unconstrained) impedance is then estimated from this by one of two techniques:
  
   “Seismic colored inversion” (SCI):
  • This technique modifies the slope of the amplitude spectrum of the zero phase corrected seismic data to  approximate that expected of the corresponding acoustic impedance profile.
  • The phase spectrum of the data is rotated by 90 degrees to reproduce the phase difference that exists between impedance and reflectivity.
  • The sub-seismic frequency component of the impedance function is not recoverable and the absolute level of impedance is not determined.
  • Wells typical of the geological basin in which the data was shot must be available in order to define the desired slope of the spectrum of the relative impedance.
   Such work can be undertaken either by TGS Imaging or under contract by ARK-CLS Ltd. using software developed by the latter for BP.
  
  Constrained sparse spike inversion:
  • A linear programming inversion algorithm is used with sub-seismic frequency constraints on the solution imposed from seismic interval velocities.
  • The broadband impedance estimate can be calibrated if the impedance of any prominent interval is known.