Seismic inversion can be classified as
- Methods based on iterative forward modeling
- Methods based on downward wave field extrapolation
Inversion by iterative forward modeling:
- Essentially in this method, elastic properties of the geological subsurface are estimated.
- Given the seismic source and an initial subsurface model, a seismic data is simulated. The simulated data is then compared to the actual measured data and using the residual, the subsurface model is refined iteratively until the residual reaches a minimum.
Benefit: Really accurate
Drawback: Computationally very expensive
Inversion by downward wave field extrapolation:
- It is the most commonly used method in the industry (migration). Here, the seismic waves (measured reflection data and the assumed/measured source) are downward extrapolated at every depth point. Thereafter, at every depth location a reflectivity is derived using an imaging condition.
- In this method, reflectivity of the geological boundaries is estimated by extrapolating the source and reflected wave fields into the subsurface using pre-specified velocity model.
- In the standard practice of migration, forward modelling of reflections is not done, neither is a residual calculated.
Benefit: Efficient compared to previous method
Drawback: The theoretical foundation is not reliable