How We Stop Leakage
PGS Ultima is a non-linear inversion process that inverts simultaneously for both velocity (FWI) and reflectivity (LSM). This is achieved with a robust imaging condition that performs the scale separation and hence removes completely the leakage between these two parameters in the multi-parameter inversion.
Internally within FWI and LSM there are several components. The first major process is creating the forward modeled data, the second is to use that to update the chosen parameters. The residual, or difference between the forward modeled data and the acquired seismic data is back-propagated from the receiver positions, whilst a source wavefield is forward propagated from the source position. This is akin to wavefield extrapolation migration (For more information see the RTM page), where the recorded data is back-propagated, not the residual.
Just as with the migration, FWI and LSM use an imaging condition to construct an image where the wavefields interact, which is based on the same one used by PGS RTM (reverse time migration). It separates the kernel into a velocity component for FWI, using both the transmission energy, or banana, and the scattered wavefield, or rabbit ears, and the migration isochrone for the reflectivity component. By performing this scale separation, the simultaneous inversion constructs an accurate background velocity model, whilst also updating the reflectivity. There is no leakage between either component. The schematic below describes the workflow.