Beyond conventional migration-Nonlinear subsalt imaging with transmissions and twosided illumination

Conventional (linear) migration algorithms use only a small portion of recorded seismic data (primary reflections) because they rely on single-scattering assumptions. Nonlinear imaging methods also use reflected multiply-scattered waves, benefiting from their additional illumination and sensitivity to the model. Primary and multiple reflections are, however, just part of the energy generated during a seismic experiment-transmitted waves are also generated but are usually not recorded by one-sided (surface seismic) acquisition systems. In theory only two-sided illumination of the imaging target would allow this energy to be recorded and used in migration. Here we use a synthetic example of subsalt imaging to show the nature of improvements to the seismic image (and extended image) from the use of multiples and transmitted waves. We then suggest a practical approach to construct the additional fields required by nonlinear two-sided imaging without the need of a velocity model with sharp contrasts and receivers (and/or sources) in the subsurface.