Robust Marchenko Focusing - Calibrating Surface Reflection with VSP Data

Marchenko focusing utilizes only reflection measurements at the Earth’s surface and an estimate of the direct wave from a focusing point in the subsurface to the acquisition level in order to retrieve the Green’s function between the two. However, the method suffers when an erroneous scaling of the amplitudes and/or a constant phase-shift is introduced to the reflection response, and artifacts caused by internal multiples are not fully suppressed in the retrieved subsurface wave fields. We propose a workflow to calibrate the reflection response, prior to Green’s function retrieval via Marchenko focusing, using additional information in the form of a VSP dataset. First a virtual VSP dataset is estimated via Marchenko focusing, to subsequently compare its upgoing component to the upgoing part of the recorded VSP. Thereby, making it possible to correct for a constant phase shift applied to the reflection data. By identifying the minimum residual energy between the virtual VSP and the recorded VSP wave fields, an erroneous scaling of the reflection response can also be corrected. This workflow leads to a more robust Marchenko focusing, where the reflection response can be redatumed to a target zone in the subsurface and used for ghost-free imaging.