Modified Reverse-Time Migration Applied To Imaging Of Offset Vsp Data With Unknown Overburden In Complex Areas

In this paper we propose a modified Reverse-Time Migration algorithm applied to OVSP data which gives a possibility to reconstruct depth structure image having no knowledge about overburden by using original registered seismograms without their redatuming. Velocity model estimated in some depth interval in vicinity of borehole can be used for migration. Its main feature is approximation of source wavefield in the target area using downgoing wavefield registered in a borehole. To do it one needs to perform back-in-time extrapolation of downgoing wavefield using finite-difference (FD) scheme for solving the scalar wave-equation (or using FD for elasticity). It is possible to migrate PP-reflections or transmitted PS-converted waves for formation boundaries or salt flank imaging. Our numerous experiments with synthetic and field OVSP data demonstrate that modified RTM provides a good quality image in complex geological conditions.