Interpreters are much more comfortable interpreting time migrated sections than stacked sections. Time migrated sections give a much clearer structural picture of the subsurface than stacked sections but are inherently mispositioned due to the limitations of time migration.

The parameters that could be observed on the time migrated sections or maps
are not reflecting the geological reality. As a matter of fact, time
migration does not always produce properly migrated time sections. This is
the reason why it is necessary to do (time) *de-migration*. This method
allows us to retrieve all the information needed to perform depth migration.
*De-migration* means cancelling the effect of time migration. The
principle of *de-migration* corresponds to the inverse operation of
time migration.

Figure 1

In figure (1), one can see the time migrated position
*A*_{tm}*B*_{tm} of a dipping reflector. *De-migration* is performed to
retrieve the observed time position *A*_{t}*B*_{t} in the inverse way that was
applied to do time migration.

In terms of ray geometry we can formulate *de-migration* as a
``ray-tracing'' from a time migrated position *A*_{tm} to the position *A _{0}*
on the free surface in order to retrieve the observed time

3-D *de-migration* equations are then expressed as Kirchhoff time
migration inverse formulation:

(1)

11/17/1997