Geology Reference
In-Depth Information
travel time to depth. Reflections are interpreted to originate along ray paths that are
normal to the reflector boundaries (Fig. 1.48a). A normal-incidence seismic ray is
deflected up the dip (Fig. 1.48a). The true location of the reflecting point is up the dip
and at a shallower depth than a point directly below the surface recording position.
When the locations of reflecting points are plotted as if they were vertically below the
surface recording stations, there will be a decrease in the calculated depth (or two-way
travel time) relative to the true depth. Two-dimensional time migration is a standard
processing procedure that corrects for the apparent dip of reflectors in the plane of the
seismic line, but does not correct for the shift of the reflector positions in the true dip
direction. Two-dimensional depth migration may give the correct depth to the reflect-
ing point, but still does not correct for the out-of-plane position shift. For example
(Fig. 1.48b), four degrees of oblique dip leads to a 400- to 600-ft shift in the true posi-
tion of the reflection points on a seismic section at depths of 10 500-10 800 ft.
