||In two-dimensional (2D) reflection seismic surveying both the sound source
and the sound detectors (numbering up to a hundred or more per shot) are moved
along a straight line. The resultant product can be thought of as a vertical
sonic cross-section of the subsurface beneath the survey line. It is constructed
by summing many compressional (pressure) wave reflections from the various sound
source and sound detector locations at the halfway sound path points beneath each
location (common depth point stacking).
In three-dimensional (3D) reflection seismic surveying the sound detectors
(numbering up to a thousand or more) are spread out over an area and the sound
source is moved from location to location through the area. The resultant product
can be thought of as a cube of common depth point stacked reflections. Advantages
over 2D include the additional dimension, the fact that many more reflections are
available for stacking at each point, which provides greatly improved resolution
of subsurface features, and elimination of the "ghost" or "side swipe" reflections
from nearby offline features that 2D surveys are prone to (except, of course,
along the outer faces of the cube).
Four dimensional (4D) reflection seismic surveying is the exact repetition of a
3D survey at two or more time intervals. The primary application of 4D is mapping
the movement of fluid interfaces in producing oil and gas reservoirs.