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puter storage and analysis, can be processed to give depth rather than time,
and to correct for geometrical effects (when they are called migrated sec-
tions).
It is important to realize that record sections are distorted, and can be
seriously distorted, representations of the geology. In the first place, the
elastic energy wave travels radially outwards from its source, as an expanding
hemispherical shell; but since the velocity of propagation is a function of
rock density, it increases with depth, in general, and the hemisphere is soon
distorted by the greater vertical velocity. We can therefore ignore the ex-
panding wave and concentrate on the path of that part that will be reflected
to a geophone from a surface.
At each reflecting surface, part of the energy is returned as a reflected
wave, part passes on. If this reflecting surface is horizontal, the echo will
come from vertically below the line of shot points. But if, as is usually the
case, the attitude of the beds is not horizontal and the dip not strictly parallel
to the line of shot points, reflections in general come from outside the line
of the record section. Because record sections are constructed by placing the
record vertically beneath the shot point, the reflection may appear too shal-
low, and displaced laterally.
The second distortion is due to the fact that different lithologies propa-
gate the energy pulse at different velocities, so distorting the hemisphere
even more. This distortion is not usually serious in uniform beds that are
nearly flat because it affects all reflections in much the same way. But if
there is a concentrated mound of material that propagates the energy wave
at a velocity different from the surrounding material, such as a fossil coral
-

NTH. EAST 4 3 KINGFISH FIELD
z0
0

I 1.0

TOP LATROBE
20
- LINE B
2 MILES

Fig. 4-8. Seismic record section across Kingfish field, Gippsland basin, S.E. Australia.
(Courtesy of Esso Australia Ltd.)