Page 110 - Petroleum Geology

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delay. Record sections can be processed to remove multiples when they
occur.
The seismic trace at a given point not only records reflections from layers
at depth, but also a considerable amount of “noise”, geological and other.
Random noise is eliminated by making repeated records in the same position
so that when the records are combined, random noise tends to cancel out
while the real events are enhanced.
Record sections are worked into seismic maps by correlating reflections
within record sections and between record sections, and plotting them either
in units of two-way time, or depth. These are contour maps, and they are
read in the same way as other contour maps (with the reservations mentioned
above for time contours); and isochron or isopach maps can be constructed.
These are usually the first maps of subsurface data in an area, and from them
the most prospective areas are chosen. Some idea of the rock types and their
distribution is also required.
The interpretation of seismic record sections in terms of probable lithol-
ogies is based largely on the apparent velocity of propagation. The length of
modern geophone spreads, commonly over 3 km, is such that the difference
in time taken for the reflected wave to travel to different geophones can be
used to compute velocities. The interval velocity between two points is the
difference between the average velocity above the bottom point, and the
average velocity above the top point, of the interval. Interval velocity
analysis is carried out by computer and shown in a plot like that in Fig. 4-9,
except that the time intervals are rather coarser when there is no well control.
Record sections can be processed and replotted in terms of interval velocities,
and from these the distribution of lithologies can be inferred.

Seismic stratigraphy

The spectacular improvements in seismic technology that resulted in record
sections of great clarity with little noise, such as in the accompanying figures,
also led to the development of what naturally came to be called “seismic
stratigraphy” (see Payton, 1977). Individual reflectors could be followed over
considerable distances, and their relationships with other reflectors, above
and below, studied and mapped. As studies extended over wider and wider
areas, certain characteristic patterns emerged, analogous to those known in
classical stratigraphy, and with them, a new terminology (Vail et al., 1977,
pp. 205-212).
It will be recalled from Chapter 1 that Barrell’s (1917) diastems, which
result from minor fluctuations of baselevel, can probably be equated to bed-
ding planes and interpreted as isochronous surfaces through a formation;
and if the fluctuations are due to global sea-level changes, and the diastem or
disconformity to a global fall of sealevel, then such surfaces will be syn-
chronous in all active marine sedimentary basins of the world. Such surfaces