Page 109 - Petroleum Geology

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oG ~ - 3 / I ~
INTERVAL VELOCITY t km/s-’i
AVERAGE VELOCITY i km/s-’i
25
30
20
02 ts
’
~
I
I(
N
I
FISH
G
1
I( I N G FISH - 1
\
14-
TOP OF LATROEE GROUP
Fig. 4-9. Velocity profiles, Kingfish field. The true average vertical velocity is less than
that indicated in a seismic record section because the reflections do not come from verti-
cally beneath the shot point. (Courtesy of Esso Australia Ltd.)

reef or a volcanic cone, the echo from the base returns sooner than that from
a reflector outside the mound, and so appears higher. This is called seismic
pull-up. A fault may have a similar effect by causing a lateral velocity discon-
tinuity in the section, so distorting the record section. A gas accumulation
may depress deeper reflectors (pull-down) by virtue of reduced velocity in
the gas-bearing sand (but not all gas sands cause pull-down).
A good example of pull-up occurs in the Gippsland basin of south-east
Australia. The seismic record section across the Kingfish field (Fig. 4-8)
shows an anticline underlying a channel. This Miocene channel is filled in
places with a dense micritic limestone, and borehole data revealed that it has
a velocity up to about 4.5 km/s, whereas the mudstones above and below
have velocities of 2.4-2.8 km/s (Fig. 4-9). The high-velocity channel fill
has distorted the crest of the anticline, “pull-up” causing it to appear imme-
diately beneath the channel. The true crest on this line is near the well King-
fish 4. The same distortion appears areally, the “time” crest being aligned
with the channel*.
Not all reflections on a seismic record section are necessarily simple reflec-
tions; some are spurious, and called multiples. There are various causes of
multiples, but all relate to reflections between layers, from the bottom of
one back to the top of a deeper reflector, and then back to the geophone,
and so appear on record sections as parallel events due to the constant time

* From a talk by D.I. McEvoy, “Velocity analysis from seismic data in the Gippsland basin,
Australia”, presented at the 44th Annual Meeting of the Society of Exploration Geophys-
icists, Dallas, 1974.