Geology Reference
In-Depth Information
water, since they then occur at a short time interval after
the primary events (Fig. 4.56). Record sections are often
difficult to interpret in areas of complex reflector geom-
etry because of the presence of bow-tie effects, diffrac-
tion events and other features of non-migrated seismic
sections.
from 3 to 12 kHz. The piezoelectric transducers used
to generate the pinger signal also serve as receivers
for reflected acoustic energy and, hence, a separate
hydrophone streamer is not required in pinger survey-
ing. Vertical resolution can be as good as 10-20 cm but
depth penetration is limited to a few tens of metres in
muddy sediments or several metres in coarse sediments,
with virtually no penetration into solid rock. Pinger sur-
veys are commonly used in offshore engineering site
investigation and are of particular value in submarine
pipeline route surveys. Repeated pinger surveying along
a pipeline route enables monitoring of local sediment
movement and facilitates location of the pipeline where
it has become buried under recent sediments. A typical
pinger record is shown in Fig. 4.57.
Boomer sources provide a higher energy output
(typically 300-500 J) and operate at lower dominant
frequencies (1-5 kHz) than pingers. They therefore
provide greater penetration (up to 100 m in bedrock)
with good resolution (0.5-1 m). Boomer surveys are
useful for mapping thick sedimentary sequences, in
connection with channel dredging or sand and gravel
extraction, or for high-resolution surveys of shallow
geological structures. A boomer record section is illus-
trated in Fig. 4.58.
4.15.1 Shallow marine seismic sources
As discussed in Chapter 3 there are a variety of marine
seismic/acoustic sources, operating at differing energy
levels and characterized by different dominant frequen-
cies. Consequently, by selection of a suitable source,
single-channel profiling can be applied to a wide range
of offshore investigations from high-resolution surveys
of near-surface sedimentary layers to surveys of deep
geological structure. In general, there is a trade-off
between depth of penetration and degree of vertical
resolution, since the higher energy sources required to
transmit signals to greater depths are characterized by
lower dominant frequencies and longer pulse lengths
that adversely affect the resolution of the resultant
seismic records.
Pingers are low-energy (typically about 5 J), tunable
sources that can be operated within the frequency range
D
0
.
1
h
SB
0
.
2
d
RH
SBM1
0
.
3
0
.
4
RHM1
SBM2
0
.
5
S
0
2 km
Fig. 4.56
Air gun record from the Gulf of Patras, Greece, showing Holocene hemipelagic (h) and deltaic (d) sediments overlying an
irregular erosion surface (rockhead, RH) cut into tectonized Mesozoic and Tertiary rocks of the Hellenide (Alpine) orogenic belt.SB= sea
bed reflection; SBM1 and SBM2 = first and second multiples of sea bed reflection; RHM1 = first multiple of rockhead reflection.
