Geology Reference
In-Depth Information
seismic surveying, single-channel profiling typically
utilizes an
oceanographic recorder
in which a stylus
repeatedly sweeps across the surface of an electrically-
conducting recording paper that is continuously moving
forward at a slow speed past a strip electrode in contact
with the paper. A mark is burnt into the paper whenever
an electrical signal is fed to the stylus and passes through
the paper to the strip electrode. The seismic/acoustic
source is triggered at the commencement of a stylus
sweep and all seismic pulses returned during the sweep
interval are recorded as a series of dark bands on the
recording paper (Fig. 4.55). The triggering rate and
sweep speed are variable over a wide range. For a shallow
penetration survey the source may be triggered every
500 ms and the recording interval may be 0-250 ms,
whereas for a deep penetration survey in deep water the
source may be triggered every 8 s and the recording
interval may be 2-6 s.
The analogue recording systems used in single-
channel profiling are relatively cheap to operate. There
are no processing costs and seismic records are produced
in real time by the continuous chart recording of band-
pass filtered and amplified signals, sometimes with time
variable gain (TVG). When careful consideration is
given to source and hydrophone array design and
deployment, good basic reflection records may be ob-
tained from a single-channel system, but they cannot
compare in quality with the type of seismic record pro-
duced by computer processing of multichannel data.
Moreover, single-channel recordings cannot provide ve-
locity information so that the conversion of reflection
times into reflector depths has to utilize independent es-
timates of seismic velocity. Nonetheless, single-channel
profiling often provides good imaging of subsurface ge-
ology and permits estimates of reflector depth and
geometry that are sufficiently accurate for many
purposes.
The record sections suffer from the presence of
multiple reflections, especially multiples of the sea bed
reflection, which may obliterate primary reflection
events in the later parts of the records. Multiples are a
particular problem when surveying in very shallow
Hydrophone array
Vessel
under way
Acoustic
source
Sea bed
Sand layer
Buried
channel
Bedrock
Fig. 4.54
The survey set-up for single-channel seismic reflection
profiling.
Peaks clipped
Threshold level
Positive values recorded
+
Time
0
-
Negative values cut off
Pattern of marks on chart recording paper
Fig. 4.55
Seismic signals and their representation on the chart recording paper of an oceanographic recorder. (From Le Tirant 1979.)
