Geology Reference
In-Depth Information
Sonobuoy
Firing/recording ship
Radio link
Water layer
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Layer 2
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Arrivals from
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Arrivals from
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Arrivals from
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Fig. 5.16
Single-ship seismic refraction profiling.
x
10-50 Hz and travel times need to be known to about
10 ms.
A large-scale seismic refraction line on land to investi-
gate deep crustal structure is typically 250-300 km long.
Seismic events need to be recorded at a series of inde-
pendently operated recording stations all receiving a
standard time signal to provide a common time base for
the recordings. Usually this is provided by the signal from
the global positioning system (GPS) satellite system.Very
large energy sources, such as military depth charges (det-
onated at sea or in a lake) or large quarry blasts, are re-
quired in order that sufficient energy is transmitted over
the length of the profile line.The dominant frequency of
such sources is less than 10 Hz and the required accuracy
of seismic travel times is about 50 ms. Such an experi-
ment requires the active involvement of a large and well-
coordinated field crew.
Along extended refraction lines, wide-angle reflec-
tion events are often detected together with the refracted
phases. These provide an additional source of infor-
mation on subsurface structure. Wide-angle reflection
events are sometimes the most obvious arrivals and may
represent the primary interest (e.g. Brooks
et al
. 1984).
Surveys specifically designed for the joint study of
refracted and wide-angle reflection events are often
referred to as
wide-angle surveys
.
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Fig. 5.17
Variation in the travel time of a head wave associated
with variation in the thickness of a surface layer.
from all refractors of interest are obtained over the same
portion of line. The importance of this can be seen by
reference to Fig. 5.17 where it is shown that a change in
thickness of a surface low-velocity layer would cause a
change in the delay time associated with arrivals from
a deeper refractor and may be erroneously interpreted as
a change in refractor depth. The actual geometry of the
shallow refractor should be mapped by means of shorter
reversed profiles along the length of the main profile.
These are designed to ensure that head waves from the
shallow refractor are recorded at positions where the
depth to the basal refractor is required. Knowledge of
the disposition of the shallow refractor derived from the
shorter profiles would then allow correction of travel
times of arrivals from the deeper refractor.
The general design requirement is the formula-
tion of an overall observational scheme as illustrated in
5.8.2 Recording scheme
For complete mapping of refractors beneath a seismic
line it is important to arrange that head wave arrivals
