Geology Reference
In-Depth Information
S/P-wave comparison
220
200
180
160
140
120
100
80
60
40
20
0
20
S-wave
P-wave
Fig. 5.26
T
-
x
graph of a seismic
refraction profile with a water-table
refractor.The rock unit is the Sherwood
Sandstone, having P-wave velocities of
800 and 2000 m s
-1
for unsaturated and
saturated rock respectively (lower line).
The equivalent S-wave plot (upper line)
shows no effect at the water-table
interface.
30
40
50
60
70
80
90
100
110
120
130
140
150 160
170
Offset (m)
N
S
GGF
HBF
SUF
Southern
Uplands
Midland
Valley
NW Highlands
Grampians
Northern England
0
6.0 - 6.2
6.40 - 6.45
5.8 - 6.0
6.40 - 6.45
20
6.25 - 6.30
6.7
7.3
?
?
Moho
40
8.0
8.0
km
0
200 km
Fig. 5.27
Crustal cross-section across northern Britain based on interpretation of a large-scale seismic refraction experiment. Numbers
refer to velocities in km s
-1
. (After Bamford
et al.
1978.) Contrast the distance scale with Figs 5.24 and 5.26.
but it is unable to provide the amount of structural detail
or the direct imaging of specific structures that are the
hallmark of reflection seismology. The occasional need
for better velocity information than can be derived from
velocity analysis of reflection data alone (see Chapter 4),
together with the relative ease of refraction surveying
offshore, gives the refraction method an important sub-
sidiary role to reflection surveying in the exploration for
hydrocarbons in some offshore areas.
Refraction and wide-angle surveys have been used
extensively for regional investigation of the internal con-
stitution and thickness of the Earth's crust.The informa-
tion derived from such studies is complementary to the
direct seismic imaging of crustal structure derived from
large-scale reflection surveys of the type discussed in
Section 4.16. Interpretation of large-scale refraction and
wide-angle surveys is normally carried out by forward
modelling of the travel times and amplitudes of recorded
refracted and/or reflected phases using ray-tracing
techniques.
Large-scale surveys, using explosives as seismic
sources, have been carried out to study crustal structure
in most continental areas. An example is the LISPB
experiment which was carried out in Britain in 1974
and produced the crustal section for northern Britain
reproduced in Fig. 5.27.
Such experiments show that the continental crust is
typically 30-40 km thick and that it is often internally
layered. It is characterized by major regional variations
in thickness and constitution which are often directly re-
lated to changes of surface geology. Thus, different oro-
genic provinces are often characterized by quite different
crustal sections. Upper crustal velocities are usually in
the range 5.8-6.3 km s
-1
which, by analogy with veloc-
ity measurements of rock samples in the laboratory (see
Section 3.4), may be interpreted as representing mainly
