Geoscience Reference
In-Depth Information
50 N
60 N
5
60 N
0.5
1
5
10
1
0.5
1
40 N
50 N
5
0.5
1
1
1
1
1
5
10
5
5
10
40 N
1
0.5
1
1
0.5
30 N
5
0.5
1
10
1
5
5
0.5
30 N
Figure 9.4.
Sediment thickness over the western North Atlantic (contours are 0.5, 1,
5 and 10 km). (After Tucholke (1986).) See http://www.ngdc.noaa.gov/mgg/image/
sedthick.jpg for a global marine sediment thickness map.
constant velocity
1
have given way to a discontinuous series of velocity gradi-
ents in which the velocity increases with depth (Table 9.2). Figure 9.5(a) shows
velocity-depth profiles for seismic-refraction lines in the Atlantic and Pacific that
were constrained by synthetic seismogram modelling. It is clear that, although
there are local differences, the general variation of seismic velocity with depth
is fairly constant within each age zone. It should be remembered when looking
at velocity-depth profiles such as these that seismic-refraction experiments gen-
erally cannot resolve structure much less than one wavelength in thickness. (For
layer 2, with velocity of 5 km s
−
1
and a 7-Hz signal, a wavelength is 0.7 km.)
Therefore, much of the fine detail and staircase-like appearance of these profiles
is a representation of velocity gradients.
1
This layering is sometimes referred to as 'Raitt-Hill layering' after the scientists who first established
that the uniform structure of the oceanic crust could be modelled as four constant-velocity layers
(Raitt 1963; Hill 1957).
