Geoscience Reference
In-Depth Information
Figure 4.2.
(a)-(c)
Examples of the
essentially linear
relationship between
seismic velocities and
density known as Birch's
law. Laboratory
measurements made on
crustal rocks at confining
pressures of (a) 0.2 GPa,
(b) 0.6 GPa and (c)
1.0 GPa, which
correspond to
approximate depths of 6,
18 and 30 km, are shown.
(1 GPa = 10
9
Pa. The
pascal, a unit of pressure,
is equal to one newton
per square metre: 1 Pa =
1Nm
−2
.Aunit of
pressure still often used
today is the bar: 1 bar =
10
6
dyne cm
−2
. Thus,
1 GPa = 10
4
bar =
10 kbar). (After Gebrande
(1982).) (d) The
relationship between
seismic velocities and
density referred to as the
Nafe-Drake curve after its
originators. Open circles,
values for igneous and
metamorphic rocks; solid
circles, values for
sedimentary rocks;
crosses show a linear
model from Birch 1964.
(After Ludwig
et al.
(1970).)
(a)
(b)
(c)
10
α
= 0.32 + 2.27
ρ ±
0.24
= 0.25 + 2.27
0.22
α
= 0.33 + 2.20
ρ ±
0.25
α
ρ ±
8
6
4
β
= 0.31 + 1.20
ρ ±
0.17
β
= 0.48 + 1.13
ρ ±
0.21
β
= 0.42 + 1.17
ρ ±
0.19
2
0
2
3
2
3
4
2
3
4
4
3
--3
Density (10 kg m )
(d)
10
8
6
4
Compressional Waves
sediments, sedimentary rocks
metamorphic and igneous rocks
2
Shear Waves
sediments, sedimentary rocks
metamorphic and igneous rocks
Birch (1964)
0
1
2
3
4
5
3
--3
Density (10 kg m )
