Environmental Engineering Reference
In-Depth Information
partly during the decay of radioactive isotopes since the formation of the primor-
dial earth. According to current perceptions, the heat-generating isotopes U
238
and
U
235
of uranium, Th
232
of thorium and K
40
of potassium are enriched within the
continental crust consisting mainly of granite and basaltic rock. In granite rock,
the radiogenic heat production rate is approximately 2.5 µW/m
3
and in basaltic
rock approximately 0.5 µW/m
3
. In addition, heat is also discharged to a small
extent by chemical processes in the earth.
Temperature in °C
100
200
300
400
0
0
Larderello
(Italy)
2,000
Soultz sous Forêts
(France)
4,000
Friedland
(North-East Germany)
6,000
Continental
deep borehole
(South-East Germany)
8,000
Fig. 2.56
Mean temperature increase with increasing depth (see /2-4/)
Terrestrial heat flow density.
The terrestrial heat is conducted within the earth
via solid rock (so-called conductive proportion (
q
.
conductive
) of the entire heat flow
q
.
) and by being transported in and with liquids (so-called convective proportion
of the entire heat flow (
q
.
convective
)). This heat flow or heat flow density is defined
as an amount of heat flowing through a unit of area within a defined unit of time.
The terrestrial heat flow density
q
.
t
, i.e. the heat flow per area in the range of depth
of the layer-thickness
∆z
, consists of these two components and the heat produc-
tion
H
, summed up along the range of depth. According to Fourier's heat conduc-
tivity equation, the conductive proportion of the heat flow density
q
.
conductive
, which
normally dominates in the continental crust, results from the product of the tem-
perature gradient
∆
θ
/∆z
and the thermal conductivity
λ
of rocks in the upper crust.
It can be described according to Equation (2.28).
∆θ
λ
∆
&
q
=
(2.28)
conductive
z
The heat conductivity of the rocks in the upper crust varies between 0.5 and
7 W/(m K). This relatively large range of values is mainly caused by the variation
of the chemical-mineralogical structure and in the textural differences (e.g. degree
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