Geoscience Reference
In-Depth Information
a)
b)
10 6
Ice
Water
Resistivity ( m)
Resistivity ( m)
Basalt
10 1
10 2
10 3
10 4
10 5
10 1
10 2
10 3
10 4
10 5
0
10 5
Active
layer
5
Permafrost
Biotite
gneiss
Permafrost
10
10 4
15
Unfrozen
Unfrozen
Gravel
20
Silt
10 3
Figure 5.25 The variation of electrical resistivity vertically through
permafrost in Quebec, Canada, during summer. (a) For marine
clayey silt and (b) for glacial till. Data obtained from resistivity
soundings. Based on diagrams in Seguin et al.( 1989 ) .
Sand
Clay-till
10 2
Clay
Clay
10 1
-10
-5
0
5
10
and/or saline water in its pore space, a larger contrast
occurs at the base of the permafrost than if the rocks have
a lesser pore volume.
Temperature (°C)
Figure 5.24 Variation of resistivity with temperature close to 0 °C for
various rocks types, illustrating the increase in resistivity associated
with frozen pore contents. Redrawn, with permission, from Scott
et al.( 1990 ).
5.4 Measurement of electrical and
electromagnetic phenomena
layers include multiple weathering fronts, salinity of the
local groundwater, depth to the water table, variations in
porosity, silici cation, lateral variations in the regolith
stratigraphy, and the nature of the protolith and trans-
ported materials overlying the ferruginous layer (Palacky,
1988 ; Emerson et al., 2000 ).
Active electrical and EM methods produce their own elec-
trical or electromagnetic fields, respectively. This requires a
source of electrical power, a control unit and an antenna.
Power is supplied from batteries or a portable generator, and
the control unit provides time synchronisation and varies
the input to the antenna with time to suit the particular kind
of measurements being made. The energy is transmitted
into the ground via the antenna. In electrical surveys the
antenna consists of a pair of transmitter electrodes in elec-
trical contact with the ground, allowing the current to flow
through the subsurface between the electrodes. They are also
referred to as current electrodes. In EM surveys the antenna
may comprise a long straight wire grounded at one end with
the return current path via the ground, or a large, usually
rectangular, loop of wire with one or a few turns, or may be
circular and comprise multiple turns of wire forming a coil.
Current is transmitted through the wire antenna, and the
associated magnetic field induces electric currents in the
subsurface (see Section 5.7.1 ). In EM methods it is common
practice to refer to the antenna as the transmitter (Tx). In
electrical surveys the term transmitter tends to be applied to
the actual control unit.
Survey measurements in both active and passive
methods are recorded using one or more detectors linked
to a recording unit. Different kinds of electrical and EM
surveys require detectors that can measure the magnitude
and direction of the electrical potential, the magnitude or
5.3.4.2 Permafrost
Since the main mechanism for the conduction of electricity
through geological materials is by ionic conduction
through groundwater, freezing the water, as occurs in the
formation of permafrost, causes their electrical resistivity
to increase ( Fig. 5.24 ) . However, the electrical properties of
permafrost are rarely consistent owing to such factors as
variations in the amount of unfrozen water, its salinity and
the presence of massive ground ice. A review of published
accounts suggests a continuum from the resistivity of mas-
sive ice towards that of fresh water.
Permafrost may vary in thickness from a few metres to
several hundred metres. Its gross resistivity structure con-
sists of a shallow zone of higher resistivity overlying a zone
of lower resistivity ( Fig. 5.25 ) . It is underlain by unfrozen
material, and in summer conditions there may also be an
overlying
of melting. Layers containing more
liquid material may also occur within the permafrost. The
resistivity contrasts between these different layers depends
primarily on the porosity and the salinity of the water
occupying the pores. If the unfrozen rock has high porosity
'
active zone
'
 
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