Geoscience Reference
In-Depth Information
Temperature becomes most significant when it is suffi-
ciently low that the pore waters are entirely or partly
enhances polarisation, explainable in terms of frozen
pore water changing the porosity and tortuosity of the
available pore space.
•
'Disseminated'
mineralisation
trend
140
120
100
80
'Vein' mineralisation maximum
60
40
Vein mineralisation
20
Disseminated mineralisation
An obvious conclusion from the above is that the propor-
tions of rock-forming minerals in the matrix have negli-
gible control on electrical polarisation and, consequently,
there is generally no correlation between it and rock type.
0
0
2
4
6
8
10
12
14
16
18
20
22
Weight sulphides (% )
Figure 5.21
Relationship between IP response, measured as a phase
shift, and sulphide content for both disseminated and vein style
mineralisation. See
Section 5.6.3
for explanation of the measurement
of induced polarisation.Note the
response when veinlet
mineralisation exceeds about 5% by weight. Redrawn, with
'
saturation
'
5.3.3
Dielectric properties
The dielectric constant is a measure of a material
is ability
to be electrically polarised, and is an important control
on the responses of high-frequency EM surveys (see
Appendix 5
)
.
Figure 5.22
shows the range in dielectric
constant for various minerals and rocks and the materials
that occur within pore space. The rock-forming minerals
show comparatively little variation. However, metallic sul-
phide and oxide minerals have significantly higher values,
indicating that occurrences of these minerals may be
detectable using radar methods. The most significant
aspect of the data is the extremely high values for water.
The data graphically illustrate that dielectric constant is
predominantly a function of the presence, or absence, of
water; compare wet and dry sands, for example. Mineral-
ogy (and therefore lithotype) exerts little in
'
varies roughly proportionally with the amount of con-
ductive material present. For vein material there appears
to be a critical amount of conductive material above
which the response ceases to increase, i.e. a
'
saturation
'
response is reached (
Fig. 5.21
).
•
The size and shape of conductive grains. Experiments
suggest there may be an optimum grain size determined
by the texture and electrical properties of the host and
the grains themselves. In general, increasing grain size
reduces the polarisation response since the surface area
available for interaction with ions decreases.
The size and shape of pores. As well as controlling
porosity and permeability, the pore geometry determines
whether conductive grains and clay minerals make con-
tact with pore
•
uence, other
than as an indirect control on porosity and potential water
content. Even though heavy minerals such as zircon and
monazite have higher values than quartz, which is likely to
make up their host sediments in beach sand deposits, it is
unlikely mineralization will be detectable in water-
saturated sediments because of the dominant in
uence of
the pore water.
fluids. Increasing permeability decreases
the polarisation response because of the larger pore
throats and/or associated decreases in clay content.
The type, distribution and volume fraction of clay min-
erals. The greatest polarisation responses occur when
the clay mineral content is about 5
•
10% by volume,
depending on the mineral species present and cation-
exchange capability. This is due to the requirement that
both clay minerals and a suitable pore channel for the
ions are present. As clay mineral content increases the
number of channels decreases.
-
5.3.4
Properties of the near-surface
Electrical and EM measurements of the ground are
strongly in
uenced by the electrical properties of the
near-surface. The electrical properties of this zone differ
markedly from deeper regions, even where bedrock is
exposed. Signi
cant lateral and vertical changes in elec-
trical properties can occur.
In geophysical terms, a conductive (low-resistivity) sur-
face layer
The type and salinity of the electrolyte in the pores and
the degree of water saturation. Enough ions are required
to allow the various mechanisms to occur, but too many
allow alternative conduction paths. Also, increasing
matrix conductivity diverts current flow away from ionic
conduction mechanisms.
•
is known as
conductive overburden, and
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