Geoscience Reference
In-Depth Information
In addition to the above complications, the location of
the electrodes with respect to the anomalous zones has a
signi
cant effect on the measured response, particularly
the shallower zones in the upper region of the pseudosec-
tion. Also, data from a survey line in rugged terrain will be
further distorted when projected onto the straight line
pseudosection plot. It cannot be over-emphasised that a
pseudosection is not a true
X
1.
X
X
a)
n
=1
Current
electrodes
Potential
electrodes
I
V
45°
45°
n
=1
'
picture
'
of the electrical struc-
n
=2
ture of the subsurface, but a
that
requires interpretation and modelling in order to trans-
form it into a (hopefully) true depth section of the subsur-
face. Interpreting pseudosections without the aid of model
responses can produce unreliable results,
Inverse modelling (see
Section 2.11.2.1
)
is routinely
used in the interpretation of resistivity/IP pseudosections
ities of the ground, and the various inherent resolution
limits of the various survey arrays and the interpretation
techniques, many complex subsurface electrical distribu-
tions can be reduced to simple but plausible models
allowing identi
cation of target zones. Limitations
imposed by mathematical complexities also affect the
accuracy of models, but non-uniqueness predominates
(see
Section 2.11.4
).
In the following examples demonstrating the application
and interpretation of resistivity/IP pseudosections, the ori-
ginal pseudosection data have been modelled using the
has been accounted for where necessary but otherwise the
inversions were unconstrained.
'
mathematical abstraction
'
n
=3
X
1.
X
n
=1
X
b)
Current
electrodes
Potential
electrodes
V
I
45°
45°
n
=1
n
=2
n
=3
X
2.
X
n
=2
X
c)
Current
electrodes
Potential
electrodes
I
V
45°
45°
n
=1
n
=2
n
=3
Example - Estrades volcanogenic massive sulphide
The Estrades Cu
Au deposit, Quebec, Canada, is
Archaean greenstone belt within the Abitibi Subprovince
and consists of a series of sub-cropping precious metal-
bearing massive sulphide lenses within a felsic pyroclastic
unit. The host sequence comprises a sub-vertical volcanic
sequence comprising mainly ma
c to intermediate lavas.
The massive sulphide body, which is conformable with the
stratigraphy, reaches 5 m in width and contains pyrite,
chalcopyrite and sphalerite. Disseminated sulphides,
mainly pyrite, occur in the footwall. The deposit occurs
beneath 12 to 40 m of glacial overburden. This example
clearly demonstrates how multiple lenses of polarisable
mineralisation (the sulphides) can produce a fairly simple
IP response. The rather accurate multi-body model can
only be developed now because of the large amount of
-
Zn
-
Figure 5.50
Creation of a pseudosection for the dipole
dipole array.
A grid of data points is created by varying the positions and spacings
(n) of the current and potential dipoles. See text for detailed
description.
-
from two adjacent sources overlap and may result in the
maximum response occurring at a location which coin-
cides with neither body. Two closely spaced sources may
appear like a single broader source. This is an important
observation since it may lead to erroneous positioning of
drillholes. A resistivity contact, representing a geological
contact also produces a pants-legs response. One
'
'
leg
is of
'
values extending across the zone of higher resistivity,
and the other
high
'
values extending across the
lower resistivity side of the contact. The surface location of
the contact is somewhere between the two sloping
'
leg
'
is of
'
low
'
'
legs
'
.
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