Geoscience Reference
In-Depth Information
5.6.8.2
Example of resistance logging - Franklin
roll-front uranium
An example demonstrating the use of single-point resist-
ance logs for facies mapping in a soft-rock uranium envir-
onment is shown in
Fig. 5.62
. The drill section crosses a
deltaic point bar near the Franklin uranium mine in Texas,
alisation in roll-front type uranium deposits is critically
dependent on redox conditions, which in turn are related
to sedimentary facies. At the Franklin Mine, uranium
mineralisation occurs in deltaic point-bar sediments. The
character of the resistance logs can be used to infer pos-
itions within subsets of the point-bar sequences, allowing
directions to uranium mineralisation to be inferred.
surveying (see
Section 5.6.4.2
)
. The technique involves
applying a potential to an electrically conductive body
and then mapping variations in the potential in the sur-
rounding rock mass to determine the gross geometry of the
conductor. Mise-à-la-masse, a French term literally trans-
lated as excitation of the mass or body, is widely used, but
we prefer to use applied potential, since it is more descrip-
tive of the method.
The target conductor may either outcrop or have been
intersected in the subsurface through drilling or mining. In
a mining context, the conductive body is usually massive
sulphide mineralisation, but other applications of the
method include mapping conductive faults and zones of
contaminated groundwater. Potential measurements may
be made at the surface, in drillholes or in underground
workings. The method can also be used to establish
whether conductive bodies identi
5.6.9
Applied potential/mise-
`
-la-masse method
ed at different locations,
usually drillhole intersections, are electrically connected,
i.e. part of
The applied potential (AP) or mise-à-la-masse (MALM)
method is a commonly used downhole form of electrical
the same electrical mass. The method is
Resistance
log
Resistance
log
Resistance
log
+
+
Resistance
log
Resistance
log
+
+
+
Bank,
overbank
Fining
upward
sequence
(point bar)
Clay channel-fill
Sand channel-fill
Crevasse
splay
Basal
point-bar
Crevasse
splay
Silt marker
10 m
VE = 2
Sand channel-fill
Accretion sets
with dipping
clay beds
Bay &
crevasse splay
Sand
Sand
Sand
Lignite
Uranium mineralisation
Oxidised sediments
Figure 5.62
Single-point downhole resistance logs from a drill section of the Franklin roll-front type U deposit. See text for details. VE
vertical
exaggeration. Redrawn from Cossey and Frank (
1983
), AAPG Bulletin (AAPG ©1983), and reprinted by permission of the AAPG whose
permission is required for further use.
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