Geoscience Reference
In-Depth Information
a)
d)
100
-100
-200
-300
-400
0
200
0
100
0
0
400
-100
-200
-300
Metres
100
200
300
400
500
0
100
Metres
90° dip
+ve
-ve
b)
e)
100
50
0
0
-50
-100
-150
-200
-100
-200
-300
-400
0
400
Metres
0
100
200
300
400
500
0
100
Metres
60° dip
+ve
-ve
c)
f)
100
-100
-200
-300
-400
0
-100
-200
-300
-400
0
0
400
Metres
-500
0
0
50
100
200
300
400
500
Metres
30° dip
+ve
-ve
+
Potential
g)
0
-
Orebody
0
100
Metres
Figure 5.29 SP anomalies associated with sheet-like bodies. (a
c) Computed potential on a cross-section of a polarised sheet for a range of dip.
(d) Sargipalli graphite mineralisation in Sambalpur, Orissa, India. Based on a diagram in Madhusudan et al.( 1990 ). (e) New Insco copper-
bearing massive sulphide deposit, Quebec, Canada. Based on a diagram in Telford and Becker ( 1979 ). (f) A copper-bearing massive sulphide
body in the Mirdita Zone of Albania. Based on a diagram in Frasheri et al.( 1995 ) . The dark and light grey areas highlight the massive and
disseminated mineralisation, respectively. (g) Contours of downhole and surface potentials (mV) in a cross-section through the Joma pyrite
deposit, Norway; see text for details. Redrawn, with permission, from Logn and Bølviken ( 1974 ) .
-
Potential variations associated with changes in lithology
are common in drillholes, being caused by liquid-junction
and shale potentials (see Section 5.5.1.1 ). As a conse-
quence,
in the adjacent formations, then SP variations of this type
will not be detected.
In soft-rock terrains, the SP log may be used for hole-to-
hole correlation and facies analysis. Galloway and Hobday
( 1983 ) describe the use of SP logs to map facies in sedi-
ments hosting various types of mineralisation including
coal and sandstone-type uranium deposits. In hard-rock
terrains, the SP log can be useful for identifying changes in
lithology (see for example Urbancic and Mwenifumbo
the measured SP tends to vary between two
'
values, known as the sand and shale baselines
in a similar way to
baseline
'
-logs (see Section 4.7.5 ) , which corres-
pond with permeable and impermeable formations,
respectively. If there is no difference between the ionic
concentrations in the water in the drillhole and the water
γ
 
Previous Page
Next Page
Geophysics for the Mineral Exploration Geoscientist
Search WWH ::




Custom Search
Home