Geology Reference
In-Depth Information
resistivity measurements are influenced by geological
noise arising from near-surface resistivity variations
which limit the resolution that can be achieved. The
more recently developed non-contacting conductivity
meters utilize EM fields and do not suffer from these
drawbacks. No ground contact is required so that mea-
surements can be made at walking pace and the subsur-
face volume sampled is averaged in such a way that
resolution is considerably improved (Zalasiewicz et al.
1985).
The secondary EM field measured in a mobile
transmitter-receiver survey (Section 9.5) is generally a
complex function of the coil spacing s , the operating fre-
quency f and the conductivity of the subsurface s .How-
ever, it can be shown that if the product of s and the
skin depth d (Section 9.2), known as the induction number ,
is much less than unity, the following relationship
results:
Transient decay curve over
peak of anomaly (447E)
1000
500
100
50
10
0.5 1.0 2 3 4 56 81015
Time t (ms)
1000
500
0
t = 1.1 ms
t = 2.3 ms
500
0
200
0
t = 4.1 ms
t = 6.1 ms
200
0
H
H
i
s
2
wm 0
s
p
(9.5)
ª
4
t = 8.2 ms
100
0
t = 10.1 ms
where H s and H p are the amplitudes of the secondary and
primary EM fields, respectively, w = 2 p f , m 0 is the mag-
netic permeability of vacuum, and i = ÷ (-1)
100
0
—— , its presence
indicating that the quadrature component is measured.
Thus the ratio H s / H p is proportional to the ground con-
ductivity s. Since d depends on the product s f , estima-
tion of the maximum probable value of s allows the
selection of f such that the above condition of low induc-
tion number is satisfied. The depth of penetration de-
pends upon s and is independent of the conductivity
distribution of the subsurface. Measurements taken at
low induction number thus provide an apparent con-
ductivity s a given by
t = 15.3 ms
100
0
436E 438 440 442 444 446 448 450 452 454 456E
Weathered zone in
conducting shale
Undifferentiated
shale
0
200 m
Hematitic quartz breccia
Quartzite
Black shale
Brecciated banded
ironstone
Tremolitic siltstone
Limestone
Diamond-drill hole
Conducting shale
4
H
H
Fig. 9.12 TDEM profiles and geological section near Mount
Minza, Northern Territory, Australia. (After Duckworth 1968.)
=
s
p
(9.6)
s
a
2
s
wm
0
This relationship allows the construction of electro-
magnetic instruments which provide a direct reading of
ground conductivity down to a predetermined depth. In
one application the transmitter and receiver are horizon-
tal dipoles mounted on a boom 3.7 m apart, providing a
fixed depth of investigation of about 6 m. The instru-
ment provides a rapid means of performing constant
separation traversing (see Section 8.2.3) to a depth suit-
able for engineering and archaeological investigations.
Where a greater depth of penetration is required, an
9.7 Non-contacting conductivity
measurement
It is possible to obtain readings of ground conductivity
by EM measurements (McNeill 1980). Measurements of
this type can be made using standard resistivity methods
(see Section 8.2), but, since these require the introduc-
tion of current into the ground via electrodes, they are
labour intensive, slow and therefore costly. Moreover,
 
 
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