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In-Depth Information
a
b
>
<
1/2
T,
s
1/2
T,
s
0
0
·
n
=
·
n
·
·
n
=
·
n
·
-15
-15
-30
-30
-45
-45
-60
-60
-75
-75
-90
-90
,deg
, deg
·
n
A
,Ohm.m
A
,Ohm.m
1000
10
··
··
n
n
100
1
·
n
T,
s
1/2
10
T,
s
1/2
0.1
1000
0.01
0.1
1
10
100
0.01
0.1
1
10
100
1000
Fig. 6.10
Curves for the apparent resistivity and impedance phase, obtained in the middle of the
dike (
y
,
=
,
=
=
0);a-resistivedike,
v
=
0
.
5km
10 Ohm
·
m
100 Ohm
·
m ; b - conductive
,
=
,
=
dike,
v
=
0
.
5km
10 Ohm
·
m
1Ohm
·
m
itself in the minimum of
E
x
and the maximum of
H
y
. Outside the dike these anoma-
lies quickly attenuate. Give your attention to the side maxima and minima of
H
y
bordering the resistive and conductive dike at high frequencies (
T
=
0
.
01 s for
the resistive dike and
T
1 s for the conductive dike). These extrema reflect the
horizontall skin effect at the dike edges. With lowering frequency the anomalies of
E
x
and
H
y
diminish. They die out provided
h
eff
=
>>
2
v
(
T
=
100 s in the case of
the resistive dike and
T
10000 s in the case of the conductive dike). We see that
resolving power of
E
x
and
H
y
decreases with depth.
Anomalies of
E
y
are of the galvanic nature. They have a jump on the dike edges
and produce strongly pronounced maxima (over the resistive dike) and minima (over
the conductive dike). These extrema are fringed by the side minima and maxima of
E
y
associated with rearrangement of the transverse currents due to different skin
depth in the dike and in the ambient medium. With lowering frequency the side
extrema vanish, while the central extrema take the rectangular form.
In closing consider the
W
zy
-profiles given in Fig. 6.13. The resistive and con-
ductive dikes manifest themselves in zigzag anomalies of Re
W
zy
and Im
W
zy
with
maximum and minimum over the dike edges. The resistive dike makes a zigzag
with left maximum and right minimum, whereas the conductive dike makes a zigzag
with right maximum and left minimum. The width of anomalies is of the order of
10
h
eff
. With lowering frequency the anomalies decay, faster for the resistive dike
and slower for the conductive dike. At the bottom of Fig. 6.13 the real inductive
arrows are shown. They converge to the resistive dike and diverge outward from the
conductive dike.
=
