Geology Reference
In-Depth Information
(a)
X
X
X
X
X
X
X
X
X
c
p
Longitudinal traverse
Transverse traverse
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
ρ
a
p
c
c
= current electrode
p
= potential electrode
Plan
0
500 m
Distance
XXXXXXXXX
Shear
zone
ρ
1
ρ
2
Section
ρ
1
Fig. 8.17
Longitudinal and transverse traverses across a series of
faulted strata in Illinois, USA. (After Hubbert 1934.)
ρ
a
ρ
2
I
k
I
Distance
C
C'
(b)
r
3
ρ
1
r
1
r
2
P'
ρ
a
P
ρ
1
ρ
2
ρ
2
Distance
Fig. 8.18
Parameters used in the calculation of the potential due
to a single surface current electrode on either side of a single
vertical interface.
cppc
ρ
2
ρ
1
face of a medium of resistivity
r
1
in the vicinity of a ver-
tical contact with a second medium of resistivity
r
2
.
In the optical analogue, a point P on the same side of
the mirror as the source would receive light directly and
via a single reflection. In the latter case the light would
appear to originate from the image of C in the mirror,
C¢, and would be decreased in intensity with respect to
the source by a factor corresponding to the reflection co-
efficient. Both the electric source and its image con-
tribute to the potential
V
P
at P, the latter being decreased
in intensity by a factor
k
, the reflection coefficient. From
equation (8.6)
Fig. 8.16
(a) A transverse traverse across a single vertical interface.
(b) A longitudinal traverse across a single vertical interface
employing a configuration in which all four electrodes are mobile.
(After Parasnis 1973.)
longitudinal traversing across a series of faulted strata in
Illinois, USA. Both sets of results illustrate well the
strong resistivity contrasts between the relatively con-
ductive sandstone and relatively resistive limestone.
A vertical discontinuity distorts the direction of cur-
rent flow and thus the overall distribution of potential in
its vicinity. The potential distribution at the surface can
be determined by an optical analogue in which the dis-
continuity is compared with a semitransparent mirror
which both reflects and transmits light. Referring to
Fig. 8.18, current
I
is introduced at point C on the sur-
I
r
p
1
k
r
Ê
Ë
ˆ
¯
1
V
P
=
+
(8.20)
2
r
1
2
For a point P¢ on the other side of the interface from the
