Geology Reference
In-Depth Information
I
Δ
V
Multicore cable
C
2
P
2
C
1
P
1
Δ
V
P
1
P
2
C
1
C
2
Fig. 11.3
The normal log.
Fig. 11.2
The general form of electrode configuration in
resistivity logging.The shaded area represents the effective region
energized by the system.
The general equation for computing apparent resis-
tivity
R
a
for any downhole electrode configuration is
difference between the electrodes reflects the current
density and resistivity in that region.The same potential
difference would be obtained no matter what the
position of the potential electrode pair. The zone
energized is consequently the region between the
equipotential surfaces on which the potential elec-
trodes lie. Figure 11.2 shows the energized zone in a
homogeneous medium.
4
V
pD
R
=
(11.1)
a
1
1
1
1
Ó
Ê
Ë
ˆ
¯
Ê
Ë
ˆ
¯
˛
I
-
-
-
CP
C P
CP
C P
11
21
1
2
2
2
where C
1
,C
2
are the current electrodes, P
1
,P
2
the po-
tential electrodes between which there is a potential dif-
ference
D
V
, and
I
is the current flowing in the circuit
(Fig. 11.2). This is similar to equation (8.9) but with a
factor of 4 instead of 2, as the current is flowing in
a full space rather than the halfspace associated with
surface surveying.
Different electrode configurations are used to give
information on different zones around the borehole.
Switching devices allow the connection of different sets
of electrodes so that several types of resistivity log can be
measured during a single passage of the sonde.
The region energized by any particular current elec-
trode configuration can be estimated by considering
the equipotential surfaces on which the potential elec-
trodes lie. In a homogeneous medium, the potential
11.4.1 Normal log
In the
normal log
, only one potential and current elec-
trode are mounted on the sonde, the other pair being
grounded some distance from the borehole (Fig. 11.3).
By substitution in equation (11.1)
D
V
I
R
= 4
CP
p
(11.2)
a
11
Since C
1
P
1
and
I
are constant,
R
a
varies with
D
V
, and the
output can be calibrated directly in ohm m.The zone en-
ergized by this configuration is a thick shell with an inner
radius C
1
P
1
and a large outer radius. However, the cur-
rent density decreases rapidly as the separation of C
1
