Geology Reference
In-Depth Information
0
64” Normal
10
Depth
(feet)
Δ
V
0
16” Normal
10
Lithology
Resistivity
(
Ω
m)
C
2
1700
Shale
Sandstone
1800
C
1
1900
2000
6650
P
1
6700
P
2
Fig. 11.4
A comparison of short and long normal logs through a
sequence of sandstone and shale. (After Robinson & Çoruh
1988.)
Fig. 11.5
The lateral log.
and P
2
increases, so that measurements of resistivity
correspond to those in a relatively thin spherical shell.
The presence of drilling fluid and resistivity contrasts
across lithological boundaries cause current refractions
so that the zone tested changes in shape with position in
the hole.
It is possible to correct for the invasion of drilling fluid
by using the results of investigations with different elec-
trode separations (
short normal log
16 in (406 mm),
long
normal log
64 in (1626 mm)) which give different pene-
tration into the wallrock. Comparison of these logs with
standard correction charts (known as departure curves)
allows removal of drilling-fluid effects.
The normal log is characterized by smooth changes
in resistivity as lithological boundaries are traversed by
the sonde because the zone of testing precedes the sonde
and the adjacent bed controls the apparent resistivity.
Examples of short and long normal logs are given in
Fig. 11.4.
siderable distance above the potential electrode pair, and
is usually mounted on the wire about 6 m above a short
sonde containing P
1
and P
2
about 800 mm apart (Fig.
11.5). For this electrode configuration
4
1
V
pD
R
=
(11.3)
a
1
Ê
Ë
ˆ
¯
I
-
CP
CP
1
1
1
2
An alternative configuration uses C
1
mounted below the
potential electrode pair.
The measured potential difference varies in pro-
portion to the resistivity, so the output can be calibrated
directly in ohm m. The zone energized extends much
farther into the wallrock than with normal logs, and the
apparent resistivity thus approaches the pristine wallrock
value more closely.
The electrode configuration causes asymmetry in the
apparent resistivity signature as the potential electrode
pair descends through one bed while the current
electrode may be moving through another. Thin beds
11.4.2 Lateral log
In the
lateral log
the in-hole current electrode C
1
is a con-
