Geology Reference
In-Depth Information
a)
b)
Figure 8.41
The basic elements of sonic
waveform logging.
Recorded waveforms
Rock
Stoneley
and fluid
T
P S
Borehole
Mudcake
R1
R2
R3
R4
R5
R6
R7
R8
R1
R2
R3
Refractions
R4
R5
R6
R7
R8
Time
with the use of the short spaced detector that primar-
ily reads the mudcake. The density correction (drho)
may be positive or negative depending on the nature
of the drilling mud and corrections of up to +/
source operating at around 3
30 kHz and an array of
receivers that detect sonic energy refracted along the
borehole wall. There are a range of different types of
sonic logging tools, essentially differing in the
number and separation of receivers and undergoing
significant development over the decades.
The basic elements of sonic logging are shown in
Fig. 8.41
. Energy from a monopole transmitter (i.e.
emitting a pulse of energy isotropically in all direc-
tions) generates three distinct arrivals: a low ampli-
tude compressional wave, followed by a larger
amplitude shear wave, followed by a Stoneley wave
(energy travelling mainly in the borehole) of even
larger amplitude. Early sonic tools simply detected
compressional wave arrivals (from the first negative
excursion of the wave train) and translated the time
difference between near and far receiver arrivals into
sonic slowness (i.e.
-
0.15g/cc are usually to be trusted.
Display of caliper (i.e. borehole diameter) and
density correction logs gives an indication of the qual-
ity of a density log.
Figure 8.40b
shows an example of a
log with both good and bad hole. The lowermost sand
(marked in yellow) has good hole with the caliper
fairly constant and the density correction very low.
Poor hole is evident in the uppermost sand, where
the caliper indicates a widened hole and the drho log
has a large correction; in this case the density log is
probably unreliable and needs editing (
Section 8.4
).
In interpreting density data, it is important to
remember that the depth of penetration into the for-
mation is low (typically 5
10 cm), so the tool is read-
ing in the zone close to the borehole wall where drilling
fluids may have invaded permeable formations, dis-
placing the original fluids and perhaps causing signifi-
cant changes in measured values. For the purposes of
seismic modelling these invasion effects should be
evaluated and corrections made (
Section 8.4.4
).
-
s/ft). Near and far receivers
were spaced 3 ft and 5 ft from the transmitter
respectively. To make corrections for poor hole con-
ditions two sets of transmitter
μ
receiver arrays were
incorporated into the tool and the measurements
related to each were averaged. This is what is usually
described as the
-
'
conventional
'
borehole compen-
sated sonic.
In uneven boreholes, for example due to altered
shale or washouts, it was found that using a
8.3.2.2 Sonic logs
The sonic logging tool is a tool which is run centred
in the borehole and stabilised by arms that extend out
to the borehole wall. It is constructed with an acoustic
'
long
spaced sonic
tool with two transmitters and two
receivers each with 2 ft spacing and 8 ft minimum
'
180
