Page 103 - The Geological Interpretation of Well Logs

- SONIC OR ACOUSTIC LOGS -

K— SEQUENCE FOR SINGLE LOG VALUE ————_- consisting of a transmitter coupled with a near receiver
and a far receiver (Figure 8.4a). Because the sonic is
INTERVAL BETWEEN
PULSES generally run hole-centred, any pulse transmitted by the
50ms (60 x 1079 5)
tool, passes first into the mud, it is then refracted at the
jo. 20 times/second

borehole wall, travels through the formation close to
the borehole wall and, at a critical (slower) velocity is
o
refracted back into the mud, so to reach the tool again
é

aly ha, Nace Y Nae V vr _ where it is detected. A significant part of the trajectory
V

, is in the borehole mud (Figure 8.42). However, if this
200 ns travel path is considered when one transmitter is used
(200 2 1075 s)
with two receivers (a near and a far), the mud effects can
SONIC PULSE
be eliminated. This is simply achieved by measuring the
Figure 8.3 Sonic tool emitter patterns (schematic). Typically
time it takes for the signal to reach the far receiver and
a pulse lasting 200 microseconds is emitted every 50
from this subtracting the time it takes to reach the near
milliseconds, i.e. 20 times a second. Four pulses are needed
for a complete (BHC) log measurement. (Re-drawn from receiver. The path from tool to borehole wall and back, in
Serra, 1979). the mud, is effectively common to both trajectories, as is
the section of the path between the transmitter and near
The borehole-compensated sonic tool has two trans- receiver: al] are eliminated on subtraction. What is not
Mitter-receiver groups (one inverted), each group common to the two trajectories is the time taken between
a) BOREHOLE b) LONG SPACING
COMPENSATED SONIC
SONIC

long

measure

Ty [Ul short

T=transmitter
L=lower U=upper
R= receiver

Figure 8.4 Sonic tools. Representations of (a) a borehole-compensated sonic tool which gives instantaneous readings with an
inverted receiver transmitter array and (4) the Long Spacing Sonic Tool (Schlumberger) which gives long and short-spaced
readings using 2 time (i.e. position} delay system: positions (1) and (2) are both relative to the same measure point. (Modified
from Thomas, 1977 and Purdy, 1982).

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