Page 104 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

the two receivers (Figure 8.4a), and this time is the second (Figure 8.3). At a typical sonic tool logging speed
formation reading; the value required. of 1500m/h (SO00ft/h), (i.e. approximately 40cm/sec or
Since tool tilt and hole size may make the common 16"/sec) each complete sequence of four readings will
parts of the trajectory unequal, a second, inverted array give one log reading for every 8cm (3") of borehole.
(with a downward moving signal), is averaged with the The borehole-compensated (BHC) sonic described
first (with the upward moving signal) to provide compen- above has a ‘static’ compensation and has been used
sation. This means that each value recorded on the sonic commonly since the 1960s. It typically has wansmitter-
log is the result of a sequence of four separate transmitter- receiver distances of three feet and five feet with two feet
receiver readings, two from the lower transmitter to its between the two receivers (Figure 8.4). In the late 1970s
near and far receivers and two from the upper transmitter it was found that longer transmitter-receiver distances
to its near and far receivers. The up and down receiver could help under certain borehole conditions and the long
sets are offset vertically to allow for the tool moving spaced sonic was designed with two receivers two feet
(Figure 8.4a). apart separated by eight feet from two transmitters also
In terms of typical values for the BHC tool, a ansmit- two feet apart (i.e. the LSS of Schlumberger, Figure 8.44,
ter pulse lasts between 100s — 200s (microseconds), Table 8.2). This too] gives a near reading with 8-10 foot
the gap between the pulses is 50ms (milliseconds) or spacings and a far reading with 10-12 foot spacings
20 pulses per second, allowing five complete sequences (Figure 8.45). Because of its length, the long spaced
of four individual transmitter-receiver readings per sonic has a ‘dynamic’ compensation system where depth

(a) BOREHOLE COMPENSATED SONIC LOG

suidap INTERVAL TRANSIT TIME
microseconds per foot

other logs
receiver-transmitter,) Ry 2Ro
3
spacings, feet
(GR, SR, caliper ete)

440.9 9a.0 ao

| 1 di

integrated
r neve
travel time”

1 millisecond 4

sS
Al

¥

Vriyy

a
(b} LONG SPACING SONIC LOG
TENS (18)
1000
delta t
cable’ tenslon
long spacing pounds
other logs
146.0 40.00
(GR, SP, caliper ete)
DT (S/F)
40.6 ‘ 140.0 0.
delta t
short spacing
asistivity
1400
7
integrated
travel tlme
10 milliseconds
DT long
Figure 8.5 Typical sonic log headings. (@) BHC tool; (6) long spacing tool (on the ISF-sonic combination of Schlumberger).
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