Page 107 - The Geological Interpretation of Well Logs

- SONIC OR ACOUSTIC LOGS -

thickness will be registered on the sonic log, but a true To use the log it is necessary to propose that when a
velocity will not be recorded. Specialist tools now exist formation has, on average, a uniform distribution of small
with much higher resolutions (i.e. the array sonic of pores and is subjected to a heavy confining pressure,
Schlumberger in certain modes, the digital array there is a simple relationship between velocity and poros-
acoustilog of Atlas Wireline, Section 8.8). ity (Wyllie et ai., 1956).

Unwanted logging effects |S
| + (1)
1-4
The conventional borehole-compensated sonic is very
robust, even in poor and over-sized holes (cf. Ellis, 1987) os V ma
due to the effectiveness of the compensation system (see
which can be written, replacing At for V, as
‘tools’ above). However, in extremely poor holes, cycle
skipping occurs (Table 8.3). This is the effect when the
first, compressional wave arrival is too attenuated (weak) At = oAt, +(1-o)At,, (2)
to activate the receiver, which is only tripped by a subse-
quent arrival: the recorded time is therefore too long
where V = tool-measured velocity; V, = velocity of the
(interval transit time too large) (Figure 8.6a). The reverse
interstitial fluid; V_. = velocity of the matrix material;
situation occurs when noise signals trip a receiver. This
= porosity; At = tool measured interval transit time;
causes noise spikes on the log and is found in hard
Ar, = transit time of interstitial fluid; and Ar, = transit
formations such as limestones (Figure 8.68).
time of matrix material.
While the conventional sonic is robust, the long spaced
Equation (2) simply states that the transit time measured
sonic is not. There are two weaknesses in the tool which
by the too] is the sum of the time spent in the solid matrix
are compounded, signal attenuation and the dynamic
and the time in the fluid: it is called the time average
compensation system. Attenuation results in a signal too
relationship (Wyllie et al., 1956). This ‘time’ is a function
weak to trigger a receiver, and causes cycle skipping. In
of the matrix velocity and constituents volumes (i.e. wave
the dynamic compensation system, each transmitter-
path length) (Figure 8.7). The relationship is best trans-
receiver reading is used twice (i.e. at two levels) and an
lated into graphic form, where it becomes obvious that
error on any one of the eight readings comprising a full
the measured interval transit time has a linear relationship
sequence, causes paired errors on the log (Figure 8.6c).
with porosity (Figure 8.8). The relationship will vary
Paired errors and serious cycle skipping are frequent on
depending on the velocity of the matrix material (see
many long spaced sonic recordings despite computer
equation 2). Some of the more common matrix velocities
‘smoothing’ (Table 8.3) (Purdy, 1982).
are shown in Table 8.4.
The quantitative derivation of porosity using the time
Table 8.3 Unwanted environmental effects — sonic log. average relationship is usually imprecise and modifications
are necessary (Raymer et ai, 1980) although these are

Factor Effect on log Severity*
often only effective very locally (Brereton and McCann,

Caving ‘Cycle skipping’ Present
Diminished At troughs to
a mud value (BHC)
High or low or alternate paired
anomalous peaks (LSS) Common

Hole ‘Noise triggering’ Rare
rugosily Increased Ar spikes (BHC)
High or low or alternate paired
anomalous peaks (LSS) Common

*When the effect makes the log reading unusable.
Ratings: frequent, common, present, rare. Mh tt é
BHC = Borehole Compensated Sonic.
volocity
Col 4
LSS = Long-Spaced Sonic.
matrix Vmg
i fl NS
i velocity
fluid V4
8.5 Quantitative uses
Figure 8.7 Diagrammatic representation of the path of P
The sonic log can be used to calculate porosities, waves through a rock, showing the relationship between Ume
although it is usually inferior to neutron or density-log spent in the matrix (V,_.) and time in the fluid (¥,) giving the
calculated values. basis for the calculation of porosity from sonic velocities.