Page 121 - The Geological Interpretation of Well Logs

- SONIC OR ACOUSTIC LOGS -

Data output, velocity picking wave arrival times and slowness values requires special
With a full, digital waveform from all of the receivers in processing. For example, the output from an eight
an afray at each sampling depth, separating the various receiver array provides 8 waveforms with different off-
sets (Figures 8.31, 8.334). To derive the separate P, S and
St wave velocities (slowness) various methods are used
comparable to stacking in seismic processing (Block ef
transmitted signal
al., 1991). Effectively, the data from the individual traces
are combined in such a way as to enhance individual
wave information and diminish noise or other unwanted
effects. Typically, at each depth, a map in a time-velocity
domain is produced (Figure 8.335) from which individual
wave slowness values can be picked and a log of slowness
against depth produced.
A complication arises using tools with only monopole
received signals and the Stoneley energy cannot be separated. In order to
transmitters in ‘slow’ formations where the shear energy
have shear velocity with this data, it is modeiled from
the Stoneley information and an interpreted shear arrival
is given. With the dipole transmitters, direct detectian
of the shear arrival is possible in both ‘fast’ and ‘slow’
time ——» size used in the processing gather or the gather technique.
formations, as indicated above.
The resolution of the array tools is related to the array

This wilt be the vertical height of the array or 1.07m
(3.5ft) for the DSI of Schlumberger (Figure 8.32). A bed
thinner than the array gather will still be indicated and in
I = incremental path
its real depth position, but it will not be fully resolved
Figure 8.31 Array sonic sampling system. At any depth,
(Hsu and Chang, 1987). Generally, alt the receiver
a series of transmitter common readings are made, with
information will be included in one gather and borehole
different offsets. Sequences of readings are gathered in
various ways (see text and Figure 8.32) (from Smith er compensation may be applied by using transmitter
al., 1991). stations through the receiver array section (Figure 8.32a).
PSEUDO
TRANSMITTER COMMGN
ARRAY RECEIVER
POSITION
_——

—_———— <——
RECEIVER
SUB-ARRAY

26
Se VERTICAL RESOLUTION
Ed
$e PSEUDO
ec TRANSMITTER
SUB-ARRAY
RECEIVER
ARRAY

A. FULL ARRAY GATHER WITH B. SUB-ARRAY GATHER WITH
BOREHOLE COMPENSATIGN BOREHOLE COMPENSATION
{16 waveforms) (40 waveforms)
Figure $.32 Data gathering methods for the array sonic used by Schlumberger. A) gather using one transmitter position and an
array of 8 receivers. B) gather using 4 transmitter positions and an array of 5 receivers. The vertical resolution of the measurements
will be the vertical height of the array or sub-array (re-drawn modified from Hsu and Chang, 1987).