Page 120 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS ~

for 8, or two for 32 waveforms and using one monopole
transmitter for 8 waveforms using either ‘a high frequen-
cy signal for P and $ waves or a low frequency signal for
Stoneley wave detection. The tool acquires digital wave-
form data with 5]2 samples per waveform. Logging
speed varies but can be at a maximum of 1100m/hr (3600
fv/hr) which is similar to the standard nuclear tools.
All tools using an array of receivers acquire a number
of receiver (or transmitter) common measurements at
each depth station (Figure 8.31), the number of common
datapoints depending on the number of receivers and/or
transmitters used. Receiver threshold detection, as used in
the standard tools, is inadequate and inappropriate for the
b)
full waveform tool. Instead, a full, digital waveform is
recorded and gathering techniques are used to collect
the common datapoints from a single depth {or selected
interval). The gathering may be made using one transmit-
Ss WA :S WAVE ter position and a full receiver array (Smith er ai., 1991)
DIPOLE
(Figure 8.31), or a sub-array as used by Schlumberger,
when several consecutive transmitter and receiver points
are used (Hsu & Chang, 1987) (Figure 8.32). Sampling
depths are normally the same as the separation between
the receivers of the array, typically 15.24cm (6").
However, since, gathering may be over an interva] cover-
ing several receivers as described, the effective interval
Figure 8.29 Array sonic transmitter types. a} monopole being measured depends on the gathering process (Figure
transmitter giving a multi-directional pulse; b) dipole 8.32), as discussed below.
transmitter giving a directed pulse {modified from
Zemanek ef ai., 1991).
dipole receiver
transmitters array
13.6% 1

(41'}
2 | cartridge
———_+

| id > i] ve ws 2 2 jinn
ix) ——+| ~—
(671
monopole recelvar
transmitter spacing
ati.
<<
tool length 15.6m (54°) —————
Figure 8.30 The Dipole Shear Imager Tool of Schlumberger (re-drawn from Schlumberger document).
Table 8.7 Full waveform acoustic tools.

Company Tool Receiver Array Transmitters & Frequency (Fq)

Schlumberger Array Sonic ASL 8 receivers 2 wansmitters, monopole, Fq = 10-15 kHz

Dipole Shear Imager 8 receivers 3 iransmitiers, | monopole, 2 dipole
DSI Fg = | kHz (diopole), Fq = variable (mononpole)

Atlas Wireline Digital Array Acoustilog 2 receivers 2 wansmitters, monopole, Fq = 9 kHz
DAC

Multipole Acoustic Tool 8 receivers 4 transmitters, 2 monopole, 2 dipole
MAC Fq = 1-3 Hz (dip), Fq = 8 kHz (mono)

Halliburton Full Wave Sonic Log 4 receivers 1 wansmitter, monopole, Fq = 13 kHz
FWS

110