Environmental Engineering Reference
In-Depth Information
Where there is no cement, the amplitude of
E
1
is large and attenuation is small.
Where cement is present, the amplitude of
E
1
is small and attenuation is large. Due to
the nature of the measuring system used, the amplitude of the fi rst arrival,
E
1
, is dis-
played on the log in millivolts. A low value of millivolts means a high attenuation rate
and good cement. A high value of millivolts means a low attenuation rate and poor
cement. Typically, amplitudes are recorded on a scale of 0-50 mV across one track.
Travel-Time Measurement
At the same time the amplitude is measured, the one-way travel time from the trans-
mitter to the receiver is also measured and displayed on the log. In casing of con-
stant size, this travel time should be constant and a function solely of the tool and
casing size. As we shall see later, this single-receiver
t
measurement is extremely
valuable in diagnosing problems such as cycle skipping and tool eccentricity.
Figure
12.9
shows the expected
∆
t
values for 1-11/16 and 3-5/8-in. OD tools. This
information is usually displayed in Track 1 of the log, with
∆
∆
t
increasing to the left.
Fig. 12.9
Single-receiver (3-ft)
∆
t values for CBL tools
Wave-Train Display
The entire wave train can be conveniently displayed in a number of ways, the most
common of which is the
variable-density display
(VDL), where bands of alternate
dark and light shading reveal the peaks and valleys of the wave train. A common
presentation is as shown in Fig.
12.10
, where the VDL display is in Track 3 scaled
from 200 to 1,200
s. As examples of alternative ways to display the same informa-
tion, Fig.
12.11
shows a full waveform display over the same section and Fig.
12.12
shows a half-wave display.
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