Environmental Engineering Reference
In-Depth Information
Fig. 12.12
Half-wave display
∆
t
Stretching
At the time the transmitter pulse is initiated, an electronic clock starts counting.
A bias level is adjusted by the logging engineer to detect the fi rst arrival at the 3-ft
receiver, at which time the clock stops.
t
is the elapsed time recorded by the clock.
The detection level must be set high enough so that the background noise does not
trigger the system but not so high as to miss
E
1
altogether and skip over to
E
3
, or
even later arrivals. This method of measuring
∆
t
exhibits two distinctive character-
istics when the amplitude of
E
1
is decreased due to good cement bonding. These are
(1)
∆
t
stretching is shown in Fig.
12.13
. Note
that the amplitude of
E
1
is decreased and, since the bias level remains in the same
position, the measured transit time,
∆
t
stretching and (2) cycle skipping.
∆
t
, is somewhat lengthened. This will be evi-
dent on the log and is a normal and acceptable phenomenon.
∆
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