ABEM Terrameter SAS 1000 / SAS 4000
- 9 -
Whenever the sum of the time windows in use exceeds the current on (off) time T, this time is
extended so that the current off time exceeds the sum of the active time windows plus the
initial time delay. Observe, that the total integration time cannot exceed 8 sec. This explains
the "white" area in the tables above.
The SAS 1000 / 4000 measures the time-domain quantity called chargeability
M
t
i
t
i+1
defined
in the following way:
( )
M
V
V t dt
t t
t
t
i i
i
i
+
+
=
1
1
1
0
[msec]
(1)
where
V(t)
is the decaying voltage,
t
i
and
t
i+1
is the start and stop time of the interval, and
V
0
is
the voltage measured before the current is turned off. The terminology refers to figure 2. The
chargeability is measured in the unit
msec
. Alternatively, the chargeability can be presented as
mV/V
:
(
)
( )
dt
t
V
t
t
V
M
i
i
i
i
t
t
i
i
t
t
+
+
−
=
+
1
1
1
0
1
[mV/V]
(2)
For further reference on the chargeability, see e.g. Parasnis
3
chapter 5.
3
D. S. Parasnis, Principles of Applied Geophysics, 5. edition, 1997. Chapman and Hall.
t
1
t
2
t
3
t
4
time
Current is
turned off at
time t=0
Transient
voltage decay
curve
Figure 3. The IP decay curve. The chargeability is measured as the area between two time
values. For example M
20,40
represents the chargeability measured in the interval
between 20-40 msec. The delay before the measurements starts are denoted by t
D
=
t
1
