ABEM Terrameter SAS 1000 / SAS 4000
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Figure 3 is a synthetic example illustrating the noise reduction effect of the chargeability. In
the figure is shown an exponential decay curve 100exp(-t/
) with the decay constant
=25
msec. Also shown is the decay curve with added harmonic power-line voltage (50 Hz, 10V)
and normal distributed noise with standard deviation 10V.
Integration over a time window, e.g. 10-30 msec, shows that the chargeability is only little
affected by the noise. In this particular example the deviation is 2.3% between the true
exponential decay curve and the curve superimposed with noise.
2.4.3
Negative readings?
Negative resistivity readings can occur, but in general, these are not caused by geological
formations if standard electrode arrays are used. Hence, negative resistivity readings are
normally a sign of serious measurement quality problems, and the cause should be
investigated. Typical causes are poor electrode grounding resulting in low transmitted power
and possible capacitive coupling. If the ground is dry the electrode contact must be improved
by e.g. watering and possibly connecting several electrodes to each electrode point. Inspect all
connectors and cable jumpers for dirt, oxide and damage that may cause data quality
problems. High noise level or the influence from close-by objects like fences or steel pipes in
electrical contact with the ground might also cause non-expected results.
Negative IP readings, on the other hand, are to be expected in many cases, even though the
intrinsic chargeability never can become negative. On a stratified ground negative IP readings
can occur in certain cases when the subsurface layer is more conducting relative to the upper
Figure 4. Synthetic example illustrating the noise reduction effect in the chargeability. In
this example the difference between the chargeabilities M
10,30
measured from 10
msec to 30 msec on the smooth exponential decay curve and the "noisy" curve is
only 2%.
Exponential
decay
curve with added
harmonic voltage (50
Hz, 10V) and normal
distributed
noise
(StDev=10V)
Time
window
