An Innovention Industries CVF2 DataLogger has been used at the University of Saskatchewan Geophysics Field School to build instrumentation for geophysics in a number of applications. The advantages for the university are low cost and greatly increased suitability to research and teaching. Most importantly, complete signal waveforms are available from the DataLogger, rather than the summary information that is often all that is available with a commercial receiver. The students benefit from seeing the raw data, as well as from performing the signal processing to yield the summary information.
This article describes an induced polarisation (IP) receiver. IP is a technique used in exploration geophysics to detect the presence of metallic conductors. Sulphide ores are a typical target. With this method, a high potential (several hundred volts) is applied to two metal electrodes driven into the ground. Currents of an amp or more energise the ground, and charge is stored wherever polariseable material is present. Metallic conductors in pore spaces and clay are common polariseable materials.
When the energising current is switched off, the charge stored in the ground decays over a period of a few seconds. The amount of charge stored is diagnostic of the surface area of polariseable material in contact with pore water. It is quantified by the chargeability
m=Vs(t)/Vp.
Here, Vs(t) is the secondary voltage Vs(t) measured between two nearby non-polariseable electrodes and Vp is the primary voltage Vp measured on the same electrodes. Chargeability may attain values of several hundred mV/V.
The rate of decay of the secondary voltage is characterised by a time constant and an exponent. The time constant and exponent relate to mineral size and size distribution, respectively. Typical values for these parameters are tens to hundreds of milliseconds for the time constant, and 0.5 to 0.9 for the exponent. The energising current is typically applied for a second or a few seconds and the decay is measured for the same length of time.
Figure 1: The Raw IP Waveform Recorded
at 1ms Samples with the CVF2
The injection is a +ON for two seconds,
followed by an OFF for two seconds,
followed by a -ON for two seconds.
A small bias voltage present in the
ground before the injection is not
evident on the scale of this figure,
nor is the OFF decay readily apparent
Figure 1 shows a raw IP waveform recorded at 1ms intervals with the CVF2. The injection measured on the receiving electrodes is labelled ON+ or ON- as the injection is cycled positive and negative with a two-second period. The primary voltage is the mean potential measured on the receiving electrodes during an ON cycle. In this case, the primary voltage is about 3,000mV. The OFF, or secondary, voltage is recorded for two seconds after each ON pulse. On the scale of Figure 1, the decay of the OFF voltage is barely apparent.
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