GEOPHYSICAL INSTRUMENTATION
WITH THE
CVF2 DATA LOGGER
J.B. Merriam
Geological Sciences
University of Saskatchewan
An Innovention Industries CVF2 data logger 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. The data logger is a fraction of the cost of any single geophysics receiver and can in principle replace several. Most importantly, complete signal waveforms are available from the data logger, 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.
Here I describe an Induced Polarization (IP) receiver. IP is a technique used in exploration geophysics to detect the presence of metallic conductors. Sulphide ores are a typical target. In this method a high potential (several hundred volts) is applied to two metal electrodes driven into the ground. Currents of an amp or more energize the ground and charge is stored wherever polarizeable material is present. Metallic conductors in pore spaces, and clay, are common polarizeable materials. When the energizing 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 polarizeable 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-polarizeable 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 characterized 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 energizing current is typically applied for a second or a few seconds and the decay is measured for the same length of time.
Figure 1 shows a raw IP waveform recorded at 1 ms intervals with the cvf2. The injection measured on the receiving electrodes is labeled ON + or ON- as the injection is cycled positive and negative with a 2 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 3000 mV. The OFF, or Secondary Voltage, is recorded for two seconds after each ON pulse. On the scale of this figure, the decay of the OFF voltage is barely apparent.
Figure 1 Theraw IP waveform recorded at 1 ms samples with the cvf2. The injection is a +ON for 2 sec , followed by an OFF for 2 sec, 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.
The processing steps required to produce theh summary information are: identify ON and OFF times, calculate the Primary Voltage in a suitable window, calculate and remove any potential that may exist in the ground before the injection, stack all the OFF cycles together (adjusting for the polarity of the injection), and finally summarize the decay by reducing it to a suitable number of windows.
Figure 2 shows the data trimmed to full ON and OFF cycles (top), the selection of a window for the Primary Voltage so that the ON transients are not included in the calculation of Primary Voltage (middle) and the two OFF cycles (bottom). The scale of the bottom figure makes the spontaneous potential (about 10 mV) and the OFF decay in time much more obvious. A bias, or spontaneous potential, existing in the grouns before the injection has been removed.
Figure 2. The initial steps in calculating the chargeability. Top figure, the raw data. Middle figure, the instantaneous Primary Voltage (- red and + blue) and the window (post dotted line) over which an average Primary will be calculated. Bottom, the measured IP decay during the OFF time.
Finally, the individual OFF decays are stacked and suitable windows are chosen to characterize the decay, Figure 3.
Figure 3. The stacked and windowed chargeabilities. The 1 ms samples are shown as blue dots and the windowed chargeabilites as red asterisks. The instantaneous chargeabilities are the ratio of the Secondary, or OFF voltages, to the Primary. In this case, the windowed chargeabilities go from about 2 mV/V to nearly zero, so the decay has not quite completed by the end of the two second observation time.
The windowed chargeabilities calculated from the CVF2 are shown as asterisks in figure 4. Also shown are the windowed chargeabilities at the same station obtained with a commercial IP receiver (circles). The agreement between the two is excellent.
Figure 4. Windowed chargeabilities from the cvf2 data logger (asterisks) and a commercial IP receiver (circles).
The windowed chargeabilities may be further summarized by calculating three parameters that characterize the Cole-Cole model for the IP decay. These are: a chargeability (effectively the chargeability at time zero), a time constant and an exponent for the time constant. At this station the IP response is weak, coming only from the clay clay content of the till.
Figure 5. The windowed chargeabilities from the CVF2 (circle) and the predicted decay based on Cole-Cole parameters inverted from the windowed chargeabilities.
At the University of Saskatchewan geophysics field school we have made extensive use of the CVF2 data logger to create specialized geophysical instrumentation. These include a seismograph, a resistivity meter, a magnetotelluric receiver and the induced polarization receiver described here. The flexibility of the data logger, as opposed to a specialized commercial receiver, is of great advantage in research and teaching.