The processing steps required to produce the summary information are as follows: 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 summarise 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 theprimary voltage so that the ON transients are not included in the calculation of primary voltage (middle), and the two OFF cycles (bottom).
Figure 2: The Initial Steps in Calculating
the Chargeability
Top figure: the raw data; Middle: the
instantaneous primary voltage
(- red and + purple) and the window
(post dotted line) over which an
average primary will be calculated;
Bottom: the measured IP decay during
the OFF time
The scale of the bottom figure makes the spontaneous potential (about 10mV) and the OFF decay in time much more obvious. A bias, or spontaneous potential, exists in the ground before the injection has been removed.
Finally, the individual OFF decays are stacked and suitable windows are chosen to characterise the decay (see Figure 3). The windowed chargeabilities calculated from the CVF2 are shown as stars in Figure 4. Also shown arethe windowed chargeabilities at the same station obtained with a commercial IP receiver (circles). The agreement between the two is excellent.
Figure 3: The Stacked and Windowed
Chargeabilities
The 1ms samples are shown as pink
dots and the windowed chargeabilites
as purple circles. The instantaneous
chargeabilities are the ratio of the
secondary, or OFF voltages, to the
primary. In this case, the windowed
chargeabilities go from about 2mV/V
to nearly zero, so the decay is not
quite complete by the end of the
two-second observation time
The windowed chargeabilities may be summarised further by calculating three parameters that characterise 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 content of the till.
Figure 4: Windowed Chargeabilities
from the CVF2 DataLogger
(green asterisks) and a Commercial
IP Receiver (red circles)
The University of Saskatchewan Geophysics Field School has made extensive use of the CVF2 DataLogger to create specialised geophysical instrumentation. This includes a seismograph, a resistivity meter, a magnetotelluric receiver and the induced polarisation receiver described here. The flexibility of the DataLogger, as opposed to a specialised commercial receiver, is of great advantage in research and teaching.
Figure 5: The Windowed Chargeabilities
from the CVF2 (green hexagons) and
the Predicted Decay Based on Cole-Cole
Parameters Inverted from the
Windowed Chargeabilities
Category:
Integrated Operations
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