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"By the mid-1990s, our research group at UBC had carried out several field experiments confirming that seismoelectric effects were real, measurable, and interpretable. We had also done a fair bit of modeling to investigate the conversion processes responsible for the piezoelectric, electrokinetic, and radio-frequency emissions we had observed. While custom-designed instrumentation and processing techniques were critical to the success of that work, we recognized that engineering seismographs were evolving quickly and could, with appropriate preamplifiers, make attractive recording platforms for the next generation of seismoelectric acquisition.
"Desirable features include large dynamic range, generous pre-trigger capability, wide bandwidth, and a large number of channels to allow simultaneous capture of signals from both geophones and antennas. The seismoelectric signals we seek to measure are very small (e.g. 1 microVolt/m) and are commonly one to three orders of magnitude smaller than the ambient electrical noise contributed by power lines and other sources. However, if the recording system has sufficient dynamic range, dramatic improvements in signal-to-noise ratio can be realized in processing by subtracting noise estimates based on harmonic noise models and on recordings made by remote reference antennas. The availability of 50 – 100 ms of pre-trigger data is very useful for purposes of formulating such noise estimates.
"A limitation of seismic recording systems in this application is the nature of their preamplifiers, which are designed to accept signals from low impedance sources (geophones). We have developed battery-powered differential preamplifiers to buffer and amplify the signals measured by our antennas prior to passing them on to a seismic cable for delivery to the seismic system. These preamplifiers prevent voltage divider effects, and ensure that recording bandwidth is not significantly affected by spatial variability in the resistance between the pair of stakes comprising each grounded dipole antenna. They are also designed to handle and reject high levels of common-mode AM radio signals which can otherwise be inadvertently demodulated by seismic preamplifiers and appear as voices in one's data! It is good practice in fact to listen to the output of any amplifier used for seismoelectric recording – either directly using headphones, or indirectly by converting the digitized record to a 'wav' file and playing it back on a computer. This has been standard practice for us since 1990 when we found that records from an Australian field experiment were contaminated by broadcasts of horse racing, and opera!
"The photograph above shows our seismoelectric recording system set up for a field trial along the shore of the St. John River in Fredericton. The silver boxes on the ground are our custom preamplifiers, connected to 24 two-metre grounded dipole antennas. The preamplifier outputs are transmitted along seismic cables to two 12-channel Geodes that are in turn networked to a notebook computer running MGOS acquisition software.
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