Comparison Tests

The goal of comparison testing is to determine the calibration, coherency, and background noise PSD characteristics of the sensors being tested. Metrozet approached the BSL with a request to install a pair of Metrozet strong motion accelerometers in a seismically quiet vault for testing. We agreed, on the condition that Metrozet perform the bulk of the comparison analysis, and on April 6, a pair of three-component Metrozet accelerometers were installed for long term testing and comparsion with the BKS FBA-23 accelerometer recordings.

A comparison of the strong motion waveforms recorded by the two Metrozet sensors and the co-sited BKS FBA-23, for a recent ML 3.0 earthquake (2005.267.1125) which occurred 5.3 km SSE of BKS at a depth of 5.4 km, are shown in Figure 9.4. The raw data, without deconvolving the instrument response, have been low-pass filtered at 20 Hz for the comparison and the data are plotted on a relative scale.

Figure 9.4: Comparison of the acceleration signals recorded by the two three-component Metrozet accelerometers and the BKS FBA-23 three-component accelerometer.
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The corresponding waveforms from all three sensors shown in Figure 9.4 are visually highly similar and there are no offsets or other aberrant behavior in the Metrozet signals. Comparing the signal levels from the Metrozet sensors in counts with the corresponding ground accelerations inferred from the BKS FBA-23, we find that the sensitivity of the Metrozet sensor coupled to the Q4120 data logger is nominally 43.6x the sensitivity of the FBA-23 coupled to the Q680 data logger or 2.30 million counts/(m/s$^2$).

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