Generally speaking, the accelerometers, being an active device, are more accurate and also more stable than the geophones so it is reasonable to assume that the most likely reason for the difference is that the assumed generator constants for the geophones are not accurate. Rodgers et al. (1995) describe a way to absolutely calibrate the geophones in situ and to determine their generator constant, free period and fraction of critical damping. The only external parameter that is required is the value of the geophones inertial mass.
We have built a calibration test box which allows us to routinely perform the testing described by Rodgers et al. whenever site visits are made. The box drives the signal coil with a known current step and rapidly switches the signal coil between the current source and the data logger input. From this information, expected and actual sensor response characteristics can be compared and corrections applied. Also, changes in the sensor response over time can be evaluated so that adjustments can be made and pathologies arising in the sensors due to age can be identified. Once a geophone is absolutely calibrated, we can also check the response of the corresponding accelerometer.
We are now performing the initial calibration tests and response adjustments for all NHFN stations as sites are visited for routine maintenance. We also plan a scheduled re-test of all sites to monitor for sensor responses changes through time.
Berkeley Seismological Laboratory
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