The NHFN station hardware has proven to be relatively reliable.
Nonetheless, numerous maintenance and performance enhancement measures
are still carried out. In particular, when a new station is added to
the backbone, extensive testing and correction for sources of
instrumental noise (e.g., grounding related issues) and telemetry
through-put are carried out to optimize the sensitivity of the
station. Examples of maintenance and enhancement measures that are
typically performed include: 1) tests on radio links to ascertain
reasons for unusually large numbers of dropped packets, 2) trouble
shooting sporadic problems with numerous frame relay telemetry dropouts,
3) manual power recycle and testing of hung Quanterra data loggers, 4)
replacement of blown fuses or other problems relating to dead channels
identified through remote monitoring at the BSL, 5) repair of frame
relay and power supply problems when they arise, and 6) correctingtenna
problems that arise due to various causes,such as weather or cultural
It is also apparent from the buzz underlying the earthquake signal in this display that the grounding schemes for stations SBRN and HERB may be in need of modification. One of the most pervasive problems at NHFN stations is power line noise (60 Hz and its harmonics at 120, 180, and 240 Hz). This noise reduces the sensitivity of the MHH detectors, and at any given station this type of noise source changes over periods of weeks to months, requiring continued vigilance and adaptability of the grounding scheme in order to maintain the desired high sensitivity to low amplitude seismic signals.
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 inaccurate. 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 geophone's 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 datalogger 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 also check the response of the corresponding accelerometer.
Berkeley Seismological Laboratory
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