SIGNAL-COIL CALIBRATION OF ELECTROMAGNETIC SEISMOMETERS

We show that electromagnetic (em) seismometers may be easily and accur ately calibrated by removing a step of current from their signal coil, and simultaneously switching the signal coil to a recorder to capture the response. A theory is developed that obtains the damped generator constant, resonant frequency, and damping ratio from the output of a system identifier used to analyze the response. Only the seismometer m ass (from the manufacturer) and the applied current (measured) need be known for a complete calibration. The coil and damping resistances ar e not required. The method is confirmed by comparing this signal-coil method with weight-lift and calibration-coil calibrations. For a GS-13 V seismometer, these results were within 1.3% of each other. The unda mped generator constant computed from the damped generator constant ob tained by the signal-coil method matched the generator constant given by the manufacturer to better than 1%. Calibration of nine new L-4C co mponents resulted in undamped generator constants all within 3% of the values given by the manufacturer. The circuit used in the signal-coil method is shown and explained.