1SIGNAL COIL CALIBRATION OF ELECTRO-MAGNETIC SEISMOMETERS By

We show that electro-magnetic (e-m) seismometers may be easily and accurately 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 which 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 mass (from the manufacturer) and the applied current (measured) need be known for a complete calibration. The coil and damping resistances are 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 undamped generator constant computed from the damped generator constant obtained by the signal coil method matched the generator constant given by the manufacturer to better than 1%. Calibration of nine new L-4C components 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. 2INTRODUCTIO