Calibration of Geophone Microseismic Sensors

This note describes a simple lab bench technique for calibrating the sensitivity and dynamics of the Mark Products L4C geophones used at SLAC for measuring microseismic ground motion and mechanical vibrations of accelerator support structures. Calibration results for SLAC’s three L4C geophones are recorded. 1 L4C geophone The L4C geophone is a seismic instrument producing a voltage proportional to the velocity difference ( ˙x − ˙xg) between its proof mass M and its housing. Voltage is generated in a pickup coil wound on the proof mass when a permanent magnet attached to the housing moves past it. The suspension system consists of a 1 kilogram proof mass suspended on soft springs with resonant frequency around 1 Hz. A photo of the instrument and the instrument’s schematic are shown below: Figure 1: Vertical L4C geophone In the righthand schematic, the proof mass M (kg) is suspended by spring stiffness k (n/m) and mechanically damped by dashpot b (n/m/sec). The motion sensing coil is mounte