Theory and Practice of Geophone Calibration In Situ Using a Modified Step Method

The transfer function of a moving coil electromagnetic geophone requires up to six constants: the suspended mass, the natural resonant frequency, the mechanical and the electrical damping factors, the inductive time constant of the coil-damping resistance circuit, and the coil transductance. If the mass is known, the remaining constants may be satisfactorily determined by displacing the mass with a known dc current, and observing the time varying output potential on removal of the steady current. The techniques outlined are suited to in situ calibration, and have a minimum equipment requirement of a CRO and a small battery. Improved accuracy is possible using a precision signal generator and a quality CRO, if the geophone natural and upper resonant frequencies are measured by the phase ellipse method. Calibration is possible for both under- and overdamped systems, using either the derived formulas or accompanying graphs. The accuracy attainable is dependent on the geophone damping and frequency range of operation, but is typically 2 percent in absolute amplitude and 2 in phase for 0.7 critical damping.